# Motor inductive impedance



## Gary B (Jun 2, 2011)

Is it true that the inductive impedance of series motor coils influences the voltage level that appears across the devices that drive that motor? - For example, in the voltage across the sources and drains of a MOSFET combination (which are in series with the motor coils)? - 

I believe it does. However, i do not see measures of that inductance in the various motor specs, such as the Warp 9 or FB1 4001. - (Or others.) - 

A related question has to do with the frequency to be used (in relation to that inductance) to calculate the effective impedance at that frequency. For example: when using PWM control circuitry, there is a base frequency of the PWM, but that does not seem to me to accurately reflect the impedance calculation because it is comprised of a non-sinusoidal waveform. - The dI/dT of the circuit could be much "faster" than that of the base frequency, increasing the effective inductance and impedance. 

This all has to do with the "safe operation" zone of the plots of the devices involved. - MOSFETS, etc. - The maximum currents of those devices generally occur at fairly narrow regions of effective delta V across the the source drain of the devices. - To determine that, one needs to know the delta V across the motor coils themselves. 

Yeah ! - If i had a motor, maybe i could measure that. - But i don't yet have such a motor. - That's why the inductance values and the frequency questions are of interest.

Any help out there, please?

Gary B.
=====


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## Rational (Nov 26, 2011)

Gary B said:


> there is a base frequency of the PWM, but that does not seem to me to accurately reflect the impedance calculation because it is comprised of a non-sinusoidal waveform. - The dI/dT of the circuit could be much "faster" than that of the base frequency, increasing the effective inductance and impedance.


A formula that may come in handy relates the 10 to 90 percent risetime tr to the max frequency in a waveform is 
MHz = 350/tr 
with tr in nS, so a 'scope with a 1 nS risetime has a bandwidth of 350 Mhz.

HP used to publish Application Notes on converting from the time domain to the frequency domain and back again and these are probably somewhere on the Web.


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## Gary B (Jun 2, 2011)

Rational said:


> A formula that may come in handy relates the 10 to 90 percent risetime tr to the max frequency in a waveform is
> MHz = 350/tr
> with tr in nS, so a 'scope with a 1 nS risetime has a bandwidth of 350 Mhz.
> 
> HP used to publish Application Notes on converting from the time domain to the frequency domain and back again and these are probably somewhere on the Web.


Hello Rational. - Your information is interesting in a way but it provides nothing useful to me regarding my "problem." - I am still interested to know what the inductances are for the main motors out there (series wound DC, etc., Warp 9, FB1 4001, etc.) Do you have that information? - Regarding the frequency, i know that it at least has to be the PWM frequency. Thanks. Gary B.


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## Rational (Nov 26, 2011)

Gary B said:


> Hello Rational. - Your information is interesting in a way but it provides nothing useful to me regarding my "problem." - I am still interested to know what the inductances are for the main motors out there (series wound DC, etc., Warp 9, FB1 4001, etc.) Do you have that information? - Regarding the frequency, i know that it at least has to be the PWM frequency. Thanks. Gary B.


For specifics I e-mail manufacturers but it's 50-50 that they answer you.


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## DIYguy (Sep 18, 2008)

Gary B said:


> Is it true that the inductive impedance of series motor coils influences the voltage level that appears across the devices that drive that motor? - For example, in the voltage across the sources and drains of a MOSFET combination (which are in series with the motor coils)? -
> 
> I believe it does. However, i do not see measures of that inductance in the various motor specs, such as the Warp 9 or FB1 4001. - (Or others.) -



I think the inductance of a Warp 9 is about 50uH IIRC. I think you should speak with Tesseract. He can tell you for sure.

cheers,


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## Gary B (Jun 2, 2011)

DIYguy said:


> I think the inductance of a Warp 9 is about 50uH IIRC. I think you should speak with Tesseract. He can tell you for sure.
> 
> cheers,


Thank you, DIYguy. - I will try to PM him. - (your info is copied and saved in my notes file.) - Admittedly, i don't know what the IIRC refers to. - Maybe i can chack that out on the WEB (?) - Gary B.


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## major (Apr 4, 2008)

Gary B said:


> - Admittedly, i don't know what the IIRC refers to. - Maybe i can chack that out on the WEB (?)


If I Remember Correctly, IIRC = If I Remember Correctly


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## major (Apr 4, 2008)

And as long as I busted in on your thread, I think it is on the source side of the switch where inductance works against you. Motor or load inductance is needed and the only worry about motor inductance is that it is enough. The FWD clamps the load side when the switch turns off. The spikes come from the bus.

And I agree, Tesseract would give a better explanation


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## Tesseract (Sep 27, 2008)

Gary B said:


> Is it true that the inductive impedance of series motor coils influences the voltage level that appears across the devices that drive that motor? - For example, in the voltage across the sources and drains of a MOSFET combination (which are in series with the motor coils)? -


No, that is not true at all. The voltage drop across the drain-source of a MOSFET depends only on the drain-source channel resistance (Rds[on]) and the _current_ flowing through it. The inductance of the motor (or anything else) is irrelevant during the conduction period.

However, during the transition periods (ie - from on to off or off to on) the stray inductance between the switches (ie - MOSFET and freewheeling diode) does determine the amount of voltage developed across the off device. 

Caveat: this subject is definitely wading into the deep end of the power electronics pool so I will only give it a cursory treatment in an off-the-cuff forum post.



Gary B said:


> ... However, i do not see measures of that inductance in the various motor specs, such as the Warp 9 or FB1 4001. - (Or others.) -


Motor inductance depends in large part on the current as all except for air core inductors will saturate at a high enough current. The WarP-9 is about 100uH below 200-250A; it drops down to below 50uH above 500A. I haven't done the (somewhat cumbersome) measurements at 1000A (or higher) because George categorically refuses to honor any warranties on his motors if Evnetics bought them... 

I'm not quoting anything else from your post because it was misguided/off-target. The motor inductance is not harmful to the controller at all; quite the opposite in fact! The inductance that IS harmful to the controller is inside of it: between the switch (MOSFET or IGBT) and the freewheeling diode. This is called "unclamped" inductance because, as major alluded to, it is not clamped to the DC bus by either the switch or the freewheeling diode in the brief period of time when one is turning off and the other is turning on. 

As I said above, however, this is definitely getting into the deep end of the pool, so to speak, and would only be of interest to someone trying to make their own controller... is that your goal?


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## Gary B (Jun 2, 2011)

O.K. Major IIRC ! ! ! (and a very good self directed chuckle.) - Thanks. - Live and Learn (LAL). 

And you, Tesseract. Thanks for your input. - YAY ! - I finally got an impedance input value for the WARP 9. - (All copied and on file away from specific forum history.) - For easier access and use. - 

Yes. - I am designing my own controller (BUT definitely NOT a big SOL !) - I am by no means trying to design a racing level controller. - Just something to putz around in around town. - 

Our viewpoints differ somewhat regarding the impact of motor coil impedance on controller design. But that's not worth getting into right now. - Regarding effects that are only applicable during the transitional phases of signal functioning. in my view there is nothing but transitions, at the PWM rate, as a minimum. - There is no real steady state. - Never mind.
Your input is still greatly respected and appreciated. Your focus on the Rsd(ON) is quite welcome and useful. However, knowing Rsd(ON) is not too helpful if one doesn't have an idea of what the source to drain voltage (Vsd) is. - The safe operating zones of these devices are defined (in part) by Vsd. The deep end of the pool is too deep for me too ! - That's what i have been struggling with. That's why i came here. - I'm definitely listening, not just talking.

Also, you guys have alerted me to other factors i have not yet considered or run into from the practical, experienced, hands on, perspective. I'm still very new at this: read many posts; and try to learn what i can. - Stray inductances and capacitances i have been alerted to. Thank you. - For all of your inputs.

Gary B.


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## Tesseract (Sep 27, 2008)

Gary B said:


> ...
> Yes. - I am designing my own controller (BUT definitely NOT a big SOL !) - I am by no means trying to design a racing level controller. - Just something to putz around in around town. -
> 
> Our viewpoints differ somewhat regarding the impact of motor coil impedance on controller design....


Sounds like you got it all figured out - carry on, then!


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## Gary B (Jun 2, 2011)

Tesseract said:


> Sounds like you got it all figured out - carry on, then!


Looking ahead: does anyone have any input on the cheapest, most reliable, controller that will handle 144 volts and 500 amps?

No, Tesseract. - I do NOT have it all figured out yet. I guess most people here (on DIY) do not have it all figured out yet, and i don't want to be seen as someone who thinks he does. 

Someone out there (for certain) (Curtis ?) is proficient doing Fourier analysis in terms of controller circuits. I am not. (Maybe you are, maybe Major is.) That's why I'm here. - For example, i did NOT have ANY inductance values to work with before i came here, and i greatly appreciate the input. - I was not deluged with responses about such values. 

However experienced they may be. Most likely, even the designers of SHIVA (etc.) have their days of "Oh ! S---T" when something they expected comes unraveled or surprises them. - Undoubtedly, i will make my mistakes or misjudgements. Who hasn't? - I'm trying to reduce them by trying to find out things here on DIY, like inductance values.

I do not ask questions here trying to "bait" someone into embarrassment or anything else. I'm just trying to get on with my project. - If i fail in my simple (less expensive) controller design project, which i consider fun and interesting, I guess i will just try to buy as used one somewhere. See lead iin question.

Thanks,
Gary B.


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## piotrsko (Dec 9, 2007)

cheapest and most reliable? possibly Paul & Sabrina's DIY kit controller. Some flavors of Curtis (used) forklift controllers, perhaps.
Per gotti and others: Synkromotive, but now you are starting to get into $$$ toys.

anything used will generally be cheaper, but the usual caveats apply.

There are google-able chinese controllers out there, but I am leery of anything not sold directly in good ol US of A.

these are all my $.02 YMMV I'd buy a used 'Zilla or a new Sol Jr.


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## EVfun (Mar 14, 2010)

I would recommend this Zilla or this Soliton. 

If you think you will want to pour on more amps in the future I would suggest the Zilla. If you feel that increasing your pack voltage down the road is more likely then I would lean toward the Soliton. Both makes of controllers are available in 300 volt 1000 amp versions for about $1000 more.


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## Tesseract (Sep 27, 2008)

Gary B said:


> ...Our viewpoints differ somewhat regarding the impact of motor coil impedance on controller design.


You mean that you don't agree with my statement (note that I did not describe this as an _opinion_) that practically all of the supply voltage will be across the load, with very little across the MOSFET (or IGBT, etc...), when it is on?



Gary B said:


> Regarding effects that are only applicable during the transitional phases of signal functioning. in my view there is nothing but transitions, at the PWM rate, as a minimum. - There is no real steady state.


This is also an incorrect assumption/viewpoint/understanding. The whole point behind switchmode power conversion is to have the active devices (ie - switches and diodes) spend as little time as possible (ideally zero) in transitioning from off to on and vice versa. Reducing that time to zero isn't practical, so a general rule of thumb is to allot 1% of the switching period to such transitions. If your strays are well-controlled (ie - your layout is good) then this goal is easily attainable; if there is lots of stray inductance, particularly in between the dc link capacitor and the active devices, and between the active devices themselves, then no amount of slowing down of the transition time will bring ringing and overshoot under control. And once again we find ourselves in the deep end of the pool, wishing for the swim flugels...



Gary B said:


> *Never mind. Your input is still greatly respected and appreciated.* Your focus on the Rsd(ON) is quite welcome and useful. However, knowing Rsd(ON) is not too helpful if one doesn't have an idea of what the source to drain voltage (Vsd) is. ...


It probably was not your intent to be snide with the above comments I put in bold, but that's how they appeared to me. My philosophy at times like this is that all are entitled to their opinions, no matter how wrong they might be... 

Setting that aside, the proper ordering of subscripted terms is very important if you want practicing engineers and technicians to properly understand you. To wit, you write Rds[on] and Vds, not Rsd(on) and Vsd. Vsd might be a haphazard abbreviation for "Variable speed drive" for all I know...

At any rate, the drain-source voltage (ie - Vds) is greatest when the MOSFET is *off*, at which time the Rds[on] is "approximately infinite" and therefore irrelevant. Rds[on] - as the term suggests - is the resistance from drain to source when the device is on, and therefore allows the designer to calculate the conduction loss (via the familiar equation, I²R). The voltage drop across the MOSFET when it is on can be calculated by multiplying drain current, Id, by drain-source resistance, Rds[on].

Finally, it is important to know what range of load (ie - motor) inductance to expect when designing a controller, but this is very much a case of asking the right question for the wrong reason. The higher the load inductance the lower the peak to peak ripple current and the lower the stress placed on the active devices. The so-called stray inductances (some of the most important mentioned above, but the list is by no means exhaustive) are also important, but for entirely different reasons.

My intent is to thoroughly discourage you - both for selfish reasons, but also for altruistic ones. Designing a switchmode power conversion circuit capable of handling more than a few tens of volts at more than a few hundreds of amperes is challenging, even for people who have done this for 20+ years (*cough*).


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## Weisheimer (May 11, 2009)

Tesseract said:


> ...Motor inductance depends in large part on the current as all except for air core inductors will saturate at a high enough current. The WarP-9 is about 100uH below 200-250A; it drops down to below 50uH above 500A. I haven't done the (somewhat cumbersome) measurements at 1000A (or higher) because George categorically refuses to honor any warranties on his motors if Evnetics bought them...


Hi Tess,

Can you elaborate on the method for measuring inductance and saturation while under load?
I am more interested in testing the saturation point of an inductor that I want to use in a high current charger, but I find the motor issue quite interesting too.

Mark Weisheimer


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## Tesseract (Sep 27, 2008)

Weisheimer said:


> Can you elaborate on the method for measuring inductance and saturation while under load?...


The inductor equation... Inductance is proportional to the reciprocal of the change in current for a given impressed voltage in a given amount of time.

In other words, use a motor controller to apply a square wave to the _stalled_ motor (or you have to subtract out the back EMF from the armature and, also, not include the armature's inductance) and then measure the rise in current over the on time to get the inductance. A greater rise in current over the time of the square wave means a lower inductance.

E.g. - the current rises 100A with a 100V square wave that is on for 100us; the inductance is 100uH.


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## Gary B (Jun 2, 2011)

piotrsko said:


> cheapest and most reliable? possibly Paul & Sabrina's DIY kit controller. Some flavors of Curtis (used) forklift controllers, perhaps.
> Per gotti and others: Synkromotive, but now you are starting to get into $$$ toys.
> 
> anything used will generally be cheaper, but the usual caveats apply.
> ...


Hello piotrsko. - Thank you. I wasn't asking just for the hell of it. - And you gave great input. - I chased down the P & S resource, which led to another "open source" resource that looks great. (Can't remember the name, but was led to it off of P & S's site. - Will find my way back. Something about a "Cougar" controller or something (open source). - I also am attracted to their "boards". - Will keep your info off line in my files. (I do that with all of the good stuff here.) Thanks,
Gary B.


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## Gary B (Jun 2, 2011)

Tesseract said:


> ======
> --- My intent is to thoroughly discourage you - both for selfish reasons, but also for altruistic ones. ---
> ======
> .


Hello Tesseract. - I am by no means discouraged (yet). - even though i know you mean me well. - I was not being sarcastic in your bolded letters of my post. - Anyway. - I am looking at the rest of your post off line. I will keep it (believe me) for what is of value in it.- I will also respond later after reading, considering, etc. - You may not believe me, but i AM here to learn. - Later, Gary B


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## Gary B (Jun 2, 2011)

Tesseract said:


> The inductor equation... Inductance is proportional to the reciprocal of the change in current for a given impressed voltage in a given amount of time.
> 
> In other words, use a motor controller to apply a square wave to the _stalled_ motor (or you have to subtract out the back EMF from the armature and, also, not include the armature's inductance) and then measure the rise in current over the on time to get the inductance. A greater rise in current over the time of the square wave means a lower inductance.
> 
> E.g. - the current rises 100A with a 100V square wave that is on for 100us; the inductance is 100uH.


 If the field and armature coils are connected in series, how is the armature inductance not included? 

Gary B


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## Weisheimer (May 11, 2009)

Tesseract said:


> ...
> E.g. - the current rises 100A with a 100V square wave that is on for 100us; the inductance is 100uH.


Tess,

So, the inductance looks like a straight forward measurement.
Getting it to saturation might be "more interesting"

Thanks for the clear explanation!

Mark


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## Gary B (Jun 2, 2011)

Tesseract said:


> You mean that you don't agree with my statement (note that I did not describe this as an _opinion_) that practically all of the supply voltage will be across the load, with very little across the MOSFET (or IGBT, etc...), when it is on?


 MC. - The matter of “opinion” depends on how one views the word “practically”. - When talking about a 144 v battery supply, on could say that “only” 10 volts will appear across the MOSFET (source to drain). - Then, “practically ALL of the supply voltage will be across the load” (motor coils). - But it may matter if the MOSFET BLOWS at 10 volts (with the current, for example, above 1 amp). - Still, practically all of the voltage (134 volts) appears across the motor. - Try hooking up a battery supply of 144 volts (or, maybe, 12 volts) to the MOSFET drain (with the gate “ON”) (without the inductive load of the motor) and see what happens. - This is back to my original question about motor inductance (and the reason for it). - 
=======


> This is also an incorrect assumption/viewpoint/understanding. The whole point behind switchmode power conversion is to have the active devices (ie - switches and diodes) spend as little time as possible (ideally zero) in transitioning from off to on and vice versa. Reducing that time to zero isn't practical, so a general rule of thumb is to allot 1% of the switching period to such transitions. If your strays are well-controlled (ie - your layout is good) then this goal is easily attainable; if there is lots of stray inductance, particularly in between the dc link capacitor and the active devices, and between the active devices themselves, then no amount of slowing down of the transition time will bring ringing and overshoot under control. And once again we find ourselves in the deep end of the pool, wishing for the swim flugels...


 MC - Actually, your comment confirms my view that the transition times ARE important and worthy of consideration. - “ON” is not as pure as it may appear to be. - Nonetheless, I do appreciate your (underlined) comment about the conventions used in regard to these transitions, together with your other comments about what influences those concerns.- (And I’m NOT being sarcastic.) - 


> It probably was not your intent to be snide with the above comments I put in bold, but that's how they appeared to me. My philosophy at times like this is that all are entitled to their opinions, no matter how wrong they might be...


MC - NO. - I was NOT being snide in that comment. And, I do not argue with your philosophy



> Setting that aside, the proper ordering of subscripted terms is very important if you want practicing engineers and technicians to properly understand you. To wit, you write Rds[on] and Vds, not Rsd(on) and Vsd. Vsd might be a haphazard abbreviation for "Variable speed drive" for all I know...


 MC - Comment accepted, certainly. - My memory fails me at times. I was just trying to save space and didn’t have the spec sheets in front of me for proper annotation. - I will try to do better. - (Aside - all of these spec sheet abbreviations like that confuse me somewhat. - Many of them, (honestly), I have NO idea of what they are talking about. - I try to pay more attention to those that seem to be more important to me. When I goof up, I’ll learn about the ones I goofed up on.



> At any rate, the drain-source voltage (ie - Vds) is greatest when the MOSFET is *off*, at which time the Rds[on] is "approximately infinite" and therefore irrelevant. Rds[on] - as the term suggests - is the resistance from drain to source when the device is on, and therefore allows the designer to calculate the conduction loss (via the familiar equation, I²R). The voltage drop across the MOSFET when it is on can be calculated by multiplying drain current, Id, by drain-source resistance, Rds[on].





> Finally, it is important to know what range of load (ie - motor) inductance to expect when designing a controller,]/Quote]
> MC - Are you saying that the load (motor) inductance does not GREATLY influence the voltage that appears across the source-drain? - What happens when you omit it? - (and turn the gate “on”.) -
> 
> 
> ...


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## Tesseract (Sep 27, 2008)

Gary B said:


> If the field and armature coils are connected in series, how is the armature inductance not included?


The armature and field are connected in series *externally*, so it is quite simple to measure the inductance of just the field (or armature). 

I see that I didn't word that sentence as clearly as I could have, though. To clarify, if the motor is allowed to spin then back EMF from the armature will oppose some of the applied voltage. This would lead to an artificially high inductance measurement from the observed dI. 




Weisheimer said:


> Tess,
> 
> So, the inductance looks like a straight forward measurement.
> Getting it to saturation might be "more interesting"
> ...


You're welcome, and yes, saturation can definitely be "interesting".... Depending on whether the core material saturates hard (ungapped ferrite) or soft (powdered iron, MPP), the current will suddenly spike upwards as the effective permeability of the core approaches that of air (~1). If this inductor is in a SMPS the usual result is the switch fails... generally something to be avoided


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## Gary B (Jun 2, 2011)

Tesseract said:


> The armature and field are connected in series *externally*, so it is quite simple to measure the inductance of just the field (or armature).


Yes, Tesseract ! - (Duh ! here.)


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