# Voltage limits of DC motors



## valerun (Nov 12, 2010)

Hi motor gurus - quick question (wasn't able to find this info here).

What is the max instantaneous voltage for the common EV DC motors (Netgain - both LV and HV), Kostov?

I know the continuous voltage limits (170/192 for Netgain LV, 288 for HV?, etc.) but that cannot also be an instantaneous. Otherwise you would not be able to run a 300VDC pack with a regular Netgain LV motor and many people do...

Just trying to understand how high we can go in pack voltage for, say, Netgain 11HV. Can I have a 400VDC pack, 500, 600, 700? A lot of things get easier with higher voltage...


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## major (Apr 4, 2008)

valerun said:


> I know the continuous voltage limits (170/192 for Netgain LV, 288 for HV?, etc.) but that cannot also be an instantaneous. Otherwise you would not be able to run a 300VDC pack with a regular Netgain LV motor and many people do...


Hi Val,

I don't think there is an easy universal answer to your question. And your logic stated in the above quote is invalid. The field in the series circuit actually reduces the voltage from the battery during the on pulse so the commutator does not see full pack voltage. The only method of which I am aware to find your voltage limit involves damage to the motor and then not quite going that high after the motor is repaired 

Regards,

major


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## valerun (Nov 12, 2010)

major said:


> And your logic stated in the above quote is invalid. The field in the series circuit actually reduces the voltage from the battery during the on pulse so the commutator does not see full pack voltage.


Good point. Do you know what the typical inductance of the field coil for something like Warp11 is? I remember Jeffrey of EVnetics mentioned 100uH for Warp9 at 500A in one of his posts but that was for the total...

...wait, I have an HV motor in my garage - can I just measure these directly?


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## major (Apr 4, 2008)

valerun said:


> ...wait, I have an HV motor in my garage - can I just measure these directly?


Sure, but where it will be of the most interest to you is when it is in saturation. So you need to bias it with 4 or 500 amps and then measure L.


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## Duncan (Dec 8, 2008)

Hi Guys,

Is this not a moot question?

The limit is rpm and current isn't it?

The motor will only need a few volts to drive the current (10v?) and the back EMF will be proportional to current and rpm

so the motor can only be fed Back EMF + 10?v - If your controller is controlling current it will limit motor voltage to that figure

Pack voltage is irrelevant - except that it will reduce battery current

At any given rpm a certain motor voltage is required for a specific current


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## valerun (Nov 12, 2010)

Duncan said:


> Pack voltage is irrelevant - except that it will reduce battery current


not quite. until certain voltage it is irrelevant but there is a point when windings insulation fails etc. Can you run a Netgain Warp11HV with a 700VDC pack? Every PWM pulse will expose the motor to full pack voltage - yes, for a short period of time during the cycte but that might be enough...


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## Duncan (Dec 8, 2008)

y_es, for a short period of time during the cycte but that might be enough._.

20Khz - 1/20,000 of a second - 
Is this not what the capacitors in the controller and the inductance of the motor kind of smooth out?

As a mechanical engineer I have no real idea - one of the electrical guys can probably answer this

My gut feeling is that if the motor can take 150v DC then it can take thousands of volts for 1/20,000th of a second


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## Coulomb (Apr 22, 2009)

Duncan said:


> Is this not what the capacitors in the controller and the inductance of the motor kind of smooth out?


The inductance of the motor smooths the current, not the voltage at all. The capacitance of the controller is needed to keep the electronics in the controller from blowing up due to voltage spikes. So parts of the motor will certainly see full pack voltage.



> My gut feeling is that if the motor can take 150v DC then it can take thousands of volts for 1/20,000th of a second


Induction motors running on 415 V VFDs see some 600 V peak. (The peak of a 415 V sine wave is some 590 V, and 415 V nominal mains could run as high as about 460 V, so that's 650 V peak). I don't believe that they have specially high insulation varnish.

So my guess would be that for the wire insulation alone 650 V would be safe, but the question is whether the brushgear and other bits and pieces have sufficient insulation. Induction motors just have windings that take the high voltage; DC motors have brushes and a rotating armature that has to withstand the instantaneous voltage, and there is conductive brush dust accumulating all the time. I'd say that most DC motors would not be designed for such voltages.

Note that those pulses of less than a 20,000th of a second get repeated 20,000 times per second, or whatever the PWM frequency is. I find it amazing that a varnish coating that gets bent on sharp corners can be made uniform enough and strong enough to withstand hundreds of volts for decades with heat cycles and vibration.


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## major (Apr 4, 2008)

Coulomb said:


> I find it amazing that a varnish coating that gets bent on sharp corners can be made uniform enough and strong enough to withstand hundreds of volts for decades with heat cycles and vibration.


Hi Coul,

Yep, that magnet wire coating is amazing. One of the difficulties in motor manufacture is removing the coating for the connections  I'm not sure what they use nowadays, but this used to be typical:

*Poly Amide-Imide Enamel*
*Trade names:* HPT, HAPT, HPAM, HTAI, GP200, MR200, HOK 
*Temperature Rating: *200C 
*Relative Dielectric Strength: *1500 Volts
*NEMA Specifications: *MW-35C, MW-36C








*Description:* Film insulation consisting of Polyimide resin. Typically red or purple in color. Typical coating thickness is .0015-.0025", but varies with conductor dimensions. 
*Applications:* All types of electromagnetic coils for motors & controls, as well as a wide variety of other applications.

*Polyimide Enamel*
*Trade names:* HML, Allex, Pyrex, Pyre ML
*Temperature Rating: *220C
*Relative Dielectric Strength: *1500 Volts 
*NEMA Specifications: *MW-16C, MW-20C








*Description:* Dual Film insulation consisting of an underlying coating of Polyester (amide) (imide) resin with an overcoating of polyamideimide resin. Typically red or purple in color. Typical coating thickness is .0015-.0025", but varies with conductor dimensions. 
*Applications:* All types of electromagnetic coils for motors & controls, as well as a wide variety of other applications. Used where higher temperature rating than Poly Amide-Imide is needed.


From: http://www.swwireco.com/bic.htm


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