PWM Drive of Universal Motor

[tom_pay]

Hi

Last week I found out how to adjust the speed of a universal motor; thanks to the members of the forum.

Can I fully rectify an AC wave and then PWM the resulting DC, at a frequency of approximately 30 kHz to adjust its speed? Would this be inefficient? Or would it even work?

Thanks

Tom

P.S When something says 240VAC or 24VAC is that the Vrms or V peak to peak?

 

[KeepItSimpleStupid]

Of course it works. It also keeps the torque relatively constant at low speeds,


I'm assuming a nameplate. Yes, RMS. They also assume a sine wave input. Also remember that 120 VAC applied to a load, the load would dissipate the same number of Watts as if 120 VDC was applied to the resistive load.

 

[tom_pay]

Thanks for that.

It is the little sticker on the actual motor.

Thanks heaps,

Tom

 

[KeepItSimpleStupid]

Universal motors are usually labeled 120 V AC/DC.

 

[tom_pay]

Yes, it works quite well on the car battery, just had very little torque.

What would normally be used to do the PWMing? I was planning to use a TRIAC, but it would be of little use.

Thanks

Tom

 

[KeepItSimpleStupid]

Triacs can be used on AC only because once they turn-on there are two ways to turn them off. Zero gate drive at (0 load current or shorted triac).

For DC operations a MOSFET or transistor can be used. If an H-bridge is employed then the enable signal can be pulsed.

 

[shortbus=]

Yes, it works quite well on the car battery, just had very little torqueTom

If your motor is for 240V it will have very little torque or speed at 12V. PWM will not help this condition, at 12V.

 

[tom_pay]

Yes, I could stall it with my fingers.

I understand that if I use a higher voltage yet PWM it I should get low speed but still some torque.

Tom

 

[KeepItSimpleStupid]

For a DC motor and it should apply to a universal motor too:

Torque is directly proportional to current.
Speed is proportional to voltage.
a) A DC motor can act s a generator when unloaded and that voltage is proportional to speed
b) It's also proportional to Vmot = Vmeas - Imotor*Rarmature

PWM allows you to use the full amount of current and the voltage, hence speed, is related to the PWM duty cycle.