# AC controller basics



## major (Apr 4, 2008)

vwdevotee said:


> Um, how does a synchronous AC controller work?


Hi vwd,

For starters, the quote in your post concerns induction motors which are asynchronous, not synchronous machines. I can't really begin to teach you how they work. But very basically, they are a 3 phase full bridge inverter capable of variable voltage, variable frequency, VVVF. Such variable frequency motor drives are quite common in industry and you should have little trouble finding web sites describing them, or text books.



> Is the torque generated by the motor controlled using PWM to vary the effective amplitude of the sine waves?


Yes, that is part of it.

Regards,

major


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## etischer (Jun 16, 2008)

An AC inverter controls motor speed by varying voltage and frequency applied to the motor. 

For example a standard motor is 230 volts @ 60 hz, this is it's base speed. If you want half the speed you apply 115v @ 30hz. You can run in extended range to get double the speed (but half the torque) by applying 230v @ 120hz. At low speed applying 4 volts @ 1 hz won't produce much power, so you need to apply "boost", maybe 40 volts @ 1 hz to get enough low speed torque. 

The commanded speed (frequency) from the inverter will be higher than the actual rpm of the motor. This difference is called "slip"

More slip means more torque. Too much slip, and the motor will fall out of sync and stall or buck.

The inverter needs to know the actual motor speed to determine slip if it is to control torque. It does this with hall effect sensors, and some times a tach or encoder is also used. The hall effect sensors help the drive determine the level of "boost" required to improve low speed torque.


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## vwdevotee (Mar 8, 2008)

I think I get it (on a really fundamental level). Thanks!


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## vwdevotee (Mar 8, 2008)

Hi! I have another simple question. I was looking at how VFDs work, and from the schematic below it looks like the high voltage side is actually a lot simpler than I though. Starting just to the right of the DC capacitor (since EV's natively have a DC supply) it looks like there is just a IGBT 6 pack to convert the DC to 3 phase AC. Other than that, the rest of the controller/VFD is just control circuits and gate drivers, right? Just so we're clear I'm not trying to trivialize the control side of them, just get a handle on what's happening inside.


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## major (Apr 4, 2008)

vwdevotee said:


> Starting just to the right of the DC capacitor (since EV's natively have a DC supply) it looks like there is just a IGBT 6 pack to convert the DC to 3 phase AC. Other than that, the rest of the controller/VFD is just control circuits and gate drivers, right?


 
Yeah, pretty simple  You still need a DC bus cap, maybe somewhat smaller than if you're using the rectifier. Just a 3 phase full bridge inverter, like I said.

major


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## samborambo (Aug 27, 2008)

major said:


> Yeah, pretty simple  You still need a DC bus cap, maybe somewhat smaller than if you're using the rectifier. Just a 3 phase full bridge inverter, like I said.
> 
> major


Hey Major, on that note, do you know how to size the DC bus caps? Trying to work it out for my AC inverter design. What's an acceptable ripple voltage % for (LiFePO4) batteries, controller, etc?

Sam.


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## few2many (Jun 23, 2009)

Hey, VWdevotee, check out this link.
http://freecircuitdiagram.com/2009/06/18/3-phase-ac-motor-speed-control/
this is what ive been trying to work on, slowly and painfully!
I've also been considering a funny little brush and stator igbt controller similar to what was listed in another thread as a way to make the three phase pulses.


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## few2many (Jun 23, 2009)

sorry, missed the link

http://www.diyelectriccar.com/forums/showthread.php/lobuck-controller-hi-amps-31134.html


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## major (Apr 4, 2008)

samborambo said:


> Hey Major, on that note, do you know how to size the DC bus caps? Trying to work it out for my AC inverter design. What's an acceptable ripple voltage % for (LiFePO4) batteries, controller, etc?
> 
> Sam.


Sorry Sam,

Don't know that one. You think it would be different than for a DC controller?

Regards,

major


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## Tesseract (Sep 27, 2008)

major said:


> Sorry Sam,
> 
> Don't know that one. You think it would be different than for a DC controller?


The cap has a much easier life in a polyphase inverter than a dc converter because the input current pulses drawn by each phase overlap so much the current is nearly DC.

How big of one? Heck, with a DC source I'd first try only using IGBT snubber caps that bolt directly onto the modules (please don't tell me you are going to parallel lots of little MOSFETs or IGBTs to make a 3ph. inverter?!) Maybe 1uF per 100A of phase current (which is motor current / 1.73).


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## major (Apr 4, 2008)

Tesseract said:


> The cap has a much easier life in a polyphase inverter than a dc converter because the input current pulses drawn by each phase overlap so much the current is nearly DC.
> 
> How big of one? Heck, with a DC source I'd first try only using IGBT snubber caps that bolt directly onto the modules


Hello Mr. Tesseract,

I'd be inclined to disagree with the above statements. My experience with AC drives built for battery sources would support the need for substantial DC bus capacitance as close to the transistors as possible.

Respectfully,

major


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## Tesseract (Sep 27, 2008)

Well, without spending too much time on this subject...

Oh, sure, you are probably still going to need some dc link capacitance, but I'd guess around half as much with batteries compared to the 3ph. mains.

As long as ultra-low ESR film capacitance is used right at the IGBT terminals to snub out spikes from stray inductance, though, there's not much harm in trying absurdly low values of dc link capacitance with batteries as the power source.


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## samborambo (Aug 27, 2008)

Tesseract said:


> The cap has a much easier life in a polyphase inverter than a dc converter because the input current pulses drawn by each phase overlap so much the current is nearly DC.
> 
> How big of one? Heck, with a DC source I'd first try only using IGBT snubber caps that bolt directly onto the modules (please don't tell me you are going to parallel lots of little MOSFETs or IGBTs to make a 3ph. inverter?!) Maybe 1uF per 100A of phase current (which is motor current / 1.73).


I think what is missing is that the LiFePO4 datasheets don't specify a recommended ripple current. Without that its fairly hard to decide what is an acceptable ripple current on the DC bus.

The IGBT module is an Infineon BSM100GD120DLC six pack, 160A 1200V no brake switch and no thermocouple. Tesseract, if you want some, surplustronics.co.nz has bulk stock of them and selling for around US$35/pc for 5+.

It is a little tricky designing for over 100A at 800V on a PCB with limited room. Very short wide tracks built up with silver solder from the module to the phase cables.

Sam.


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## samborambo (Aug 27, 2008)

Here's a thought: you know those BLDC controllers for model aeroplanes and helicopters that are designed to run off LiFePO4 cells at around 17V and over 100A? If they have a similar switching frequency, they'd have a similar capacitance across the bus.

Anyone ripped one apart before?

Sam.


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## Tesseract (Sep 27, 2008)

samborambo said:


> I think what is missing is that the LiFePO4 datasheets don't specify a recommended ripple current. Without that its fairly hard to decide what is an acceptable ripple current on the DC bus.


They sure don't, and I doubt any manufacturer has (or ever will) test this spec. Testing the ESR of batteries at anything besides DC is difficult to do.



> It is a little tricky designing for over 100A at 800V on a PCB with limited room. Very short wide tracks built up with silver solder from the module to the phase cables.


Obviously, standard "1oz" (35um) pc boards are not really meant for this sort of thing but there are "heavy copper" laminates with up to 350um of copper which, as you might imagine, can a lot more current for a given trace width (not exactly a linear relationship between Cu thickness and current carrying capacity). 350um (10oz) copper clad is expensive and hard to come by, though, so one old trick you can do is to solder desoldering braid onto the traces that carry high currents. In general, trying to build up silver solder over a larger area of a board results in delamination of the copper, so I wouldn't try it, but best results will be obtained with solder paste and heating the board up all at once with, for example, an electric skillet (or griddle, or whatever you guys call them).

***

Back to the input capacitor... those little BLDC controllers for RC models sure are cute, aren't they, but I wouldn't try to extrapolate too much from their component selection to a much larger inverter - after all, weight and volume are probably the most important criteria in an RC bldc controller, not long life for the motor, controller or battery...

A rule of thumb for mains-operated VFDs is to aim for 10% ripple at full continuous load current. If you assume the batteries contribute no current during switch on-time (which is, of course, a worst case scenario) then the capacitance required is simply a function of the switching frequency, load current and tolerable ripple (e.g. - 10% of the pack voltage).

This is an extremely pessimistic calculation for a battery-powered inverter or converter, though. Much better is to select the capacitance based on the ESR necessary to stay at or below the target ripple voltage.

Then you have to consider the amount of time spent at the highest power versus the average power and whether it is appropriate to size the capacitance to withstand the highest power continuously or to, perhaps, make some allowance for the fact that the inverter/converter won't spend all day at 100% output...

The moral of the story is that quite a bit of subjective judgment goes into sizing the dc link capacitor. I, personally, selected a capacitor for the "BMF" controller that can handle the worst case full ripple current at 1000A output continuously (actually, can handle nearly twice that), but the _amount_ of capacitance is arguably marginal as it will result in 25% peak to peak ripple (assuming the battery contributes no power during the switch on time, once again). This is the judgment call I made and given the BMF's ability to maintain a stable motor current desite ripple being 40-50Vpp it would appear to be a good one if I do say so myself  .


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## major (Apr 4, 2008)

Tesseract said:


> Oh, sure, you are probably still going to need some dc link capacitance, but I'd guess around half as much with batteries compared to the 3ph. mains.


Hi Tesseract,

My experience is using electrolytics for the DC link and it was less than half compared to the uF with the 3ph mains. Maybe a forth to a third.

This is when you can do laminated bus attaching the caps within a few inches of the trannies. And the equivalent 3ph mains rectified model had cap bank like 10 inches or longer away connected with bus bar. A great deal depends on the DC bus design and the stray inductance on the source side of the switch. Better to not have the spike at turn off than to have to use huge snubbers to deal with afterwards. 

Regards,

major


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## samborambo (Aug 27, 2008)

Tesseract said:


> They sure don't, and I doubt any manufacturer has (or ever will) test this spec. Testing the ESR of batteries at anything besides DC is difficult to do.
> 
> 
> 
> ...


Good tip on using solder wick for beefing up the solder traces, Tesseract. I'll keep that in mind.

Your idea of leaving the source impedance out of the equation definitely simplifies things - calculate based on the max I*s during one switching period and go with a "rule of thumb" ripple voltage on the caps. You're right, it appears to be a rather conservative calculation but a good starting point nonetheless. The same guys who do the surplus IGBTs also do a 50 pack of [email protected] electrolytics for US$50. I've no idea what the ESR of the caps would be as I can't find a datasheet but I'll put them on the meter and find out. With high ESR caps, I may need more anyway to overcome power dissipation from I^2 * R losses.

Sam.


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## Morf (May 29, 2009)

Hi Major,
I am looking at a 45KW Axial Flux/Gap motor from Finland. For right now I am wondering what is meant by ``Any standard PWM or DTC drive aimed for induction machines`` means following the word ~drive~. They are clearly saying what can be used on their 45KW pancake motor. Can you decipher `PWM and DTC drive` please. Thanks, and best wishes.


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## Tesseract (Sep 27, 2008)

Morf said:


> ...Can you decipher `PWM and DTC drive` please. Thanks, and best wishes.


Keeping in mind this is w/r/t ac motor controllers (aka inverters)...

DTC = Direct Torque Control, or, more commonly, Field Oriented Control*, or even more commonly, some variation on Vector control. This is, IMO, the vastly preferred control scheme if you want decent starting torque out of an induction motor.

PWM = V/Hz control, in which the stator volts and frequency are proportional. This is fine for controlling fan and pump loads.

* - actually, DTC and FOC are different control schemes, but for this discussion consider them both to be superior to plain old V/Hz


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## samborambo (Aug 27, 2008)

Morf said:


> Hi Major,
> I am looking at a 45KW Axial Flux/Gap motor from Finland. For right now I am wondering what is meant by ``Any standard PWM or DTC drive aimed for induction machines`` means following the word ~drive~. They are clearly saying what can be used on their 45KW pancake motor. Can you decipher `PWM and DTC drive` please. Thanks, and best wishes.


An axial flux induction machine? That's rare. Have you got a web link to the motor I could look at?

Sam.


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## Morf (May 29, 2009)

Hi Tesseract,
Your highly valued information went directly into my working file. How fortunate we are to have easily available resources of this caliber. Thanks again.


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## major (Apr 4, 2008)

samborambo said:


> An axial flux induction machine? That's rare. Have you got a web link to the motor I could look at?
> 
> Sam.


Hi Sam,

I'd say uncommon, not rare. Here is a link. Primarily used as a generator, but also as a motor. Motor, generator, same thing.

http://www.aurasystems.com/pages/prod_tech.html

Regards,

major


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## major (Apr 4, 2008)

Tesseract said:


> DTC = Direct Torque Control, or, more commonly, Field Oriented Control*, or even more commonly, some variation on Vector control. This is, IMO, the vastly preferred control scheme if you want decent starting torque out of an induction motor.
> 
> PWM = V/Hz control, in which the stator volts and frequency are proportional. This is fine for controlling fan and pump loads.
> 
> * - actually, DTC and FOC are different control schemes, but for this discussion consider them both to be superior to plain old V/Hz


Thanks for fielding this one, Tesseract.

In most cases, the DTC and FOC, Vector in general, will use PWM. There are a couple of other ways to synthesize an AC waveform, but are not often seen in garden variety VFDs.

I guess I'm not sure why Morf likes these axial machines. Not that there is anything wrong with them. But I'd think he'd get a better value all around with the standard approach, radial motors. But it is his project. I actually like seeing guys use unusual approaches.

Regards,

major


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## vwdevotee (Mar 8, 2008)

Hi major. Would you mind talking about the other methods of generating the AC signal? I've only heard about PWM and a slight varient called a magic sine wave (but that's still pwm). Thanks.


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## major (Apr 4, 2008)

vwdevotee said:


> Hi major. Would you mind talking about the other methods of generating the AC signal? I've only heard about PWM and a slight varient called a magic sine wave (but that's still pwm). Thanks.


Sorry, vwd. Probably shouldn't spoken of something of which I know little to nothing. But what I had in mind was hysteretic control and cycloconvertes and maybe soft switching or resonant PECs. Can't elaborate further due to inadequate knowledge. If you investigate and learn something, let us all know.

Regards,

major


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## Morf (May 29, 2009)

Hi Samborambo,
When looking for axials/Flux motors, the use of keywords `pancake` and `Lynch` whose name is associated with Axials quite commonly, gave me choices among the tiny racing motors in cars about 11 inches long, hard drives, and small tools with servo requirements. 
If you want motors in the 37, 45, and 250 KW range, put <AXCO-motors Oy> in your search engine. There is an interesting story how a technology oriented university began developing Axials back in 2000 and the industry followed, right there in the same town. There is likely a mistake about the weight of the 250KW engine, if you have been reading some of my posts. It is shaped like a piece of a barrel, is 15 inches deep, and 30 inches wide(and high). It likes to run at 6000 rpm, and according to my figures has 300 # of torque. I heard first that it weighs 600#, now I hear 600kg. I don`t see how it could be be near 1300 # even if it were a solid block of something. Anyway the 45KW is 122Kg. I am working with the company at this moment on pricing and other `stuff``. The smaller two motors efficiency charts stay above 90% at 3000rpm, but they rev up to, I believe, 9000 rpm. Interesting --!! Wait till you see the photo of the 250. Tell me what you think. There is a trick to emailing them. They have an info(at) address that you will find. Forget the (at), replace it with the standard @ or it will bounce back to you. Best Wishes,


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## samborambo (Aug 27, 2008)

Morf said:


> Hi Samborambo,
> When looking for axials/Flux motors, the use of keywords `pancake` and `Lynch` whose name is associated with Axials quite commonly, gave me choices among the tiny racing motors in cars about 11 inches long, hard drives, and small tools with servo requirements.


We were talking about induction motors. Axial flux permanent magnet brushless motors are very common.

Sam.


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## Tesseract (Sep 27, 2008)

major said:


> Sorry, vwd. Probably shouldn't spoken of something of which I know little to nothing.


Bah! As JRP3 quipped to me a while ago, why deprive us of the entertainment?! 



> But what I had in mind was hysteretic control and cycloconvertes and maybe soft switching or resonant PECs....


Hmmm... definitely showing your age on this one 



vwdevotee said:


> Hi major. Would you mind talking about the other methods of generating the AC signal? I've only heard about PWM and a slight varient called a magic sine wave (but that's still pwm).


Major bailed on you so I'll toss in my two cents...

PWM covers a wide territory but major was correct in stating that virtually all VFDs today use it. The type of modulation scheme specifically used, though, varies somewhat, with the predictable result that some schemes create a more sinusoidal current in the motor than others. The voltage waveform produced by a modern VFD (aka - 3ph. inverter) will always be a train of pulses, but the mark/space ratio of the pulses is what determines how closely the current (which produces torque) will resemble a sine wave.

So called "magic sine waves" are simply one such modulation scheme which can create fairly low distortion sine waves. However, they require an absurd numbers of bits to generate them and still suffer from the occasional high harmonic so there is little impetus to use them in VFDs.

One of the most popular means of similating the RMS power of a sine wave is called "quasi square wave". This scheme is simply a square wave with the same peak amplitude of the sine wave it is mimicing but only a 70.7% duty cycle. This does a surprisingly good job with inductive and resistive loads, but not so much capacitive ones (like a computer's power supply).

There are even more elaborate schemes, some which have theoretical promise, most which are just plain goofy, but in the end plain PWM or quasi square wave methods cover 99% of applications just fine.


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## samborambo (Aug 27, 2008)

Morf said:


> Hi Samborambo,
> If you want motors in the 37, 45, and 250 KW range, put <AXCO-motors Oy> in your search engine. There is an interesting story how a technology oriented university began developing Axials back in 2000 and the industry followed, right there in the same town. There is likely a mistake about the weight of the 250KW engine, if you have been reading some of my posts. It is shaped like a piece of a barrel, is 15 inches deep, and 30 inches wide(and high). It likes to run at 6000 rpm, and according to my figures has 300 # of torque. I heard first that it weighs 600#, now I hear 600kg. I don`t see how it could be be near 1300 # even if it were a solid block of something. Anyway the 45KW is 122Kg. I am working with the company at this moment on pricing and other `stuff``. The smaller two motors efficiency charts stay above 90% at 3000rpm, but they rev up to, I believe, 9000 rpm. Interesting --!! Wait till you see the photo of the 250. Tell me what you think. There is a trick to emailing them. They have an info(at) address that you will find. Forget the (at), replace it with the standard @ or it will bounce back to you. Best Wishes,


I can see you confusing yourself trying to relate power to weight in an electric motor. Motor size and weight is loosely proportional to the torque, not power, it produces.

eg: You may find a 250kW industrial 12 pole, 500RPM induction motor weighs 3 tons - very high torque. At the other extreme a 15kW 60,000RPM model aeroplane BLDC motor weighing 2kgs - very low torque.


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## Morf (May 29, 2009)

Hi Samborambo,
The first quote was to those(other than yourself) who have been on the same search I have, stating what not to do to get get to the land of larger axials. So the next line after my quote that you copied(that you didn`t copy) gives you the web address where you can find what you are looking for. They have an excellent section, find it on the left hand side of the page, explaining that they carry both, and there is a bold headline over the section describing their induction motors, maybe you gave up to early in your reading. There is a section on the advantages of multiple stators, discussions about rotor problems from early on, and much more. Let me know what you think. Best Wishes,


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## samborambo (Aug 27, 2008)

Morf said:


> Hi Samborambo,
> The first quote was to those(other than yourself) who have been on the same search I have, stating what not to do to get get to the land of larger axials. So the next line after my quote that you copied(that you didn`t copy) gives you the web address where you can find what you are looking for. They have an excellent section, find it on the left hand side of the page, explaining that they carry both, and there is a bold headline over the section describing their induction motors, maybe you gave up to early in your reading. There is a section on the advantages of multiple stators, discussions about rotor problems from early on, and much more. Let me know what you think. Best Wishes,


I did check out the page briefly.

I'm not knocking the idea of an axial induction machine. Like they say on that website, axial designs, especially double stators, have a shorter flux path. 

However, I think that the rotor eddy current path in an axial induction machine may be longer and of higher impedence than a radial induction machine. That's definitely not a good thing.

One problem with high power axial machines is the axial forces on the bearings. Typically, the axial force acting on the bearings from the attraction in the magnetic circuit is at least 10 times the radial force. Not such a big deal on a small windmill generator but probably a big issue for bearing life in an automotive sized machine.

I'd be very keen on an axial induction motor if it actually offered some advantages over radial induction motors or BLDC/PMSMs. Its just a little gimmicy at the moment.


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## Morf (May 29, 2009)

Hi Samborambo,
Thanks for your comments on the AXCO axials. On the topic of bearings, I noticed where their 250KW has `ceramic grease lubricated bearing that have a 2 year service interval`. One is led to think that would mean working the motor at some job with considerable hours, probably continuous duty. 
To try to figure out if the weight issue was some misprint or other, I did some scaling on the picture of the 250. Though the proceedure was crude due to the motor showing a 3/4 angle view, I did what I could do using the vertical, which was also slightly skewed. I deduced the drive shaft having a diameter of 7 inches, possibly 6, which makes that chunk of metal weigh about 200 pounds if 18 inches long. The page of information from axco mentions the many patents this organization holds on axials. In my next letter to them , I will try to encourage the engineers to place the 250 in their test facility and show the dials when they turn it on. I would watch and would also like to hear the sound when it is given a load. Best Wishes,


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## Drew (Jul 26, 2009)

samborambo said:


> I think what is missing is that the LiFePO4 datasheets don't specify a recommended ripple current. Without that its fairly hard to decide what is an acceptable ripple current on the DC bus.
> 
> The IGBT module is an Infineon BSM100GD120DLC six pack, 160A 1200V no brake switch and no thermocouple. Tesseract, if you want some, surplustronics.co.nz has bulk stock of them and selling for around US$35/pc for 5+.
> 
> ...


Have you considered just getting your track pattern water jet or laser cut out of copper sheet 1-2mm thick then gluing it to a fibreglass board, this would give you a pretty significant advantage in terms of track width to section.

If you used a single sided board and glued your high power tracks to the underside you could have a low power side and a high power side.


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## Amberwolf (May 29, 2009)

Drew said:


> If you used a single sided board and glued your high power tracks to the underside you could have a low power side and a high power side.


Better be careful with that layout, though, because with currents like that, you could induce some heavy interference in stuff on the low power traces on the opposite side of the board. 
________
LadiDi


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## vwdevotee (Mar 8, 2008)

This might be more of a general AC question, not necessarily a control question, but is there somehting fundamental about AC systems that make them prefer high voltages? I've noticed most DC systems seem to be content between 96 and 156 volts or so, but most AC systems are 300+. Thanks.


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## samborambo (Aug 27, 2008)

Amberwolf said:


> Better be careful with that layout, though, because with currents like that, you could induce some heavy interference in stuff on the low power traces on the opposite side of the board.


I've gone with surface mount components for the control area of the board for that reason. The underside of the board (closest to the IGBT module) is a ground plane.

I'll post up some pictures of the new design on my blog tonight hopefully.

Sam.


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## samborambo (Aug 27, 2008)

vwdevotee said:


> This might be more of a general AC question, not necessarily a control question, but is there somehting fundamental about AC systems that make them prefer high voltages? I've noticed most DC systems seem to be content between 96 and 156 volts or so, but most AC systems are 300+. Thanks.


Excessive arcing on the commutator with a high voltage DC motor would be a show stopper. Once the air around the commutator becomes ionised from a flashover, the arc can be supported with fairly low resistance through the ionised gas.

There's definitely an upper limit to voltage on an AC motor too. You don't find many induction machines above 3300V as the copper winding diameter becomes too small to be reliable (for smaller motors) and the insulation adds considerable bulk to the winding window. Even the containerised 1MW diesel generators we sometimes use employ 400V alternators(1500A). Huntly power station here in NZ uses 250MW 11000V alternators (13kA each!) for plants 1 thru 4. That's about the practical voltage limit for any design I can think of.

As far as EVs go, I think AC motors may be higher voltage because they can. From an efficiency/cost point of view, its far more preferable to go with a high voltage, low current system.


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## yarross (Jan 7, 2009)

Seen many of 6.3kV voltage. This is standard in 300-2000kW range. Nothing extraordinary.


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## coulombKid (Jan 10, 2009)

Tesseract said:


> They sure don't, and I doubt any manufacturer has (or ever will) test this spec. Testing the ESR of batteries at anything besides DC is difficult to do.
> 
> 
> 
> ...


NutsVolts magazine did a soldering oven project with a converted toaster oven with a BS2 controller. It was designed to do surface mount boards.


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## vwdevotee (Mar 8, 2008)

I was looking at commercial VFDs and I noticed that the rated current in and out are pretty similar, which I would expect, but since we would be bypassing the rectifier and hooking onto the DC link directly, how does the current compare in it? If it were single phase I could probably muttle my way through figuring it out, but with a three phase output I just keep going in circles; which usually means I'm missing something fundamental. What I'm looking for is an explanation or a link to one that ends in I_DCbus=x*I_motorphase.

Thanks!


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## samborambo (Aug 27, 2008)

vwdevotee said:


> I was looking at commercial VFDs and I noticed that the rated current in and out are pretty similar, which I would expect, but since we would be bypassing the rectifier and hooking onto the DC link directly, how does the current compare in it? If it were single phase I could probably muttle my way through figuring it out, but with a three phase output I just keep going in circles; which usually means I'm missing something fundamental. What I'm looking for is an explanation or a link to one that ends in I_DCbus=x*I_motorphase.
> 
> Thanks!


Power in a 3 phase circuit:

P = Vptp x I x sqrt(3)

where Vptp is the RMS voltage phase to phase. I is the RMS current in each phase.

The DC bus voltage should be at least Vptp x sqrt(2).

Current in each winding of a star connected motor is the same as the phase current but the voltage across the winding is Vptp / sqrt(3). In a delta connected motor, the inverse is true - current in each winding is I / sqrt(3) and winding voltage is Vptp. Draw a diagram of star and delta connected circuits, look at where the currents branch and you'll find it easier to understand.

Sam.


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## samborambo (Aug 27, 2008)

Here's a worked example:

My MR2 project is two 415V 15kW induction motors.

Motor phase current = 15,000 / [ 415 x sqrt(3) ] = 20.87A

Minimum DC bus voltage = 415 * sqrt(2) = 587V

If your bus voltage was 587V, phase current on the AC side would equal the DC bus current.

My nominal battery voltage is 640VDC (200x LFP 40Ah in series).

Battery current at 30kW = [ 15000W x 2 ] / 640V = 47A

Thats just the continuous rate of course. Peaks are around 4 times higher.

Don't forget losses along the way. There's typically 5V dropped at the inverter.


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## coulombKid (Jan 10, 2009)

For my little bench top experimental inverter I'll have a full bridge diode pack driven from a varactor feeding a bank of DC capacitors. When my varactor has been slowly turned up to 240 VAC RMS I should see a charge level of (240 VAC RMS)* 1.414= 339.4 VDC on the capacitors. A true RMS current meter on the DC bus would allow me to calculate power to the IGBTs by (339 VDC) * I rms in watts. An efficiecy multiplier of .9-.95 would give a fair estimation of watts delivered to the motor terminals. In an EV drive neither voltage nor current would be near name plate values much of the time. You'd have to do a fair job of instrumenting your motor to allow your controller to display motor Kw usage instant.


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## coulombKid (Jan 10, 2009)

samborambo said:


> Here's a worked example:
> 
> My MR2 project is two 415V 15kW induction motors.
> 
> ...


Wouldn't the AC current for a single phase be DC buss current/1.72 under steady state conditions?


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## samborambo (Aug 27, 2008)

coulombKid said:


> Wouldn't the AC current for a single phase be DC buss current/1.72 under steady state conditions?


Nope. Sounds weird, I know. Looking at each phase individually, the phase is carrying 57.7% of the current to the motor. Since the phases are shifted 120deg from each other, the return current of that 57.7% plus the other 42.3% of the current is being carried by the other two phases.

This is really hard to explain in text - and even harder when you take into account power factor and harmonics.

The phase currents and voltages, at any instant in time, add to equal zero. The RMScurrent in one phase is always 57.7% [ 1 / sqrt(3) ] of the total current being supplied to the motor.

If it still doesn't make sense, convert each side to power and they should be equal.


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## samborambo (Aug 27, 2008)

Maybe this animation will help:










The centre point of the star connection is the neutral. In a balanced system (all phases instantaneous current adding to zero) there is no current flow through neutral so it doesn't need to be connected from the source to the load.


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