PWM DC MOTOR at low frequency...

[Greenplaid]

Does anyone know the pitfalls of operating a low power DC motor with a PWM at a lower frequency?

For example:
12VDC Motor @ .250-.500 Amps... yes, 250-500 milli-Amps

Things I do know:
If the PWM frequency is too low, the ripple on the current waveform will be very high. Higher frequency PWM will provide less ripple in the current waveform.

Changing duty cycles would still provide speed control at low PWM frequencies.

Does this type of operation create a life cycle issue for either the motor or the driver if both are adequately heat sunk and/or back emf protected?

GP

 

[sreid]

Higher ripple current in the motor causes extra heating. PWM below 16 kHz is audable (and can be very annoying).

 

[BigLe]

What's the rational to change the pwm frequency? if it's a dc-dc converter type of drive.. switching frequency would be fixed for most applications.

 

[UKpete]

I don't think it matters very much. A lot of dc motors operate off simple thyristor controllers from the mains so there is a lot of 50/60Hz ripple. The inductance of the motors also helps smooth out the current.

 

[blacksea]

As wrote sreid,
servo amp has constant (!) and higher than 16kHz PWM frequency. Than current riple value rather depends from motor inductance and PWM method (modern servo amp has advanced unipolar one).
UKpete: you told about SCR controlled amp with higher (vs PWM controlled from DC line) current ripple.

 

[UKpete]

hi blacksea, I meant that dc motors were used in many applications with the older type of controller (scr) with much worse low frequency ripple without ill-effects, hence I wouldn't expect there to be problems with PWM. DC commutator motors are suprisingly resilient, look at Universal motors, they can even run with ac.

 

[Greenplaid]

Thanks for all the replies guys!

So, the concept of excessive heating becomes the issue.

If the frequency is too low...
The OFF time of the PWM DC is sufficiently long enough to allow the back EMF to conduct throught the reverse protection diodes of the drive mosfets... thus during this time the magnetizing current decays all the way down to 0.
This creates the "maximum ripple" in the current waveform.

So, other than being horribly ineffiecent, the motor system still functions.

For you motor experts... does this prematurely destroy the motor brushes at a rate faster than normal DC operation?
Even if: 12VDC Motor @ .250-.500 Amps...