# Controller compatability



## EVfun (Mar 14, 2010)

Maybe. It makes the controllers job of limiting current more difficult. The field winding of the series motor is the biggest inductor in the controller buck circuit. You should consult the controller manufacturer for advice.


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## Tesseract (Sep 27, 2008)

Well, you might still be a stupid beginner, Gary B, but this is a good question all the same... 

EVfun is correct that the inductance might be too low so I am merely going to expound on that a bit more both for your benefit and the many others who have the same question but hadn't asked.

Modern motor controllers use a technique called Pulse Width Modulation (PWM) to vary the power delivered to the motor. More specifically, they chop up the battery voltage into rectangular pulses and vary the percent on time from 0-100% (the percent on time is called _duty cycle_).

The average voltage of the rectangular pulses of current results in an average current to flow through the load, but the nature of the load strongly affects how the current waveform looks compared to the otherwise always rectangular voltage waveform from the controller.

The current waveform through a resistive load - like an incandescent light bulb or a water heating element - will look exactly like the voltage waveform: if it's a sine wave (like the AC from a wall outlet) then the current will also be a sine wave; if it's a series of rectangular pulses (like from a motor controller), then the current will also be a series of rectangular pulses. 

Motors, however, have very little resistance but a considerable amount of inductance, and the singular property of an inductor is that it will resist changes in current. So when a rectangular pulse of voltage is applied to an inductor the current will ramp up linearly over time until the voltage pulse once again drops to zero - and provided there is a path for current to continue flowing (usually provided by a "freewheeling diode") - the current will then ramp back down to zero. The rate at which current ramps up (or down) is directly proportional to the applied voltage and inversely proportional to the amount of inductance, so the current flow through a highly inductive load will look like DC _no matter what the voltage waveform looks like_!

Which gets to the heart of the matter, as the most commonly used DC motor in DIY EVs is the series DC traction motor, and these motors tend to have a lot of inductance. A typical value for such a motor is around 100uH (uH = microHenry) which means that if 100V were applied across it the current would increase linearly at the rate of 1A/us (microsecond).

PM motors don't have a field winding - that function is taken over by the permanent magnet, hence the name - so they tend to have a much lower inductance relative to the amount of resistance for a given output power rating. There is another name for a pathway for current which has little resistance or inductance: a short circuit. 

Some of the more common PM motors large enough to be used in EVs (e.g. - Lemco) have a total inductance of around 10uH, so current will rise 10x faster for a given pack voltage in a given amount of time as it would in the typical series DC motor with 100uH of inductance. The average current might be the same, but the peak current reached during each switching cycle might be 10x higher (depends on the switching frequency and the control loop bandwidth). Thus, life can be much harder - indeed, fatal - for a motor controller when driving a PM DC motor, though not necessarily so.


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## Salty9 (Jul 13, 2009)

Thanks Tesseract. It is the most lucid explanation of inductance that I have seen.


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## Gary B (Jun 2, 2011)

Thanks, indeed, Tesseract. - A lucid (and gentle) response. - much appreciated.
For a given throttle input level "A", the controller anticipates (is designed for) an output current (IA). (For a series DC motor). - When the controller is used for a brushed PM DC motor, the same throttle input level will result in a current level of 10 times IA (due to the difference in inductive impedance, as your described). - Whatever "current level feedback" (if any) is used in the controller circuit would be defeated. And, with or without such current feedback, the design intentions would NOT be properly balanced per design. - So much throttle: so much current. If i depress the throttle one inch to get a certain response, say 100 amps to the motor. When i use it with a brushed PM DC motor (as you say) i will get 100 amps when i have depressed the throttle only 1/10 as much: i.e., 0.1 inches. ! - I cannot possibly control my motor on that basis, by moving the throttle only 0.1 inches per 100 amps. - - The controller is designed as part of a system, including the kind of motor it is intended to drive. - I thank you for focusing on the huge impact of the differences in inductive impedance between those two kinds of motors and how they impact controller design. - Best respects and regard, Gary B.


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## Tesseract (Sep 27, 2008)

Gary B said:


> ...For a given throttle input level "A", the controller anticipates (is designed for) an output current (IA). (For a series DC motor). - When the controller is used for a brushed PM DC motor, the same throttle input level will result in a current level of 10 times IA (due to the difference in inductive impedance, as your described). - Whatever "current level feedback" (if any) is used in the controller circuit would be defeated.



That's not what I said, so perhaps my explanation was not lucid enough. I didn't say the average current would be 10x higher, I said the *peak* current would be 10x higher IF the inductance were 10x lower (and everything else was the same). The average current will be the same in either case.

For example, say the average current through a series motor with 100uH of inductance is 100A. If you were to look at the actual current waveform on a scope you might see it trough at 95A and peak at 105A. The average is 100A, but the peak to peak ripple is 10A (105 - 95). Under the same conditions a PM motor might vary from 50A to 150A - the average current is still 100A but the ripple current is 100A! The semiconductors see a peak current of 150A and have to be sized to withstand that, hence why I said driving a low inductance motor is hard for some controllers.

FWIW - and since you asked in another thread - our controllers CAN drive any of the common PM motors that can fit into a car, we just advise not pushing the pack voltage or the motor current to their limits, but that is more to protect the motors than our controllers: none of the popular EV-size PM motors can take anywhere near 300V and/or 1000A, at least not for long nor without risk of fatally demagnetizing the PMs.


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## major (Apr 4, 2008)

Hi Gary, Tess and all,

I have used the exact same controller for both a series motor and then on a brushed PM motor and it essentially behaved the same. So much so I never needed to scope it so I can't tell you if rate of rise was different or anything like that. But it responded to throttle input and current limit the same.

On the other hand, I have had some real problems with motor/controller compatibility using AC drives  Which tells me there could be cases where a DC controller would have difficulty with certain motors. I don't think there is a universal answer here. Sometimes it can work; sometimes it gives you real headaches 

Regards,

major


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## Gary B (Jun 2, 2011)

major said:


> Hi Gary, Tess and all,
> 
> I have used the exact same controller for both a series motor and then on a brushed PM motor and it essentially behaved the same. So much so I never needed to scope it so I can't tell you if rate of rise was different or anything like that. But it responded to throttle input and current limit the same.
> 
> ...


Thanks, Major. - I wasn't trying to quote Tess, i was just trying to present something further that i thought might be a related concern regarding such a design, considering his comments. - My further research showed that some controller designers (sellers) DO indicate what motors they are appropriate for. As you also indicate, "SOMETIMES" - (things work) - That underlines my caution. - What works for series wound DC may (or may not) work for a PM brushed DC type of motor. I happy that yours did both. - Also, i sent you a PM of thanks for a previous input on a proposed purchase. - I hope you got it. - (It did not appear in my "sent" message box.) That concerned me that i don't yet know how to do these things. (?) Still, thanks, - I appreciate that some of these things are more appropriate for PMs. (and I'm still scratching my head.) - 
Gary B.


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## PhantomPholly (Aug 20, 2008)

Great writeup Tesseract. I'm not quite certain I fully get the whole inductance thing, but what I did glean was that current ramp-up and ramp-down may not work as expected.

So, knowing that you use square-wave DC my first thought is that perhaps another way of putting it is that the motor "doesn't like it because it is noisy." Is that a good way of anthropomorphizing it? 

If that is the case, would a "noise filter" help (e.g. a minimal sized capacitor pack just large enough to smooth out voltage & current)? I get that it would not be ideal because when you let up on the accelerator there might be a moment of "trail-off" acceleration (dropping quickly) as the capacitor bank discharges, and likewise perhaps a barely perceptible lag from the time you hit the accelerator until full acceleration is available. But, it seems as though sizing the unit properly could make that barely perceptible / not a safety concern.

And, as I say this I fully expect that in my ignorance such a "solution" might cause more problems than it solves...


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## Tesseract (Sep 27, 2008)

PhantomPholly said:


> Great writeup Tesseract.


Thanks... would you believe some one gave me negative reputation for that post?



PhantomPholly said:


> ...So, knowing that you use square-wave DC my first thought is that perhaps another way of putting it is that the motor "doesn't like it because it is noisy." Is that a good way of anthropomorphizing it?
> 
> If that is the case, would a "noise filter" help (e.g. a minimal sized capacitor pack just large enough to smooth out voltage & current)?...


You are actually correct... Go figure! 

More specifically, inductance smooths out the current waveform while capacitance smooths out the voltage waveform. With enough of both you can get arbitrarily close to pure DC. Note that you must place the capacitor across the motor's armature, not across the entire motor, otherwise the capacitor will present a low impedance to the controller and potentially destroy it. At any rate, the only real benefit to adding capacitance across the motor armature is a (dramatic) reduction in electrical noise emissions - it won't really affect the controller at all.


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