# Open Revolt with IGBT driver blew IGBTs



## cpate (Nov 21, 2011)

Hey guys, for the past few months I have been putting together an Open Revolt controller with a VLA500 IGBT driver and 3 Fuji 2MBI300L-060 IGBTs. I've read about a few other people doing the same thing and I've read the documentation on driving IGBTs and minimizing inductance. However it seems that I've done something wrong because today we tested the controller and 2 or 3 of the IGBTs blew. We first tested it with no load or battery hooked up, just to test the IGBT driver. The signal looked fine, with the low Base-emitter signal being -8v, and the high being 16v. We then hooked it up to a 12v battery and a 30-ohm burner, and it ramped the current up and down just fine from 0 to .5 amps. I did not notice anything weird about the gate drive signal on my oscilloscope. Then we hooked up our 144v traction pack and when we opened the throttle up, a high whine and then a pop was heard, and current went up to about 7 amps through the burner.

Here are some pictures if anyone can spot any stupid mistakes I might have made. The gate resistor on each IGBT is 5.6 ohms, as per the IGBT spec sheet. Unfortunately, I didn't bother to implement any voltage clamping diodes on the gate wires. Could that have been my mistake?


































Any advice or comments are appreciated! Thanks!


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## z_power (Dec 17, 2011)

This might be not very important but shouldn't be the current sensor on M- busbar? Looking at pictures it's on B-, please correct me if I'm wrong. It doesn't sense current flowing trough freewheeling diode (E1->C1) during switchoff phase of PWM.

edit: it's hard to see, are E1-G1 connected on each module? are gate resistors parallel to g-e?


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## cpate (Nov 21, 2011)

Thanks for the response!
Maybe you are right. I just assumed that the current sensor is supposed to measure battery current. 
As for the resistors, there is a 10k ohm resistor between E1 and G1 and another one between E2 and G2 on each module. The 5.6 ohm gate resistors are between the driver and G2 on each module. The E2's are all connected to the driver.


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## bjfreeman (Dec 7, 2011)

cpate said:


> Unfortunately, I didn't bother to implement any voltage clamping diodes on the gate wires. Could that have been my mistake?
> Any advice or comments are appreciated! Thanks!


can't find a spec sheet but if you don't have FWD across the Source and Drain of each, the inductive EMF will be many time 144 volts. I am guessing about 1,440 volts.

Wire dressing to keep inductive pick up.


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## subcooledheatpump (Mar 5, 2012)

Across the gate wires? wouldn't adding a diode create a short circuit during negative gate turn off? I think he is using the FWD of the upper IGBT across C1 and C2E1

By the way, what switching frequency are you running those at? 

I've got the same IGBTs, I use them for a low frequency induction heater, I don't run them above 10 kHz.

Also with those crazy gate connections you've got going on, I would recommend increasing the Rg (gate resistor) to about 10 ohms. You'll get alot of voltage buildup between the gate driver board and the IGBT gate terminal with that low of a resistor.


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## MPaulHolmes (Feb 23, 2008)

motor amps * pwm duty = battery amps. So, if you tried, say, 20% of full throttle, the pwm duty would ramp up until the current feedback was 20% of the max current. I'm not sure what the software was, but on a 0.75" x 0.375" bus bar, the full range of that current sensor (assuming the melexis HB) is 0 to 1200 amps or so. That would mean you commanded 20% of 1200 amps from the Battery pack. The motor amps can be very very large when the battery amps are small. So, while the pwm was ramping up, in the vain attempt to see 20% of 1200 amps (or whatever), the motor amps would have been very large near zero rpm.


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## cpate (Nov 21, 2011)

@bjfreeman: if you mean freewheel diodes across the motor, I am indeed using the upper half of the IGBT module for this purpose.

@subcooledheatpump: I'm not sure what exactly you mean by "crazy gate connections." I will try switching out the resistors for ones with higher values.

I am using the firmware modified for a hall throttle and 8khz PWM.

Thanks for answering, paul! The thing about the motor current is, we were testing the controller with a stove burner, which is basically the same as a resistor as far as I know. The 30 ohm resistance of the burner we were using limits the current that can pass through it at 144v to about 5 amps. Is it possible that the heating coil has enough inductance to blow the IGBTs?


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## badfishracing (Dec 4, 2009)

The side of the IGBT that you're using as the freewheeling diode, the side that you didn't connect to the driver, should have the gate and emitter tied with a jumper instead of a 10K resistor? Might not be an issue though. The 10K might be enough to keep it off.


Darin


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## MPaulHolmes (Feb 23, 2008)

Oh, the burner was from B+ to M-?


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## badfishracing (Dec 4, 2009)

badfishracing said:


> The side of the IGBT that you're using as the freewheeling diode, the side that you didn't connect to the driver, should have the gate and emitter tied with a jumper instead of a 10K resistor? Might not be an issue though. The 10K might be enough to keep it off.
> 
> 
> Darin


Probably not a big factor, but I believe the normal inductance in the motor is an important part of the igbt/cap/diode power trinity. So straight resistance isn't really the load you want to use.


Darin


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## swoozle (Nov 13, 2011)

MPaulHolmes said:


> Oh, the burner was from B+ to M-?


Yes, the burner was across B+/M+ and M-


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## subcooledheatpump (Mar 5, 2012)

What I mean by crazy gate connections; The wires are a little long and aren't twisted tight, Could lead to inductance problems. 

I got to looking and was wondering, are you using any other capacitors besides the one mounted on the busbars in the pictures?


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## swoozle (Nov 13, 2011)

subcooledheatpump said:


> What I mean by crazy gate connections; The wires are a little long and aren't twisted tight, Could lead to inductance problems.
> 
> I got to looking and was wondering, are you using any other capacitors besides the one mounted on the busbars in the pictures?


Ya, we need to tighten those up. The curious thing is, we had the power side hooked up to a 12V source first and ran the PWM to 100% just like we did when the IGBTs blew. It was fine that first time. Doesn't that imply that the driver circuit/wires are good and that whatever is wrong is in the high-voltage circuit?

Yes, it's the only cap. Not trying to throw Paul under the bus(s), but that cap was Paul's recommendation.

In retrospect we should have looked closely at the driver and high voltage waveforms before charging ahead so quickly. Any suggestions on the best stepwise test sequence moving up to full pack and motor is welcome. Is it better to go straight to pack(full or partial?)/motor than to try and jury-rig some low current load?


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## Weisheimer (May 11, 2009)

cpate said:


> Thanks for the response!
> Maybe you are right. I just assumed that the current sensor is supposed to measure battery current.
> As for the resistors, there is a 10k ohm resistor between E1 and G1 and another one between E2 and G2 on each module. The 5.6 ohm gate resistors are between the driver and G2 on each module. The E2's are all connected to the driver.


I almost hate to post because I won't be able to follow this thread much over the next week or two...

The E1-G1 needs to be a jumper wire to keep the FWD side IGBT turned OFF.

The wiring needs tended to as well. Your drive signal from the control board to the driver board is hanging right over the top of your bus bars.
You might end up with an oscillator 

The common reference signal from the control board to the driver board appears to be one of the twisted pair wires used to drive the IGBT.
It should be from the control board and twisted tightly with the PWM FROM the control board.

The current sensor should be on the M- bus bar.

Your link cap is a decent film cap and should work fine.

You are almost there! (and I think that there is some sort of rule that you have to blow stuff up first anyway...)


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## swoozle (Nov 13, 2011)

Weisheimer said:


> The E1-G1 needs to be a jumper wire to keep the FWD side IGBT turned OFF.
> .....
> You are almost there! (and I think that there is some sort of rule that you have to blow stuff up first anyway...)



Fantastic, this is great! Thanks much.

And yes, I've already done the esplodin' IGBT trick on the charger. I was hoping we could avoid it for the controller. Ebay, here I come....


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## subcooledheatpump (Mar 5, 2012)

I agree the film cap should be enough for the local bus for the IGBTs, but really a few larger caps would offer some peace of mind, especially when switching hundreds of amps. 

Definitely short out G1 to E1. I always use a short copper wire and solder it to the terminals when using the IGBT as a chopper


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## jackbauer (Jan 12, 2008)

Just to add to what others have said. When driving multiple igbts in parallel you need emitter resistors. these should be connected between the kelvin emitters and the driver board.Should be about 10% of the gate resistor value. Have a look at some of my youtube videos on building the liquid cooled controller.


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## cpate (Nov 21, 2011)

Thanks for all the responses!

okay, so I'll set up kelvin emitter resistors, put a jumper on the FWD gate, shorten the gate wires, put the current sensor on the motor bus bar, and twist the pwm signal wire together with the controller ground. Hopefully this stuff will keep it from blowing up again!


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## evimarn (Jun 26, 2008)

jackbauer said:


> Just to add to what others have said. When driving multiple igbts in parallel you need emitter resistors. these should be connected between the kelvin emitters and the driver board.Should be about 10% of the gate resistor value. Have a look at some of my youtube videos on building the liquid cooled controller.


Hi any links please as doing a search seems leading to nowhere.
Thanks for the expert help .


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## z_power (Dec 17, 2011)

http://www.evbmw.com/igbt.pdf
"Paralleling - application of driver circuit"


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## swoozle (Nov 13, 2011)

evimarn said:


> Hi any links please as doing a search seems leading to nowhere.
> Thanks for the expert help .


http://www.evbmw.com/

http://www.youtube.com/user/pooey1911


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## evimarn (Jun 26, 2008)

swoozle said:


> http://www.evbmw.com/
> 
> http://www.youtube.com/user/pooey1911


Hi thanks just what I was looking for.);


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## JRoque (Mar 9, 2010)

Hi. Someone already mentioned but I would add your bulk capacitance bank even when just testing. Good luck!

JR


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## z_power (Dec 17, 2011)

There's a lot of useful information about driver circuits in Semikron's application notes AN7002 and AN7003, easy to find in internet.


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## swoozle (Nov 13, 2011)

z_power said:


> There's a lot of useful information about driver circuits in Semikron's application notes AN7002 and AN7003, easy to find in internet.


Yes, those look very useful, thanks.


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## swoozle (Nov 13, 2011)

OK, new IGBTs and some experiments. Please take a look and help us understand what's going on. 
These are all one IGBT with a 12V battery source running a starter motor. 
All have the same time scale and V scale except the last trace (V scale increased to show the driver signal, time scale the same). 

C-E trace, no snubbers, increasing but low PWM, amps probably around 20








One snubber added. 








Two snubbers added (though one had long leads)








same with stable PWM somewhere around 50%, current at ~35amps








The drive signal with 12V source and starter motor load.








The snubber. They came with the used IGBTs and we can't find any info on it.


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## cpate (Nov 21, 2011)

So we don't really understand why our voltage traces don't look like those shown in other people's pictures of their oscilloscopes when testing their controllers. But we're pretty confident that the snubbers are helping. The first snubber that we added, with extra short leads, helped a lot. The second one, which had the standard leads that were about .75" longer, helped less. I looked around for better snubbers and I found these:

http://www.digikey.com/product-detail/en/C4BTHBX4500ZAFJ/399-6240-ND/2783740









2.9 mOhm ESR, 29 amp ripple current rating, 5 microfarads. These have about 5x the capacitance of the ASC snubbers that we have, but I don't know if their ripple current rating/inductance/resistance is any better. Do you guys think that these will quell the voltage spikes to acceptable levels?


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## subcooledheatpump (Mar 5, 2012)

Am I reading that scope correctly? It seems you are jumping up to 60 volts during turn off

Have you added any bulk capacitance?

What gate resistor this time? It looks like it's not fully saturating the IGBT


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## cpate (Nov 21, 2011)

subcooledheatpump said:


> Am I reading that scope correctly? It seems you are jumping up to 60 volts during turn off
> 
> Have you added any bulk capacitance?
> 
> What gate resistor this time? It looks like it's not fully saturating the IGBT


Yup 60 volts or higher seems to be the spike level when the motor is starting. We haven't added any bulk capacitance, just stuck with the round 400v 380uf cap and added the snubbers. The gate resistor is up to 11.2 ohms + .56 ohms on the emitter. Hmm what does a non-saturated IGBT look like? The curve downwards after the spike?


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## subcooledheatpump (Mar 5, 2012)

The C-E should look roughly like the gate drive signal

The IGBT Should be on for the duration of the pulse, that should look like a flat line. Then it should almost instantly fall to zero, another flat line. Just like the gate signal. 

Personally, I would add some big capacitors to that, keep the snubber, but add some 6000 microfarad capacitors, rated at 400 volts. change the gate resistor to a slightly lower value, 10 ohms, then try again with the same setup. You really shouldn't be spiking that high with only 12 volts. It should really not spike above 15 volts.


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## cpate (Nov 21, 2011)

subcooledheatpump said:


> The C-E should look roughly like the gate drive signal
> 
> The IGBT Should be on for the duration of the pulse, that should look like a flat line. Then it should almost instantly fall to zero, another flat line. Just like the gate signal.
> 
> Personally, I would add some big capacitors to that, keep the snubber, but add some 6000 microfarad capacitors, rated at 400 volts. change the gate resistor to a slightly lower value, 10 ohms, then try again with the same setup. You really shouldn't be spiking that high with only 12 volts. It should really not spike above 15 volts.


OK, I am just wondering what are the signs of partial saturation? I know what the C-E signal is supposed to look like, but I just assumed that ours looked strange because of inductive spikes.

I will look at some big electrolytics to add to the DC bus, thanks for the tip.


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## PStechPaul (May 1, 2012)

It would really help to have a detailed schematic and a set of specifications for this. I looked at the BMW conversion website and watched some of the videos but there were still many details missing. I looked up the Open Revolt controller and found this:
http://www.paulandsabrinasevstuff.com/HelpFileMcontrollers.html

The schematics and the PC board and components look OK, but it is still unclear if there are any external snubbers to take care of voltage spikes on the MOSFETs. And if you are using IGBTs then you must have a modification to the circuitry, as you show in the video, but a schematic would be a lot more helpful.

Also, I don't know what sort of motor is being controlled, although I assume it is a PM DC motor, or perhaps a series wound motor. If it is a PM motor, I think it should have some dynamic braking or regen or load dump capability.

But if you were testing this on a resistive hot plate, there should be no significant inductance, and the waveforms should be very clean rectangular waves. If you are blowing IGBTs on a resistive load, and if you are getting messed-up waveforms as shown, then something is very wrong. 

Is it possible that the scope is set for AC input? That would explain same of the curved waveforms, and what appears to be some negative and positive portions.

Good luck, and HTH


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## cpate (Nov 21, 2011)

We're controlling a car starter motor and we are using IGBTs. The necessary modifications to the control board to use IGBTs have been applied. I don't think the scope has an AC input mode, it's a very simple portable oscilloscope. All the signals have been above 0v so far, ground is down at the bottom of the screen.


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## jackbauer (Jan 12, 2008)

The first thing i would do here is go back to basics. Drive one igbt and one cap. No snubbers. Also look at the 15v supply to the gate driver. Does it contain a lot of ripple? Is it dipping on load ? That 15v rail must be stiff as a board. Then start with a gate resistor that is too high. Say 33R and drive a pure resistive load and watch the C-E voltage on the scope. The miller plateaus should be quite obvious. Then connect up the starter. You should get no turn off kick with that high a gate resistor. If it does kick then the free wheel diode is not recovering in time. Look at the datasheet for the igb and make sure the the reverse recovery time for the fwd is faster than the turn off (toff) of the igbt.

Now , assuming its working ok at this point the igbt should be getting warm as its not switching fast enough due to the high value gate resistor. the trick is to get the gate resistor low enough to ensure the device switches as fast as possible but not so fast as to exceed the recovery time of the fwd. The vla502 driver switches on with +15v but off with -8v so you may need to use two gate resistors and a fast diode to shape the turn on and turn off to get best results. Only when you get this worked out on one igbt should you proceed to parallel up the others.


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## PStechPaul (May 1, 2012)

cpate said:


> We're controlling a car starter motor and we are using IGBTs. The necessary modifications to the control board to use IGBTs have been applied. I don't think the scope has an AC input mode, it's a very simple portable oscilloscope. All the signals have been above 0v so far, ground is down at the bottom of the screen.


The last two traces are the only ones that make any sense. And it seems that the gate drive does dip below ground (as I think is recommended). But the C-E trace above that shows the voltage well above ground and then passing through 0V and going negative. If you are measuring across the upper IGBT then it shows desaturation and a lot of power being dissipated when it should be solidly ON. If you are measuring to GND then you are seeing the supply rail. And if you are measuring across the lower IGBT, which is used as a commutating (or free-wheeling) diode, then you are seeing the motor voltage, but the waveform is more indicative of a low PWM than the 80% or so that is indicated by the gate drive.

I'd really need to see a schematic to provide any more than an assumtive guess. I can't figure out the modifications from the video. But it would help to try a resistive load so inductive effects will not be present.

I had to look up the VLA502. It should have desaturation detection built-in, and its own DC-DC converter for the gate drive. This app note shows the details: http://www.pwrx.com/pwrx/app/vla502_applnote.pdf


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## cpate (Nov 21, 2011)

jackbauer said:


> The first thing i would do here is go back to basics. Drive one igbt and one cap. No snubbers. Also look at the 15v supply to the gate driver. Does it contain a lot of ripple? Is it dipping on load ? That 15v rail must be stiff as a board. Then start with a gate resistor that is too high. Say 33R and drive a pure resistive load and watch the C-E voltage on the scope. The miller plateaus should be quite obvious. Then connect up the starter. You should get no turn off kick with that high a gate resistor. If it does kick then the free wheel diode is not recovering in time. Look at the datasheet for the igb and make sure the the reverse recovery time for the fwd is faster than the turn off (toff) of the igbt.
> 
> Now , assuming its working ok at this point the igbt should be getting warm as its not switching fast enough due to the high value gate resistor. the trick is to get the gate resistor low enough to ensure the device switches as fast as possible but not so fast as to exceed the recovery time of the fwd. The vla502 driver switches on with +15v but off with -8v so you may need to use two gate resistors and a fast diode to shape the turn on and turn off to get best results. Only when you get this worked out on one igbt should you proceed to parallel up the others.


OK I will try bringing the gate resistor extra high and see what happens. Thanks! The gate drive signal looks fine so I don't think that's the issue. But I haven't checked on the freewheel diode turn off time.


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## PStechPaul (May 1, 2012)

cpate said:


> @bjfreeman: if you mean freewheel diodes across the motor, I am indeed using the upper half of the IGBT module for this purpose.


If you are using the upper half of the IGBT module for freewheel diodes, then the motor must be connected between B+ and the E1-C2 connection, while E2 is at GND. So you are modulating the bottom half with the PWM? I don't believe it's a good idea to have the motor float at the top of the battery pack. Even if the frame is isolated from the M+ and M- connections, there is still some leakage, and the possibility of metal or carbon particles to create a conductive path to the frame. 

And *swoozle* also confirmed that the hot plate was connected from B+/M+ to M-. But the Open Revolt controller apparently has the motor connected from the common sources of the MOSFETs to the motor, and the diodes across the motor to GND. But I don't know if there is a reason to reverse those connections when modifying the circuit for IGBTs. I'd love to see a complete circuit diagram!


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## cpate (Nov 21, 2011)

PStechPaul said:


> If you are using the upper half of the IGBT module for freewheel diodes, then the motor must be connected between B+ and the E1-C2 connection, while E2 is at GND. So you are modulating the bottom half with the PWM? I don't believe it's a good idea to have the motor float at the top of the battery pack. Even if the frame is isolated from the M+ and M- connections, there is still some leakage, and the possibility of metal or carbon particles to create a conductive path to the frame.
> 
> And *swoozle* also confirmed that the hot plate was connected from B+/M+ to M-. But the Open Revolt controller apparently has the motor connected from the common sources of the MOSFETs to the motor, and the diodes across the motor to GND. But I don't know if there is a reason to reverse those connections when modifying the circuit for IGBTs. I'd love to see a complete circuit diagram!


I'm fairly sure that most if not all brushed motor controllers connect M+ to B+ permanently. I don't think that this is an issue. You are correct about the motor and battery connections on the IGBTs. I am not sure exactly what you mean about the MOSFET sources? That is true that the diodes should be across the motor but I don't think that they should be connected to ground.

Sorry I do not have a full circuit diagram. It's a pretty simple setup, you pretty much described it with your post. But you can take a look at the photos that we've previously posted if that helps!


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## PStechPaul (May 1, 2012)

OK, I found a simple schematic for an EV, and I was very surprised to find that, indeed, the battery (+) is connected to the motor (+), and the other (-) goes to the drive, which modulates the DC to the negative (B-). Well, I suppose this is OK, but it's not what I expected - or what I would like to have in an EV, but I'm not going to use a brushed DC motor anyway. I suppose this design may be an extension of simple low power PWM motor controllers where the switching element was a BJT or MOSFET which is on the low side, and the motor is the load on the collector or drain. And a simple commutating diode across the motor completes the circuit. Here's what I found:
http://www.evconvert.com/eve/ev-schematic










I don't know why it was so hard to find. It seems like a schematic would be the first thing to have when making an EV. Well, at least now I have one, and the only thing that needs to be added is the actual IGBTs and the driver circuits. The Open Revolt website has some schematics, but they are for their MOSFET version. It does not actually show the MOSFETs - or the freewheeling diodes - so I'd have to dig deeper to see how they are connected to the motor. I guess it really doesn't matter, electrically, but I just like the idea that the terminals on my motor will be at the same potential as the frame when it is not running. So, OK, I learned something! 

PS: And here are more schematics and other resources:
http://evhelp.com/Wiring_Diagrams.htm
http://evdrives.com/
http://waynesev.com/ev/ev_wiring_dia.html
http://visforvoltage.org/topics/schematic
http://www.evworks.com.au/tech/?section=circuits


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## TigerNut (Dec 18, 2009)

PStechPaul said:


> OK, I found a simple schematic for an EV, and I was very surprised to find that, indeed, the battery (+) is connected to the motor (+), and the other (-) goes to the drive, which modulates the DC to the negative (B-). Well, I suppose this is OK, but it's not what I expected - or what I would like to have in an EV, but I'm not going to use a brushed DC motor anyway.
> <snip>
> I guess it really doesn't matter, electrically, but I just like the idea that the terminals on my motor will be at the same potential as the frame when it is not running. So, OK, I learned something!
> <snip>
> http://www.evworks.com.au/tech/?section=circuits


The traction (144V) pack should be completely isolated from the chassis at all points - neither the positive or negative end is tied to the chassis of the vehicle or the motor frame. So it doesn't matter whether the B- is tied directly to M- or B+ tied directly to M+ - either way, there should not be a current path between the chassis and the motor terminals. The only thing that can happen here is that commutator and brush dust will accumulate in the motor and this may form a low-grade conductive path to the frame; but in that case it still doesn't matter which end is permanently connected to the traction pack.


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## MPaulHolmes (Feb 23, 2008)

PStechPaul said:


> The Open Revolt website has some schematics, but they are for their MOSFET version. It does not actually show the MOSFETs - or the freewheeling diodes


Here you go:

Capacitor bank + ------------------------- Freewheel diode Cathode
Freewheel diode anode ------------------- MOSFET drain
Mosfet Source ---------------------------- Capacitor bank minus.

-Paul


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## subcooledheatpump (Mar 5, 2012)

This is what he's doing 


This is what PSTechPaul says he should do 










Only difference is, a higher voltage is required to drive the upper IGBT, thats the positive IGBT (Collector on positive, emitter to the motor negative)

And the motor wouldn't be connected to the high voltage battery positive at all times.


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## PStechPaul (May 1, 2012)

Yes, I see now. I just ASSumed that the B- was at chassis potential as it is for ICE cars. If it is isolated and floating, then the IGBT drive must also be isolated and floating, and it does not matter if it is on the top rail or the bottom. The gate reference can be connected to either E1 or E2, and the isolated PWM drive will still provide the same +15V and -8V as specified for IGBTs.

How is reverse handled in such DC drives? A full H-bridge would take care of this nicely, and it could also short out both motor terminals for dynamic braking (although for series wound motors I don't think this would apply). And an H-bridge requires four IGBTs and drivers for each, so complexity is increased. 

Thanks for teaching me this fact about EVs. I still don't know why the waveforms are screwed up and the IGBTs are blowing. Hopefully the OP can find out why and fix it!


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## cpate (Nov 21, 2011)

Well yesterday we got some fancy new snubbers and a big electrolytic cap! Per Jack Bauer's advice I installed a 33 ohm gate resistor on the IGBT. With a resistive load and no snubber, there's basically no spiking and the miller plateaus are very clear. I guess that is a good indication that this is working ok.

Then we hooked the starter motor back up and added the new snubber. Unfortunately it still spikes to 20-30 volts upon turn off. As Jack Bauer pointed out, the IGBT did get warm. We were doing about 30 amps peak, with the motor.. We then hooked up 24v to see how big the spikes got and they jumped to 50-70 volts. It seems that the freewheel diodes are not doing their jobs very fast, or at all. However, the spikes are much smaller and we can now see the oscillations in the voltage, instead of the turn off and turn on being a huge spike up followed by a curve down. We will post oscilloscope pics soon to illustrate what I'm talking about.

I guess I expected the new snubber and high gate resistor to solve our problem, but at least we're making progress.


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## MPaulHolmes (Feb 23, 2008)

Sometimes the spikes can actually be noise being picked up on the oscilloscope. A good way to see what the spikes really are is to put a diode from B+ to another capacitor + (like 330uF or whatever you feel like using), then connect B- on the controller to B- on the new capacitor. It will act like a peak detector. Or a crab trap! Little electron crabs go into the capacitor, and they want to leave, but they can't! The stupid diode won't let them. Then you boil them for 20 minutes in ocean water, and eat them.


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## PStechPaul (May 1, 2012)

One easy way to see if the scope is picking up noise rather than a real signal is to connect the probe to the same point as its ground reference on the device. It should not show anything significant. Otherwise, what you see is probably inductively coupled or transients in the grounds. Sometimes a full differential reading is better, using two identical probes, and using the scope's A-B mode. Sometimes you can adjust one of the gains for minimum noise.

Sounds like good progress. Turn-off transients can be a bear!


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## cpate (Nov 21, 2011)

Thanks guys I will definitely try those things! That would be nice if it turned out to be interference. Oh and Paul your crab metaphor made my day


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## cpate (Nov 21, 2011)

Today we put on a 2200uF electrolytic capacitor and used that in conjunction with the new 5uF snubber. Here is the result










This is with one IGBT, one electrolytic and one snubber cap, inductive load, 33R gate resistor, 8khz switching circuit. Purple line is ground and green is signal. 10 volts per vertical division and 20us per horizontal division. The battery pack that it's switching is about 24v. 

Does anyone have any idea why the IGBT is switching off twice in the time it's supposed to switch on once??? and we have these huge capacitors and gate resistor but the spikes are still large and there is this big curve down after the turn off? I connected the oscilloscope probes to the same point on the ground and there were only small ripples in the signal. I don't think this is enough to create the voltage spikes that we see. I will do the diode/cap test next, but even if it's reading too high this waveform makes no sense 

Perhaps there is something very wrong with the Freewheel diodes... I guess I know what I should do now
a. augment/replace freewheel diodes??
b. do diode/cap test to verify oscilloscope results.

Any input is appreciated as we are stumped


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## PStechPaul (May 1, 2012)

I assume the waveform is the voltage from the bottom IGBT collector to GND? What is the gate signal under the same conditions? It appears that there are two fairly solid turn-ons, but one is about 20 uSec while the other is about 30 uSec. And these have 5 volts drop, so if you are running 50 amps that's 250 watts! Then there are the second turn-ons 6-12 uSec later, but there is a much higher voltage drop of 20-15 volts, which is an even higher amount of power.

You may be exceeding the safe operating area (SOA) of the IGBT, which can happen when there is a very high power dissipation for even a short time. It is generally destructive, but maybe you've been lucky so far.
http://en.wikipedia.org/wiki/Safe_operating_area
http://en.wikipedia.org/wiki/IGBT
http://www.electronics.dit.ie/staff/ypanarin/Lecture Notes/K235-1/2 Transistor-Thyristor.pdf

Apparently older devices were more susceptible to second breakdown, so if you have IGBTs from old drives that may be a problem

It would be very helpful to show your complete schematic, especially the base drive components, FWDs, and any snubbers or capacitors or inductors you have in the circuit.

If you have a very high snubber capacitance across the IGBT, the turn-off may cause a transient due to the motor inductance which could result in a breakdown of the IGBT, especially if the gate drive is too "soft". Thus the IGBT will act as part of the snubber and it will dissipate the inductive energy rather than the FWD. And the FWD also might be too slow, or defective.

So if you can show exactly what you have, I might be able to help.

Oh, and another clue is that the saturation voltage seems to be decreasing with time, which also hints at slow gate drive. If the IGBT is turned on full, the voltage should start very low and then rise as the inductance saturates. And it should be no more than 1 or 2 volts. If this is to be used for a motor that will draw 500 amps, you want to limit the peak dissipation to maybe 500W. If the IGBT has a 5V drop on a little starter motor, it will surely blow up on an automotive traction motor.


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## swoozle (Nov 13, 2011)

Let's see if i can get this right. 
It looks sooo simple...


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## PStechPaul (May 1, 2012)

That helps. But I don't see the freewheeling diode. And a proper snubber is a resistor and capacitor in series, across the motor terminals, or even better might be across the IGBT. Do you have part numbers for the IGBTs? And the specifications for the driver circuit. I'd like to see the gate drive waveform as well. And I think you said the motor was a 12V starter motor, and I think you said you had 30A peak. Is that the peak of the PWM pulses, or the maximum RMS? 

Also, your symbol for the battery is reversed. The wide line is positive, at least in my experience. And previously you said the battery pack was 24V, but the schematic shows 12V. 

I found another good reference about IGBTs and their characteristics and optimum drive techniques:
http://www.irf.com/technical-info/appnotes/an-990.pdf


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## swoozle (Nov 13, 2011)

Gate is tied to emitter on the other IGBT to keep it off and act as FWD.

Woops on the battery 

We are performing experiments with both 12V and 24V sources to try to understand this.
The amp numbers are RMS.

The IGBTs are Fuji 2MBI300L-060 (datasheet attached).

The gate waveform has been consistent, looks good as far as we can tell and is pictured in one of the above posts.

Thanks

View attachment 2MBI300L-060.pdf


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## PStechPaul (May 1, 2012)

I assume this is the gate drive? It looks like the peak gate voltage is almost 25 volts, while the absolute maximum is 20, so you should be around 15V. And this waveform looks like about 120 uSec out of 160 for a duty cycle of 75% and a frquency of 6.25 kHz.










But your more recent screen shots of the C-E voltage look more like about 20% duty cycle at 8 kHz, and the saturation voltage looks way to high, especially if you are running only 30 amps RMS. But at 20% duty cycle that's 150 amps peak.

You really should not be seeing such high spikes if the FWD of the upper IGBT is working properly. And the other transition about 30 uSec after turn-off may be some additional inductive ringing or noise that is getting to the gate and causing the IGBT to turn on again.

If you can measure the actual current through the IGBT then it might be more clear as to what is going on. And it may help to put about a 1k resistor and a 15V zener from the base to emitter. If you want to keep the -5V turn off drive then you will also need a 5V zener in series.

Have you tried again with a resistive load? IIRC even that was not quite right. You should get a very clean waveform and very low VceSat. Until you do, and until you provide the correct base drive, you should not attempt an inductive load.


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## swoozle (Nov 13, 2011)

Ya, simple little scope that's hard to decipher: the 0V on that trace is the small purple triangle to the right. The turn on is ~ +15V and the turn off is ~ -8V.

The duty cycle is unfortunately all over the place with our pics. Apologies.

We did do the resistive load and thought it looked pretty good.

Here's a better view of the recent progression: 12V, resistive load (low current level), 30R gate resistor, old 3uF snubbers








Same 12V and old snubbers but with starter motor in place of resistive load. The screwy double-turn-offs (or is it ons?) kick in. The voltage spikes are understated in this trace capture. They were worse.








Same 12V and starter motor but with new 5uF snubbers. Lower spikes on turn-off, flatter off-voltage but still the screwy double-kick. And upping to 24V seriously messed it up as in the set of pics posted a couple of days ago.


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## subcooledheatpump (Mar 5, 2012)

Leads me to think either the frequency is too high for the diode, it can't recover in time, or something is just wrong with the diode regardless. Those IGBTs are supposed to work at 300 amps 600 volts, no way they'd work like this at that power level, something is wrong

Could you try 1 ot 2 kHz or is that not adjustable with the PWM controller that you're using?


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## PStechPaul (May 1, 2012)

Is the first picture with the 15V signal the gate drive or the C-E waveform? The gate drive should be rock solid and should not spike or sag as it appears to. Are you sure it is DC coupled, and the probe is properly compensated?

You really should have a dual trace scope for this. And I don't understand why the PWM duty cycle (and frequency) are not constant. It's important to know if the IGBT stays fully ON during the high gate drive. And when it is off, the voltage should quickly reach the battery voltage and stay there. Is the battery voltage solid? Also if there is a large inertial load on the starter, it may act as a generator, but only if it is a PM motor and not series wound. However I have no experience with these motors and controllers so I'm just tossing ideas into the mix. 

Also, do you have the other components for the VLA500 IGBT driver, as shown in the following?:
http://www.pwrx.com/pwrx/docs/bg2a_application_note.pdf

I see that you are using the P&S controller, which IIRC has been modified for IGBTs. I found a schematic on their website:
http://ecomodder.com/wiki/index.php...r_Series_1000_DC_motor_controller_.28Rev2A.29

It would be helpful to see a modified schematic showing the changes that were made for the IGBTs.


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## cpate (Nov 21, 2011)

The PWM duty cycle is not constant because it is a constant-current controller, so it is adjusting the PWM to get the motor current to match the throttle input. The first picture with the 15v signal is the C-E waveform in swoozle's latest post. 

Unfortunately I don't have the programming know-how to change the firmware to a 2khz drive signal. I agree there is something about the diode. I don't think we are out of the safe operating area, because we are measuring the current and voltage across the IGBT and neither are near the maximum rated. The saturation voltage is definitely something to look into, as a symptom that will tell us more about our problem. Our gate drive is very slow right now because of the 30 ohm resistor. However, with the 5.6 ohm resistor, we seemed to be having the same double-switching issues. 

I think the gate drive is fine but I will check again. You are right about the resistive load though, the voltage graph was surprisingly spiky for very small current and voltage.

Looking at the diode reverse recovery time, it says 300ns for 900a/us di/dt and -10v Vge. So this should switch more than fast enough for our uses. Unless it's not switching fast enough because Vge is 0v for the side of the IGBT that we're using as the diode, because G and E are directly connected?


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## PStechPaul (May 1, 2012)

Maybe the problem is with the current/torque mode control. An unloaded series wound motor is very difficult to control, because there will be very little current to work with. Can you put some sort of load on the motor shaft, like a fan? Can you take a scope reading on the output of the LEM current sensor? Are you monitoring the speed of the motor? And how are you setting the throttle?


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## z_power (Dec 17, 2011)

For low current testing of P&S controller i took LEM off the busbar and routed a few turns of cable trough it - it reads actual current multiplied trough loops number, it makes PID more stable with low power levels; I used 12V 50W lightbulb, then 80W wiper motor and 160W radiator fan, the last one was quite easy to control trough ~3/4 of throttle range.


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## cpate (Nov 21, 2011)

We have tested the output signal of the controller and of the IGBT driver and determined that the strange double-turn off is not coming from the controller! It is coming from the IGBT driver. We are not sure why the IGBT driver is doing this though. I have placed the oscilloscope on the 15v supply and it is as solid as a rock. I am thinking it my be because I disabled the short-circuit protection so I will re-enable it and see if that gets us anywhere. This is very strange behavior from the VLA500. The presence of the motor doesn't matter, it just always does this at low duty cycles, when you first twist the throttle. It even does the double-turn off with no load.


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## swoozle (Nov 13, 2011)

OK, we got the double-tap taken care of. It was a couple of things, one of which is a weird interaction between the netgain hall throttle and the revolt board. Whenever the revolt 5V power is connected to the throttle, the 5V line takes on a 8khz roughly-sinusoidal 4-6V pattern which is then superimposed on the throttle output. Needless to say the VLA500 doesn't like that. Couldn't figure out the root cause so I took care of it with an off-board 5V regulator to supply the throttle. Problem solved.

The one that remains and is giving us fits is this long tail on turn-off. With a 30R gate resistor the switch time should be around 2us. The FWD reverse recovery time is supposed to be .3us. And yet the tail looks like the FWD isn't working at all over 40 or 50 us. What the heck? This is only 12V and 60 or 70 amps.

The behavior is the same using another IGBT of the same model (Fuji 2MBI300L) as well as another different IGBT (2MBI200N, same as in the EMW charger). I can't help but think it must be something obvious but we aren't finding it. Ideas are very welcome.


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## jackbauer (Jan 12, 2008)

My original controller used a pair of 1mbi800 fuji parts and worked fine even with a unipolar 12v gate drive signal. Next thing to do is start dropping down the gate resistor and see if the the spike magnitude increases with switching speed.


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## swoozle (Nov 13, 2011)

jackbauer said:


> Next thing to do is start dropping down the gate resistor and see if the the spike magnitude increases with switching speed.


Yes, it does. I dropped back to 10R with the same setup and the spikes got worse.

The gate on the FWD is just grounded. If that switch is turning on (as has been mentioned in other threads), would it exhibit this behavior? Doesn't seem like it, but I'm barely a novice here. Would a negative applied voltage on the FWD gate help by making sure the FWD switch stays off?


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## PStechPaul (May 1, 2012)

Trying to analyze this... If you are getting faster, higher spikes at turn-off, that shows that the IGBT is working properly. The magnitude of the spike should not be much higher than the bus voltage if the upper IGBT as FWD is working properly. A properly designed RC snubber across the motor (or across the lower IGBT) should reduce that spike and the ringing, although it may eat some power.

But your scope trace awhile ago also shows a decaying voltage that seems to start at a voltage substantially higher than the nominal DC bus voltage, and this cannot be if the upper FWD is working. The motor connection between the two IGBTs cannot go any higher than one diode drop above B+. So, either the diode is defective, or your bus voltage is not solid. If you are isolating the B+ from the battery with a diode, then it could rise to a much higher voltage, which would then bleed off through the bus capacitors. This could also be caused if the motor was generating during the OFF cycle. I don't think a series DC motor will do this, but if your starter motor is a PM type, it may do so. In that case, you may need much larger capacitors from B+ to B-. And you should be able to see this effect by scoping the B+ line.


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## cpate (Nov 21, 2011)

I have not heard of anyone putting snubbers across the load or freewheel diodes. Are you sure that this is a common practice? I think that the next step may be to attach some external diodes directly to the motor and try to stamp out the spike at the source.


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## PStechPaul (May 1, 2012)

The snubber is used to absorb the transient that happens when current through an inductance is interrupted. Once the freewheeling diode conducts, it is no longer needed, but without an RC snubber there can be a very high, fast spike that can damage the IGBTs. I found a lot of information and posted some links on Stiive's thread:
http://www.diyelectriccar.com/forums/showthread.php/3-phase-dtc-svm-induction-motor-74151p18.html

Particularly useful might be the following:
http://www.cde.com/tech/design.pdf

It does show snubbers across the DC voltage supply but only when there is series inductance from the original source (batteries). Even lengths of wire have inductance, and the insulation of the windings of the motor have distributed capacitance which can "ring", which is shown by the high frequency damped oscillations. If you do not have a really good scope, it is possible that you are not seeing all of the transients, and what you don't see can damage the semiconductors or degrade the insulation.

But the main problem appears to be the decaying voltage with a rather long time constant that occurs at turn-off. This indicates a much larger source of stored energy, which could be a large inductance, or, more likely, the rotational inertia of the motor armature which is acting as a generator. However, if the voltage as shown on the scope is more than a couple volts above the B+ voltage, either the FWD is open, or there is little capacitance on the B+ line. That is easy to check with a scope.


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## cpate (Nov 21, 2011)

How to check B+ line capacitance? And yes we seem to be sure that the FWD is simply not working. So I believe the next step is to put an oscilloscope across the FWD and see what is going on with that, and if it's not working, put some external diodes directly across the motor.


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## PStechPaul (May 1, 2012)

The first thing I would do is put the scope from B- to B+, right at the IGBTs. It should be just about rock solid, with maybe just one or two volts of ripple depending on the drive for the motor. If you see variations similar to what you see when the scope is from the motor (-) to B-, then the capacitance is too small. How much capacitance do you have? IMHO it should be at least 1000uF, and of course it needs to be rated for the maximum B+ voltage. 

If you want to measure the actual capacitance, you can just connect the batteries to charge it up to the pack voltage. Then you can disconnect the batteries and put a resistor across the capacitor (it should have a bleeder anyway), and measure the time for the voltage to drop to 37% of the full charge. This is the RC time constant:
http://en.wikipedia.org/wiki/RC_time_constant

So since T=RC, C=T/R. If you have 100 volts and with a 10,000 ohm resistor it drops to 37 volts in 10 seconds, the capacitance is 1,000 uF.


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## cpate (Nov 21, 2011)

I remember now that we have done a measurement of B+ to B-. We only did it once and I don't remember under what circumstances. I do remember that it was very wavy, with a sinusoidal thing going on starting at each turn-off.

The thing with the increased capacitance is - we tried a 400v 2200uf cap and it didn't damp much better than our 380uf 400v cap. Installing a small snubber close to the IGBT terminal made more of a difference than using a larger main bus capacitor.


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## PStechPaul (May 1, 2012)

To be really effective, the capacitor must have a very low ESR at the frequencies involved. The capacitors I have which are from a VFD are 3300 uF 400V with an ESR of 10mOhm ot 100kHz. They are Kemet PEH200 series. A Vishay/Sprague 36DY series capacitor of similar size is about 40 mOhms at 100 Hz, and not specified at 100kHz. You can get effectively lower total ESR by adding smaller film type capacitors in parallel.

Also, make sure your scope isn't picking up noise. Try shorting the probe and touching it to the point you are trying to measure. Anything you see is noise and not valid signal. Don't make any assumptions. If you saw any significant ripple on the DC bus you should take a snapshot or at least draw it, with measurement of time and amplitude.

Here is a good reference of capacitor terms and properties:
http://wiki.xtronics.com/index.php/Capacitors_and_ESR

Do you have a really good, complete schematic for the entire circuit? You should include any capacitors, resistors, diodes, snubbers, and even long runs of wire and connectors. Also the driver circuit, especially the changes you have done for the IGBTs.


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## swoozle (Nov 13, 2011)

Tried a few things today, let's see if I can tell the story.
In general the CE traces are 10V per division, everything else is 5. 0V is indicated by a little purple triangle on the right axis.

The main cap is a Kemet 380uF, .8 mOhm ESR at 10khz. Confirmed the capacitance using your time RC time constant measurement.

Here's the setup. 12V batt to the left, controller board center-ish (w VLA500 driver & one 300A 600V Fuji IGBT). Using 30R gate resistor to slow everything down. Motor is out of view to the right. Wires to and from the batt/motor are 10 or 12 AWG.









Here's the gate drive signal. Looks clean, roughly +17V to -10V swing. The 15V power supply line is steady (not shown).








Tried a couple of things that are not shown. Applied -12V to the FWD gate to keep it off. No change. Tried reducing the gate resistor again (to 10R) and yes, it gets MUCH worse. Turn-off spikes to 100V on a 12v supply. Back to 30R.

Here's CE for a small resistive load on 12V supply.








Here's a low duty cycle CE trace, 12V supply, starter motor load. Bleh.








Here's a medium duty cycle CE trace, 12V, starter motor. Better. In general low duty cycle is a mess with the long tail on turnoff. That goes away as the duty cycle increases. The turnoff spike stays but is diminished. But everything goes to crap using a 26V supply. Turn-off spikes are outta sight, the tail is much worse and never goes away regardless of duty cycle.








Here's a low duty cycle B+/B- trace (still 12V). Not very stable.








Medium duty cycle B+/B-.








Now things start to get interesting. Baffling (to me), but interesting. Added a 2200uF electrolytic cap, but it has about 80mOhm ESR (all I have to play with at this point). This is in addition to the kemet.
Here's the B+/B-. Significantly more stable.








But here's the CE with the added cap (high duty cycle). Again, this is on 12V. On turnoff, the voltage spikes to ~40V and never gets even close to 12V. That yellow horizontal line is at 24V. This was NOT using a 24V supply.
When the switch turns back on the voltage has problems getting close to 0v. This is under no load stable running. At this high of a duty cycle and without the added cap, the CE looked fairly good (see above). The high ESR cap made it much worse.








So how do I interpret this? B+/B- instability indicates insufficient main cap size? The added cap didn't help because the ESR was too high?


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## PStechPaul (May 1, 2012)

swoozle said:


> Tried a few things today, let's see if I can tell the story.
> In general the CE traces are 10V per division, everything else is 5. 0V is indicated by a little purple triangle on the right axis.
> 
> The main cap is a Kemet 380uF, .8 mOhm ESR at 10khz. Confirmed the capacitance using your time RC time constant measurement.
> ...


It's hard to tell if the setup has any major problems. It's best to keep all connections short, or at least twisted together to minimize inductance. The battery leads seem to go all over the place. But if you have a really big bus capacitor as it seems, that should take care of those current surges. And it seems like you're running on 7.8 kHz, so that's not terribly high frequency.



> Here's the gate drive signal. Looks clean, roughly +17V to -10V swing. The 15V power supply line is steady (not shown).
> View attachment 13431


I don't see any problems there. 



> Tried a couple of things that are not shown. Applied -12V to the FWD gate to keep it off. No change. Tried reducing the gate resistor again (to 10R) and yes, it gets MUCH worse. Turn-off spikes to 100V on a 12v supply. Back to 30R.
> 
> Here's CE for a small resistive load on 12V supply.
> View attachment 13430


There is some indication of inductance even there. And the voltage should bang solidly between 0 volts and 12V, within maybe 0.5V. I can't tell where the zero reference is. So, first sign of possible trouble...



> Here's a low duty cycle CE trace, 12V supply, starter motor load. Bleh.
> View attachment 13428


That looks really bad. Still, I can't tell where zero reference is. I assume it's about 2-1/2 divisions from the bottom. So it's jumping to about 42V on turn-off, and then after about 100uSec it settles to about 15V, so still above battery voltage. This indicates that the upper IGBT used as a FWD is not functioning.



> Here's a medium duty cycle CE trace, 12V, starter motor. Better. In general low duty cycle is a mess with the long tail on turnoff. That goes away as the duty cycle increases. The turnoff spike stays but is diminished. But everything goes to crap using a 26V supply. Turn-off spikes are outta sight, the tail is much worse and never goes away regardless of duty cycle.
> View attachment 13429


It still appears that the voltage is rising to about 30V, and then settling to about 20V. Seems like the starter motor is acting as a generator - and the FWD is open.



> Here's a low duty cycle B+/B- trace (still 12V). Not very stable.
> View attachment 13427


Hmmm. It looks like it's a triangle wave alternating between about 7V and 17V. Just what would be expected with a very large inductance, or a motor running as a generator.



> Medium duty cycle B+/B-.
> View attachment 13432


So your B+ starts rising at turn-off, and then falls during turn-on. Seems like the B+ has a huge amount of inductance or the battery is sagging greatly. Your battery connections might be bad. Read the voltage from the battery to the end of the cable where it's clipped onto the B+ and B-. If you get more than one volt, it's bad, and you need go no further until you get a solid connection. Or, if the battery voltage is jumping that much, you have a bad battery. (Unless you're drawing over 100 amps!)



> Now things start to get interesting. Baffling (to me), but interesting. Added a 2200uF electrolytic cap, but it has about 80mOhm ESR (all I have to play with at this point). This is in addition to the kemet.
> Here's the B+/B-. Significantly more stable.
> View attachment 13426


OK, a large capacitor will compensate somewhat for bad connection or bad battery, but not completely.



> But here's the CE with the added cap (high duty cycle). Again, this is on 12V. On turnoff, the voltage spikes to ~40V and never gets even close to 12V. That yellow horizontal line is at 24V. This was NOT using a 24V supply.
> When the switch turns back on the voltage has problems getting close to 0v. This is under no load stable running. At this high of a duty cycle and without the added cap, the CE looked fairly good (see above). The high ESR cap made it much worse.
> View attachment 13425


If the motor is acting as a generator, it will charge the capacitor to whatever it puts out, and then that charge will bleed back through its ESR. I think you have a multitude of problems.



> So how do I interpret this? B+/B- instability indicates insufficient main cap size? The added cap didn't help because the ESR was too high?


It's been quite a challenge interpreting these scope shots, but I think if you reconnect your test setup with short lengths of cable and take care to take the scope measurements directly on the terminals of the IGBT, you will find that things make more sense and it should work fine. But I'm also not sure if your setup is good with a starter motor instead of a series wound traction motor. If you spin the starter motor by hand and get voltage out, then it's a PM motor and it might not like being connected to a FWD, and it may have produced enough power to have damaged the IGBT. These are all things that can be checked individually, so that when they are all connected together as a system you should have few surprises.

HTH. Good luck! 

BTW, here is a presentation on the use of IGBT snubber and DC link capacitors: http://dkc1.digikey.com/us/en/tod/Kemet/film-caps-for-power-apps/film-caps-for-power-apps.html


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## swoozle (Nov 13, 2011)

Great, thank you. I checked quickly and it is a PM motor.
I think you mentioned that previously and I forgot to check.

We'll work on the other things as well but I'm hoping that's the big, basic mistake I was expecting to find.


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## cpate (Nov 21, 2011)

Today we hooked up the controller to our big motor hoping to make some progress. This didn't work though.

We tried measuring the voltage across the freewheel diode - from M+ to M- there were positive spikes but only very short negative ones every once in a while. So I guess the freewheel diodes work alright.









*Freewheel Diode Collector-Emitter with 12V in-out*









*12V main IGBT C-E graph with 30 ohm resistor*









*26V voltage testing graph*









*12v B+ to B- Graph*









*26V B+ to B- Graph*

What's interesting is that the voltage across the battery cables in both directions looks a lot like this graph. It actually oscillates between positive and negative which makes me think that there's a lot of stray inductance in the cables.

Anyways do you guys think that more capacitors with lower resistance will solve our woes? Is there anything else we can try? This frustrating occurence is holding up our project 

Here is a picture of our setup in case there is something obviously wrong.


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## Stiive (Nov 22, 2008)

Hi,
Haven't read your whole thread, just the last post where you've implemented some changes.

Don't know if you have before, but you should show a close-up of your hardware... From afar the leads looks long and cross over high EMI parts of the controller. This could be causing problems.

How long is the motor lead? Is it looped on the floor? Maybe you have high inductance here causing these voltage spikes...


What I think has happened here is somewhere along the way you have damaged your FWDs in the IGBT. Its easy to check with a multimeter with diode setting. Check between collector and emitter of each gate, probe leads one way should read open circuit, the other way around should read diode drop. Check this value against spec sheet of IGBT to see if you have damaged the FWD.
Both diode drops should be very close (within a few mV), otherwise there's something wrong.


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## Stiive (Nov 22, 2008)

Well, your other post arrived while i was writing mine.

Can you provide more pics of the hardware? How many IGBT bricks are you using? From the lack of screws it looks like its set up for 3 but your only using 1 at the moment

From a very brief look i would suggest place a EMI shield between IGBT and drive board and look at turning the IGBT around so the gate leads can be minimised and don't have to cross over the DC bus/motor leads.


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## JRoque (Mar 9, 2010)

Hi. That's strange, I can't see the bank of bulk capacitors or the heavy wires from the pack/to the motor. Need to look closer I guess.

JR


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## Stiive (Nov 22, 2008)

JRoque said:


> Hi. That's strange, I can't see the bank of bulk capacitors or the heavy wires from the pack/to the motor. Need to look closer I guess.
> 
> JR


He's got a film cap there, no need for bulk caps - that will actually make things worse. Reducing inductance is the name of the game here

I also looked for the motor leads... The very thin power leads I can see are perhaps being used as PTC current limiters  Maybe we have low voltage/current testing here.


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## PStechPaul (May 1, 2012)

If you are getting any significant voltage across the length of any of the cables, then the resistance and/or inductance is too high. Only one of the cables from the battery appears to be heavy gauge. The green wire looks like it's only about 12 AWG. Even if you are expecting only 20-30 amps RMS, and the wire may handle it without overheating, it will affect the high current surges of the PWM. A much larger capacitor may help a little bit, but it needs to be able to handle surge currents as high as a couple hundred amps, even if the average (or RMS) current is much less.

It's still difficult to determine where the zero volt reference is on your scope traces. But it's pretty obvious that there is a great deal of inductance in the circuit. Remember that inductance increases with the enclosed area of a conductor loop. A big circle is worst. If you twist the leads together or at least bundle them in pirs with ty-wraps, you should see some improvement. 

I know this can be frustrating but there are definite steps you can take to determine where things are going bad, and there are ways to make improvements. Good luck!


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## cpate (Nov 21, 2011)

JRoque said:


> Hi. That's strange, I can't see the bank of bulk capacitors or the heavy wires from the pack/to the motor. Need to look closer I guess.
> 
> JR


We used 8 gauge wires and the motor only pulls 75 amps max, are you saying we need to go bigger? 
I would think that 8 gauge would be big enough for that current!

And with the large 2200uf cap wired up, it smoothed the battery signal but seemed to distort the C-E signal as seen previously.

The diodes were checked earlier today and they look fine.
Here is our setup when we were at 12v


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## Stiive (Nov 22, 2008)

More hardware pics please from different angles... I cant see where all the leads go.



cpate said:


> The diodes were checked earlier today and they look fine.
> Here is our setup when we were at 12


You need to do the test I listed before. The diodes might not be 100%.


Also, the probes you are using with the scope are not great. You possible need non-intrusive probes to measure the signals without causing problems.


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## PStechPaul (May 1, 2012)

I finally I found the little purple triangle on the scope traces indicationg 0V ground reference. It's a bugger that it's midway between grid lines, but at least I can tell a little more about what seems to be wrong. The C-E voltage of the controlling IGBT should not be as high as it is. It seems like the voltage goes to zero for a couple microseconds, then jumps up to about 4 volts and then decays to 2 volts. Usually, switching an inductive load, the C-E voltage would initially be very low, and then perhaps rise as current in the inductor increases.

But perhaps a motor load acts differently, and as the motor speeds up its back EMF rises so the current actually drops. However, I think it is more likely that there is external inductance, and also possible noise into the scope leads causing a false reading. If you clip the leads together and to one point of measurement if you see anything, it is noise and will affect your reading accordingly. 

Another consideration is that you may be running the motor with no load, and I think a seriesw wound DC motor will become unstable under those conditions. That might explain the crazy waveforms at 24 volts where the motor starts to spin much faster. I don't have experience with series DC traction motors so maybe someone else can help.


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## cpate (Nov 21, 2011)

Sorry I was unclear about the diode test. The voltage drop test was the test that was done.

Today we tested with doubled up 6awg cables to the batteries and motor. It did not really make a difference for the spikes.

We also poked the oscilloscope leads everywhere. It didn't change the readings much either. On 26v we were still getting spikes to 60v+ on turn off.




edit:
Then we tried a bunch of small fast diodes to augment the FWDs. This didn't work either. I will post more pictures of our setup soon so you guys can feast your eyes on our failure 

Here is a link to the datasheet of our 400v 380uf cap. Do you guys see anything wrong? The esl is 40nH and the [email protected] is .81mOhm.

http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/C4DE%20Series/$file/F3303_C4DE.pdf


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## PStechPaul (May 1, 2012)

cpate said:


> Sorry I was unclear about the diode test. The voltage drop test was the test that was done.
> 
> Today we tested with doubled up 6awg cables to the batteries and motor. It did not really make a difference for the spikes.
> 
> We also poked the oscilloscope leads everywhere. It didn't change the readings much either. On 26v we were still getting spikes to 60v+ on turn off.


If you are getting scope readings showing spikes and unusual waveforms "everywhere", then the signal is coming from "everywhere", most likely high level magnetic fields inducing current in the scope leads. Rather than just "doubling up" the battery and motor leads, twist them together and keep them as short as possible. 

The entire circuit is an inductive loop, essentially a transmitting antenna or an induction coil, and anything inside the loop, or near it, will be affected by the field. Try to keep the high current leads as close together as possible, and keep any measurement leads outside the loop. The high voltage spike you are seeing may be the inductance of the motor or the inductance of the leads. 

Once you are sure your measurements are reliable, then you can work on reducing the spikes and protecting the IGBTs. A proper RC snubber across the IGBTs will reduce the spike, and then you can also add a TVS zener diode rated at just below the IGBT breakdown rating. You can tell if there are destructive spikes by running the system for a little while and feeling the TVS. If it gets hot, you will need to work on the snubber to reduce the transients. The snubber resistor might get hot, but that means it's limiting the transients, which is what you want. But even better is to work on wiring and adding bypass capacitors and possibly even inductor filters to reduce the transients.

Here are some references on IGBTs and snubbers:

http://www.fe-frontrunners.eu/Semi/Datenblatt/Application/ApplicationIGBT.pdf
http://www.pwrx.com/pwrx/app/UsingIGBTModules.pdf

Edit: 
I looked at the link for your capacitor http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/C4DE%20Series/$file/F3303_C4DE.pdf
If you think about it, a 60V spike across a capacitor with 0.8 mOhms ESR means a current of 75,000 amps. I think not. And there are other clues that point to measurement error. You should buy or borrow a good oscilloscope with good probes and repeat your measurements. You should get a dual trace scope so you can perform some measurements in differential mode, and for other measurements you should show things like gate drive and Vce simulataneously. If you are investing thousands of dollars in an EV, and hundreds of hours of time fooling around with dubious measurements, you should invest in a couple hundred dollars worth of decent test equipment.

Good luck!


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## cpate (Nov 21, 2011)

Sorry I meant that we masured C to E on several points on the bus bars. If you don't put the probes there you get nothing reading. I believe that our scope readings are correct. We have done the diode-capacitor trick that paul suggested and the cap was charged to the voltage of the spikes that the oscilloscope was reading.

We kept the battery leads much shorter than they are in most other people's cars, less than 4'. It would be possible to make them shorter but what is the point when they would have to be longer just to connect the batteries in our car together? Twisting them together would also be difficult because of their large size and they would have to be a fair amount longer.


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## PStechPaul (May 1, 2012)

Make sure you connect the two probes together when you check for no signal. Your probes appear to be really long and unshielded, and are bundled together in a loop. If it is magnetic coupling, the loop needs to be completed so that a current can be induced.

If you can't twist the battery and motor leads together, at least ty-wrap them so they run together.

And you should also twist the scope probe leads so that any inductive pickup will be mostly canceled. I think you'll see a big difference and more reasonable readings.


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## Stiive (Nov 22, 2008)

PStechPaul said:


> Make sure you connect the two probes together when you check for no signal. Your probes appear to be really long and unshielded, and are bundled together in a loop. If it is magnetic coupling, the loop needs to be completed so that a current can be induced.
> 
> If you can't twist the battery and motor leads together, at least ty-wrap them so they run together.
> 
> And you should also twist the scope probe leads so that any inductive pickup will be mostly canceled. I think you'll see a big difference and more reasonable readings.


+1. 
And when you connect the leads together and do a reading, place them on top your bus bars (but no electrical connection) and have the controller running. This will tell you if the switching noise is effecting your output.
I don't think this is the complete solution as the readings don't look random enough, but I don't like those leads 

I havn't read the thread, but have you reason to believe the drive is misbehaving? Or are you just going from the scope? I see the title is that you have previously blown an IGBT, but have you fixed anything major since? Just a thought.


p.s MOAR PICS


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## Stiive (Nov 22, 2008)

Oh, and have you tried turning the IGBT around? Seems silly to have the controller on one side, then run the signals across the EM field to the gate drivers. I would move all electronics to the one side and put in some shielding. Also keep the cables twisted tight.

What are the gate resistors you are using? Did you size their power rating correctly?


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## swoozle (Nov 13, 2011)

Diode measurement (fluke meter) for both sides of the IGBT measured the same....at least until I blew up the IGBT. It's amazing how high the spikes can get when the busbars area laying on the IGBT but the bolts aren't tightened down. Even on 24V supply. Kablammo.

We appreciate all the help you guys are offering and will continue to implement your advice as we work through this, but I have to tell you I don't think it's inductance or EMI in the various wire bits. We've moved stuff around, gone up significantly in gage, made wire runs shorter etc., and it has produced no significant change. The gate signal is clean and always has been. The CE signal stays stubbornly and consistently screwed.
But that's just an opinion and meant to explore other causes.
Changing the motor from a PM to the traction motor, that made a significant change. At 12V, that is. At 24V: still a mess.

Unfortunately I've only got the one low-ESR capacitor, because it's getting down to be the last thing that could be hosing up things so badly. Maybe the early tests messed it up. What happens when you over-volt a cap? Wouldn't it cause an obvious short?
The capacitance measured out in the right ballpark using a 150FV initial charge. Actually it measured out higher than expected, but I put that down to my timing accuracy. I don't suppose there's an easy way to test capacitor ESR?

I could scavenge a cap from my EMW charger to swap in, but it's about 14mOhm ESR. Is that worth trying?

And to answer one of the questions, we've been at 30R gate resistor, R6 emitter resistor for awhile.

More details later on yesterday's fun...


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## Stiive (Nov 22, 2008)

swoozle said:


> Diode measurement (fluke meter) for both sides of the IGBT measured the same....at least until I blew up the IGBT. It's amazing how high the spikes can get when the busbars area laying on the IGBT but the bolts aren't tightened down. Even on 24V supply. Kablammo.


Yes you must have the bolts tight!




swoozle said:


> And to answer one of the questions, we've been at 30R gate resistor, R6 emitter resistor for awhile.


What is the power rating. Have you checked it resistance lately? Do you have separate resistors for turn on/off?


While sizing up gate resistors on my new project I came across this:

"Tests should be started from higher inductances going down to the lowest. The highest voltage overshoot typically occurs when the IGBT switches off just before desaturation occurs. This is at low short circuit inductances when the over current detection switches off the IGBT just
before desaturation occurs. The test should be carried out at low junction temperature and high junction temperature."

AND 

"Diode switch off;
Note: Voltage spikes can occur at diode switch off, which can lead to high blocking voltage on the diode and the parallel connected IGBT. The worst case is mostly at low current (<10%*IC) and low temperature. The voltage has to be measured on the diode which is switching off or on the parallel connected IGBT. Sometimes the snubber capacitor is more necessary for the diode switch off than for the IGBT switch off. Short on times of diodes can also cause voltage spikes if the chip is not fully floated with carriers."

This is referring to a test procedure for testing the gate driver setup. Perhaps you should follow the steps on this. http://www.ppi-uk.com/pdfs/AN-7006_IGBT_Peak-Voltage_and_Snubber.pdf


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## swoozle (Nov 13, 2011)

Connected the test leads together and laid them every which way all over the controller, busbars and cables. At most got a +/- .5V reading.

Gate resistors are 2W. We aren't playing with the on/off resistor tweaking until we figure out what's causing the larger problem.


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## swoozle (Nov 13, 2011)

PStechPaul said:


> You should buy or borrow a good oscilloscope with good probes and repeat your measurements. You should get a dual trace scope so you can perform some measurements in differential mode, and for other measurements you should show things like gate drive and Vce simulataneously. If you are investing thousands of dollars in an EV, and hundreds of hours of time fooling around with dubious measurements, you should invest in a couple hundred dollars worth of decent test equipment.


That point hit home (though I'm sure others have suggested a real scope as well).
Here's the C-E trace off of my "new" scope w 10X probe.








vs the old one for the SAME SETUP.










sigh....

Now on to more constructive tweaking. 

To you and the many others that took the time to offer their advice, thank you for your patience and apologies for wasting your time.

To everyone else thinking of doing this: buy a real scope.
And WOW is it fun playing with a real scope


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## PStechPaul (May 1, 2012)

I am really amazed at the difference. I had thought it would be just a bit better but it's totally different and much more believable. Actually I can't quite believe those two traces were for the same setup, since the duty cycles appear different, but maybe the sweep time was different. Anyway, it does prove that you can't always trust your equipment, but there are usually ways to validate readings or at least use a couple of different instruments as a sanity check. 

I'm glad you finally have the equipment needed to continue and now you should be able to go back and try some of the suggestions for cleaning up the spike as shown on the new scope. I think an RC or RCD snubber across the IGBT will help a lot, and there are also ways to capture some of that otherwise wasted energy using a fast diode and a capacitor. If nothing else, you could just power a string of LEDs for general lighting or as an indicator that the motor/controller are running. 

Yay!


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## MPaulHolmes (Feb 23, 2008)

Wow!! I was completely baffled by the other traces. I'm so glad you are figuring this thing out. Sorry I wasn't much help. I've been working 14 hour days at the college this summer.


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## swoozle (Nov 13, 2011)

I think this is a trace and question that makes a little more sense than the phantoms we were chasing. 

Here's the CE trace for a 26V pack. See the slope on turn-off? At low duty cycle that is horizontal. As the duty cycle increases, it gets a pronounced slope. The voltage is rising from ~20V to 36V in this trace.








Here's the B+ / B- trace. Approximately the same duty cycle (note that the time axis is different). The peaks and upward ramps correspond with the CE trace. So while the switch is on, B+ goes down about 6V (to 20V) and when the switch turns off B+ rebounds ~15V to ~35V.








This is only pulling ~100 amps on an unloaded traction motor.
I haven't seen this on other traces posted. Why is this occurring? Is it a problem?
Thanks


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## PStechPaul (May 1, 2012)

100 amps seems like a lot for an unloaded motor. But assuming that is normal, the B+/B- voltage waveform seems to indicate that the peak current could be as much as 3-4 times that. If you are using the lead-acid batteries as shown in your photo, they may very well drop to 20V at several hundred amps peak load. I think I saw a shunt in your setup so you should put the scope on it and verify what the currents are.

The rising voltage seems to indicate that the motor is generating current back into the batteries, but they really shouldn't go higher than about 28 volts. You should put the scope across the battery terminals and see if it's more solid. Then you can track down where the voltage drops are. It might be a bad connection or just high resistance cables.

The scope readings are great! Begone, phantoms!


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## swoozle (Nov 13, 2011)

PStechPaul said:


> The rising voltage seems to indicate that the motor is generating current back into the batteries, but they really shouldn't go higher than about 28 volts. You should put the scope across the battery terminals and see if it's more solid. Then you can track down where the voltage drops are. It might be a bad connection or just high resistance cables.
> 
> The scope readings are great! Begone, phantoms!


You were right. Here's the scope at the battery terminals: significantly more stable. Resting pack voltage is 26.4 (8 x 130Ah lifepo cells). Voltage variance is +/- a couple of volts and it's not going above about 28 volts.









By the way *Damien*, tried your switch snubber as a quick test and it whacked the top off of the turn-off spike very nicely (15V overshoot --> ~4V) at the expense of some weird ringing. But this was just a quicky; I don't get a replacement for the last esploded IGBT until Friday, so no serious homing in on Rg-on, Rg-off and snubber setup until the weekend. But it looks like we are set to make some serious progress this weekend.

No switch snubber ____________________________________switch snubber














Damien, are you still running switch snubbers?


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## swoozle (Nov 13, 2011)

*(#@$((#$*#

http://missoulian.com/news/state-an...cle_235edb7a-d137-11e1-a31d-001a4bcf887a.html


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## PStechPaul (May 1, 2012)

At first I didn't see the caption in the picture. So I assume it blew up? Time for a post-mortem forensic analysis. Any particular clues as to the chain of events leading up to the failure? 

I don't have experience with series-wound traction motors, and only a little with IGBTs, and at much lower power levels, so I can only offer condolences and maybe some theoretical concepts to consider. The MOSFETs in my DC-DC booster failed, twice, but in each instance there were definite clues and warnings, and I've been doing simulations and discussing the problem with other electronic design experts and I think I have a good handle on it and several design changes that should make it reliable.

There are three major failure mechanisms that I know of. First is overvoltage transients, but usually you can and should have TVS diodes as well as snubbers and MOVs, and you can usually tell if they are doing their job, and identify the fact that you do have serious voltage spikes, by observing the TVS or the resistor of the RC snubber getting hot, or by measuring the degradation or observing the eventual failure of an MOV. 

The second failure mode is simple overcurrent. This can actually be caused by having low ESR capacitor snubbers without series resistors. If a fully charged capacitor is across the IGBT and it is turned on, The current can be extremely high. Consider if you have 96 VDC in a 1000 uF capacitor with 1 milliohm ESR. When the IGBT is triggered, the peak current could be as high as 96,000 amps, although in reality the ESR of the IGBT and parasitic inductance will limit it to much less, although very likely several thousand amps. 

The third mechanism is second breakdown. I don't fully understand it but apparently it is a concentration of power in small areas of the IGBT because of insufficient current sharing, and it will cause irreversible damage and eventually destruction of the device.

Slowing down the gate turn-on and turn-off can help in some cases but it can also result in higher power dissipation. I have been experimenting with this in my simulations and it can be very tricky. Sometimes the simple trick of a series gate resistor is not enough. The "Miller capacitance" from collector to gate can cause a dramatic slowdown of the transition because it tends to drive the gate in the opposite direction and inhibits proper switching. It's probably better to switch the gate with a high-current driver and then deal with the resulting inductive transients externally with snubbers, rather than using the IGBT itself to quench the energy. 

Much of the problem I have experienced seems to be due to the "leakage inductance" of the transformer I am driving. A DC motor may have similar effects. And it is also due to the layout of the wires and bus bars. At higher frequencies, there is the "skin effect" which means that the current will be conducted only in the outer surface of the conductor, so making it much larger does not help. What is usually done is providing many separate connections in parallel, and sometimes even braiding them rather than simply twisting or running in parallel. Bus bar connections are often laminated to reduce the skin effect.

And the inductance of connections is also a major factor, even at 8kHz PWM frequency. Tightly bundling the conductors as much as possible can greatly help. I hope I've given you some ideas and inspiration as you move forward. Hang in there!


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## swoozle (Nov 13, 2011)

PStechPaul said:


> At first I didn't see the caption in the picture. So I assume it blew up?



Woops, sorry Paul. That wasn't a metaphor, I mean it literally is in that train car (ok, maybe not that particular one). A replacement from an earlier esplosion is on its way to me and UPS is showing it as delayed because of that train wreck. So I'm stuck from making anymore progress until it gets rescued.

But thanks for the info, very informative.


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