How come more ripple in PMG DC motor causes more heat??

[zuq]

I have been searching around on the net and I see a lot of people saying that the more ripple you have in Motor current the more the motor will heat up. So, if you increase your frequency ripple should go down right??
but, by changing the frequency the average current does not change then how come it affects the heating?? I^2xR?? Only way I see this working, is if you change the electrical time constant of the motor, then it will reduce heating.

Somebody care to help me on this please?

 

[mtwieg]

Heating is caused by rms current, not average current. In a switching drive, the average current can be zero but the rms current will not, which will cause losses. The higher the ripple the higher the losses. However when operating at a heavy load, the ripple current should be an insignificant contributor to total rms current.

 

[zuq]

Hi
Thanks a lot once again mtwieg, this totally slipped out of my mind.
I have another question, if you happen to know please do suggest.

I am trying to determine the switching frequency for my drive. The way I see is that I should make a balance between the conduction losses and switching losses.
What would be the worst case scenario to compare different frequencies at?? (50% duty?? at full load??)

Also to be able to find switching losses, I need to know the rise and fall time of my currents, how do I calculate that?

Thanks again

 

[mtwieg]

Optimal switching frequency is somewhat hard to pin down. It depends on the motor winding inductance, your desired control bandwidth, power level... you should make a design where operating frequency is an easily tunable parameter, so you can experiment with it.

For switching losses, most power IGBTs and MOSFETs will specify rise and fall times (for a given test setup). They will also usually define the switching energy losses under some test setup. Look for plots of Eon/Eoff vs collector current, collector voltage, gate resistance, etc. You should use those to get an idea of what your switching losses will be.