# Series wound DC controller - IDEA?



## major (Apr 4, 2008)

Stiive said:


> can you just bypass your controller


Hi Stiive,

Forklift controllers do this. It is called 1-A bypass. A contactor which is controlled thru the controller logic. I seldom see it done in electric cars.

Regards,

major


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## Stiive (Nov 22, 2008)

why not? is it a viable option for heavy acceleration without the need for purchasing a bigger controller? If you can program the controller to accelerate the motor as fast as it and the motor can handle up until it hits its maximum amp output then bypass the controller to continue the acceleration. Assuming that by the time the controller has hit its maximum output the motor is at adequate speed to control itself.

why isnt this done in EV's? sounds like a cheap alternative to a large motor controller 
Arnt curtis controllers forklift controllers anyway? does that mean the 144v 500A controller has this function inbuilt?


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## TheSGC (Nov 15, 2007)

Stiive said:


> why not? is it a viable option for heavy acceleration without the need for purchasing a bigger controller? If you can program the controller to accelerate the motor as fast as it and the motor can handle up until it hits its maximum amp output then bypass the controller to continue the acceleration. Assuming that by the time the controller has hit its maximum output the motor is at adequate speed to control itself.
> 
> why isnt this done in EV's? sounds like a cheap alternative to a large motor controller
> Arnt curtis controllers forklift controllers anyway? does that mean the 144v 500A controller has this function inbuilt?


They aren't really used in EV because bypass are anything but efficient. They provide direct power to the motor, and that leads to excessive AMP draw and actually decreases your range. Also, the reason for a motor controller is that a motor does not "control itself". If given unregulated power, it will draw power as fast as possible until it either over revs and grenades, or burns itself out. The EVs they are used in are drag racing where control is not an object, but outright power is wanted, and they aren't in use enough to burn out the motor.

If you want an EV to actually be drivable, you really need a big controller. The 144 Curtis 500A is a PWM controller with no bypass contactor. And I don't think it's considered a forklift controller because of the high voltage.


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## Stiive (Nov 22, 2008)

TheSGC said:


> They provide direct power to the motor, and that leads to excessive AMP draw and actually decreases your range.


Of cause this is the case - i am talking about using this function for HEAVY acceleration.... This function would only be enabled during WOT




TheSGC said:


> . Also, the reason for a motor controller is that a motor does not "control itself". If given unregulated power, it will draw power as fast as possible until it either over revs and grenades, or burns itself out.


As previously stated once the speed of the motor is sufficient (will be at a certain armature voltage) that the armature resistance can limit the amp draw to a "safe" amount which is below 200% of is max continuous power rating? Say the armatures resistance was .2ohm for 144v input the max current draw is 720A which should be okay to provide to the motor for acceleration for a limited amount of time. Even though the power is unregulated it cannot possibly draw more amps than this. ohms law you know.
There should not be a problem unless your tires lose traction - a series wound dc motor will spin itself to death if the load is removed and at that amount of power, once the wheels start slipping if you don’t get traction quickly the motor will just keep speeding up until it blows.



TheSGC said:


> If you want an EV to actually be drivable, you really need a big controller. The 144 Curtis 500A is a PWM controller with no bypass contactor. And I don't think it's considered a forklift controller because of the high voltage.


I will have a controller for normal driving - just want the option to be able to smoke some ICE's off at the lights. Again the controller will be used for acceleration up until the motor is at sufficient speed to limit its current draw to a "safe" amount.

Its been a while since ive studied motors so im happy to be wrong on this. Just making sure you understand my argument.

As for the curtis controller - thanks for the info, i have not been looking at EVs for long but vaguely remember someone saying they were built for forklifts. hence my comment.

Anyway thanks for your time to discuss my idea


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## major (Apr 4, 2008)

Stiive said:


> Its been a while since ive studied motors so im happy to be wrong on this.


Hi stiive,

Your concept is technically achievable, but it does not sound like you have the proficiency to pull it off. You can try the by-pass with a Curtis or the like, but be prepared to fry a couple of controllers in the process of getting it right. In applications where by-pass is used, like the forklift GE EV1 controllers, they have been designed to do it. The Curtis has not.



> once the speed of the motor is sufficient (will be at a certain armature voltage) that the armature resistance can limit the amp draw to a "safe" amount which is below 200% of is max continuous power rating?


The armature resistance does not change with RPM. 



> Even though the power is unregulated it cannot possibly draw more amps than this. ohms law you know.


Incorrect.



> 450A continuous, which shouldn’t be a problem?


From your first post referring to a motor. 450 amp continuous would make for an awfully large motor. A lot bigger than you'd want for a car. Equal to 2 or 3 normal motors.

Regards,

major


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## Stiive (Nov 22, 2008)

major said:


> Your concept is technically achievable, but it does not sound like you have the proficiency to pull it off. You can try the by-pass with a Curtis or the like, but be prepared to fry a couple of controllers in the process of getting it right. In applications where by-pass is used, like the forklift GE EV1 controllers, they have been designed to do it. The Curtis has not.


Why would the controller care if I bypass it or not? Will there be some surge in current when I switch back to the controller or something? I’m sure small amount of circuitry could fix something like this. 
I would have my own microprocessor determining when the bypass rules have been met and the controller would just see its input power removed and turn off.




major said:


> Incorrect


Well if ohms law doesn’t apply any more I don’t know what to believe in…. Things must of greatly changed since I was at uni.. Which wasn’t actually long ago.




major said:


> The armature resistance does not change with RPM.


Obviously. Sorry I mustn’t have been making myself clear.
I think you obviously know how a series motor works and the logic behind it. The way a series DC motor varies from other DC motors is that its armature, field and line current is all the same.
Kirchoffs voltage law for a series DC motor is

V[Terminal] = E[Armature]+I[Armature]*R

Obviously based on ohms law. Since when a motor starts its armature voltage E[Armature] is 0V which means the motor will be subject to a current I[Armature] equal to

I[Armature] = (V[Terminal] - 0) / R

Hence my comment before, the max armature current is equal to the voltage applied divided by the internal resistance of the machine. I don’t see how it can get any higher?

From the equation it is easy to see there are 2 ways of effectively changing the armature current. 1 is to change the terminal voltage applied (ie with the use of a PWM controller) and 2 is to change the armature resistance by adding/removing resistors in series. The latter is an inefficient way of changing the current and therefore the first is used.

What I mean by the resistance can limit the speed is that as the motor speeds up E[armature] increases and therefore the armature current decreases. Obviously at startup, when E[Armature] =0V, the internal resistance is not sufficient to limit the current to a safe level (and therefore the PWM will not supply full terminal voltage so the motor doesnt blow up), but as the armature voltage nears the terminal voltage (near full speed) the armature current is limited by Kirchoffs equation to a safe amount without the need for voltage regulation or adding resistance to the circuit (speed regulation as described above).

At 144V with the 500A curtis controller you cannot apply full power to the motor (say internal resistance of about .1) until the armature voltage is

E[Armature] = 144 – 500*.1 =94V

Which depending on the amount of windings in the motor, will be at a certain speed for the motor. But as this is the motors max voltage and the controller cannot supply any more amps, this is where the power ends, and the only way to keep accelerating is to reduce the torque load.




major said:


> From your first post referring to a motor. 450 amp continuous would make for an awfully large motor. A lot bigger than you'd want for a car. Equal to 2 or 3 normal motors.


http://www.belktronix.com/WarpComparison1.jpg shows the torque vs amps of some motors. It shows the ADC motor upto 500A. I think you should be able to push 700A for brief amounts of time without causing irreversible damage. If the maximum you can take one of these motors is 400-500A then what was the point of ever buying a Zilla (especially the 2k)? If you could never use the power they supplied I don’t know how anyone justified spendnig double for potential that can never be reached.




major said:


> Your concept is technically achievable, but it does not sound like you have the proficiency to pull it off.


Thanks mate….? 


Well anyway hopefully I have made myself a bit clearer again. But yes I am still happy to be wrong with my working, after all that’s why I started this post. If I knew I was right, I wouldn’t have to. Please someone tell me if/where I have gone wrong. Don’t simply write ‘incorrect’ without explaining your reasoning.

Thanks 4 the help so far guys


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## TheSGC (Nov 15, 2007)

I think the thing you are missing is that actual power capability of the motor. Those Warp motors with ratings up to 500A is for an extremely short time, the motors themselves can probably take more like 250 AMPs continuously. 

You are basing your motor math purely on electronic laws, and you are missing the thermal and mechanical failure of these motors. These motors will never reach a self "controlled" state because at those levels, the motor wire insulation melts, causings shorts and destroying the motor. Also, depending on how its winds, that self "controlled" state is most likely way past the motors max RPM due to the bearings or strength of the motor material. 

The point of having a Zilla (besides have one of the most reliable controllers) is that most larger motors can handle 2000 AMPs for a brief time, thus better acceleration. But your still limited to the thermal and mechanical limits of the motor.


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## major (Apr 4, 2008)

Stiive said:


> Kirchoffs voltage law for a series DC motor is


Hi Stiive,

So you do realize there is more to it than Ohm's law.

I've told you that there are 1000's of PWM controllers out there with millions of hours of service using 1-A by-pass. So your concept is doable. Why don't EVs use it? Probably isn't worth the trouble. But you think it is a good idea for your project, then by all means, go for it.

Good luck,

major


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## Stiive (Nov 22, 2008)

TheSGC said:


> I think the thing you are missing is that actual power capability of the motor. Those Warp motors with ratings up to 500A is for an extremely short time, the motors themselves can probably take more like 250 AMPs continuously.





TheSGC said:


> You are basing your motor math purely on electronic laws, and you are missing the thermal and mechanical failure of these motors. These motors will never reach a self "controlled" state because at those levels, the motor wire insulation melts, causings shorts and destroying the motor. Also, depending on how its winds, that self "controlled" state is most likely way past the motors max RPM due to the bearings or strength of the motor material.
> 
> The point of having a Zilla (besides have one of the most reliable controllers) is that most larger motors can handle 2000 AMPs for a brief time, thus better acceleration. But your still limited to the thermal and mechanical limits of the motor.


Hey guys thanks for your feedback. Sorry i havnt been looking at EV's for long but someone told me there is no need to really go above a warp 9 motor. If say a warp13 could handle such power maybe i need to investigate one of those. I hadn’t even put any thought into which motor im going to use, I just mentioned warp 9. Anyway if another bigger motor can handle upto ~2000A then my application in my view would definitely be useful. 
Like I said this bypass would be for a very brief time (5-10secs). I just don’t want to end up disappointed with acceleration limited to ~500A.





TheSGC said:


> Also, depending on how its winds, that self "controlled" state is most likely way past the motors max RPM due to the bearings or strength of the motor material.


Now that’s a valid point… id have to look into that, cant imagine why it would be designed in such a way but worth finding out. Most cheap power tools and applications where efficiency isn’t needed that much use start-up resistors for the motor. Ie runs off mains power and its easier to upgrade the motor to handle the efficiency loss than spend money designing a controller. Before the technology advances in PWM and before they got affordable all motor applications used start-up resistors so I don’t see why its such a hassle to get the best of both worlds.




TheSGC said:


> You are basing your motor math purely on electronic laws, and you are missing the thermal and mechanical failure of these motors.


Granted but it is a very good theoretical indication. I actually hated this subject at uni because there were wayyyy to many pracs testing theoretical versus actuals of motor specs. They were always extremely close though, the only time the gap between the results started to slip is when the motor was old/tired/worn out. In most cases this still didn’t matter because we use ‘no load tests’ and the like to work out the actual values for the motors and not using the specified values which obviously change as the motor gets old. Another things worth noting is that we pushed those motors to their limits, we would push in about 400% of its rated amps for ~5seconds and they were fine. I obviously wouldn’t do that with a motor that I had paid for, but worth noting that you can give them abuse, as long as its not for long periods of time to let them heat up. The same applies with the batteries, they cant handle their max output for >10seconds anyway.




major said:


> So you do realize there is more to it than Ohm's law.


Kirchoffs law basically is ohms law though. Do you now agree that the max theoretical amperage the motor can accept is V / R? Sorry I didn’t really explain myself the first time, but there was method in my madness. Even though I hadn’t revisited kirchoffs law I knew that it must be the case.




major said:


> Why don't EVs use it? Probably isn't worth the trouble.


Well maybe if I go ahead with this and show how simple it is for the extra buck you (should) get, people might start using it? Maybe the comment someone told me about the warp9 is true because theres no point going any bigger as there are no controllers available to satisfy their thirst. The only limitation with upgrading to a bigger motor is that is has to be able to reach its “self controlling” state by 500A. Even if it didn’t, just add another resistor to limit the current until it does reach speed. Sure this decreases the efficiency, but id sacrifice efficiency for acceleration. I don’t care if this car doesn’t have a high top speed (as long as its >100km/h) but I do care if it’s slow off the mark.


Can you please give suggestions on some possible threats by doing this on the controller? I don’t know a lot about them or their limitations. There shouldn’t be huge voltage spikes when switching between it, and if there are, its nothing a bit of circuitry cant handle. Anything else?


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## CPLTECH (Sep 14, 2007)

Stiive,
_"Can you please give suggestions on some possible threats by doing this on the controller? I don’t know a lot about them or their limitations. There shouldn’t be huge voltage spikes when switching between it, and if there are, its nothing a bit of circuitry cant handle. Anything else?"_

On this forum we have discussed the reason a free wheeling diode is placed across the motor – to protect the controller. Perhaps this is what “Major” is insinuating. Altho, I am not formally educated as some on here, from practical knowledge, I know that EE’s design a circuit to have a clamping diode across a relay’s coil to stop electrical spikes from affecting the circuit. Since motors have coils that make magnetism & when that magnetism collapses, I’d hate to be holding the motor wires. So your idea to switch across the controller may be hazardous to its health. This seems to be a problem that is plaguing emerging designers, from what I hear. Of course, I have had the privilege if being in the vehicle when things went up in smoke, because those details had not been worked out. So, too, have a multitude of others. So it is not all that simple when you get down to it…


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## major (Apr 4, 2008)

Stiive said:


> Do you now agree that the max theoretical amperage the motor can accept is V / R?


Stiive,

It depends on the RPM. The only case where I = V/R is at stall, zero RPM. And also realize that the motor resistance can vary quite bit with temperature. And also that the example resistance values you've used could be an order of magnitude or two higher than typical EV motors.

Regards,

major


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## Stiive (Nov 22, 2008)

Yeh at zero RPM the current is V / R and this is the highest theoretical current the motor can draw. I believe you are right that the resistance would easily be about half the value i used before, i havnt been able to find any values for motors (didnt look very hard though) and obviously dont have one to obtain the value experimentally. 
Can you comment on CPLTECH's comment? I really need some solid comments and facts as it seems like i will be attempting this. Unless someone has a zilla they want to sell me?
BTW - is flywheel diode and free wheeling diode the same thing? i always refered to it as free wheeling. same same but different?


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## major (Apr 4, 2008)

Stiive said:


> Yeh at zero RPM the current is V / R and this is the highest theoretical current the motor can draw.


Hi Stiive,

Not really. The generated voltage can be negative if rotation is backwards. Such as in a plug reversal. Then currents can exceed the normal stall current.



> is flywheel diode and free wheeling diode the same thing?


I have never heard of a flywheel diode.

I think you need to take a hard look at what is to be gained against the trouble implementing such a system. Suitable contactors will be costly, and large, and probably arc at these higher voltages. Sure, maybe diodes, snubbers and the like can help. And I don't know what spikes will be presented to the controller from a by-pass coming in and dropping out. Chances are, something will happen, probably not good. And you think the controller dealer will honor warranty with your application?

Here's an idea. Buy an old electric golf cart. See if your by-pass will work on it. Be a lot cheaper experiment than a full size EV.

Hey, I'm not trying to discourage you. Just callin' them as I sees them.

major


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## Stiive (Nov 22, 2008)

Thanks for your help Major. Well, how good of an acceleration am i looking at with a 1300kg (2800lbs) converted weight with good aerodynamics (1.8m^2 frontal area and .32 coef drag) running 144V of 160Ah thundersky lithiums and a 500A curtis controller? Im guessing the motor doesnt really matter as i cant put huge amount of power into it anyway? If its going to have slower acceleration than my '92 mazda 626 then its unacceptable.

Yeh i didnt want to experiment on my $20k AU battery pack so was thinking of alternative testing equipment. 

I was thinking of going the easy way out and just having a relay switch on that bypassed the controller. Sure i could find something like a 1.5kA relay somewhere. I prob wont even disconnect the controller but in effect have it in parrallel with the bypass as the current will take the path of less resistance anyway. Not sure if this is worse for the controller but its cheaper for me  from there i can implement my 'safety' circuit to limit huge voltage spikes etc.

With the amount of money i have budgeted for this project - if its not fast ill kill myslef


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## 300zxev (Dec 5, 2008)

Hi All

Can we open this post up again to some positive suggestions ...
I seem to have hijacked another thread discussing this exact topic.
From further researching this topic, I have found where bypassing the controller has been used on multiple different occasions achieving the desired effect.

Now I have far less technical knowledge than Major and Stiive but i can see this as a posible way of saving the current range of controllers instead of just destroying them.

I was just about to post the question to DIY Guy on what he thought would be a suitable contactor for this application off this list ... http://relays.tycoelectronics.com/kilovac/ ... (the LEV200 is currently used, but is there an alternative that would be better suited to the greater current ?) (why do they describe some as "make and break load switching ?" is that what the LEV200 is anyhow ? ) And I would probably need the same large starting contactor as the bypass contactor right ?

Comming off the current draw of the motor while in "Bypass mode" is what the controller has to be protected against right ???
And alot of other discussions involved this free wheeling diode that is placed between the battery pack/controller ... and motor/controller ...
What does it do and will it protect the controller ?


Once the extra protection of the controller was sorted, couldn't you initially set this up with large resistors and monitor the current to the motor until you are happy with the performance ??? 
Your first attempt could be even supplying less amps to the motor than the controller does, but at least it is somewhere to start right ?
I realise the power is lost reducing range when involving resistors, but I don't think for a minute that driving an EV is all about the range you can get ...

Well to "hyperthetically" get this working what would you need and where ... 

Any input is appreciated ... pointing out safety concerns is good ... shutting down an idea is not going to help anyone ... Afterall, don't we want people to share their experiences whether good or bad ???


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## Stiive (Nov 22, 2008)

Yeh im still thinking of doing this if i go DC... 
Would certainly help if i got a bit more feedback though

glad to see someone else is interested in the topic 

I know the formulae are right - just need some values. Unfortunately i dont have a motor on which i can experimentally get some values to work out whether this is a viable idea. Anyone with a mulitmeter interested in doing some load tests? lol


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## major (Apr 4, 2008)

Stiive said:


> Yeh im still thinking of doing this if i go DC...
> Would certainly help if i got a bit more feedback though


Hi guys,

I told you that forklifts used by-pass. Maybe you should study how they do it. Here is a link to a GE controller manual. 

http://picasaweb.google.com/rodnhower/EV10# 

Rod from the EVDL was kind enough to scan and post it.

Beware. It is an older SCR PWM control and low voltage. There may issues with implementing by-pass with higher voltage FET controllers. But learning how it was done in the past would be a good starting point for you guys.

Regards,

major


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