Help needed in PWM motor control

[nimeshasilva]

Hi All,

I wrote a program to control a small brushed 12V DC motor from PIC16F627A through a USART via PC serial port. now its working properly.

but the problem is, I've set 10KHz as the PWM frequency, and in lower speeds, an audible noise, (a high frequency noise) is emitting from the motor. but the motor is running smoothly, accelerating, decelerating properly. all in all, the motor seems to mechanically has no problem.

will the 10KHz damage the motor windings?? coz I have heard high frequencies are likely to damage the insulation of windings?? is it true?? if it is likely to damage the motor, how can I find the safe PWM frequency??

and finally, pls tell me what are the considerations if I'm going to implement this to a 220V DC Shunt wound 8.3A motor. I'm going to drive it from a power MOSFET. and I'm very thankful if somebody can provide me with a schematic of the power section.

Thank you
Nimesh

 

[Tahmid]

Hi,
I think you hear the sound as 10kHz is within our audible range (~20Hz to ~20kHz). Maybe you should lower the frequency. I'm not sure.
But I usually use around 1kHz for DC motor control.

For the 220V version, you'll need a power MOSFET and it needs to be driven by a MOSFET gate driver IC (eg TC427/426).

And make sure you apply a flywheel diode (anti-parallel with the motor).

Hope this helps.
Tahmid.

 

[nimeshasilva]

Thanx for the reply.

I already put all those things. and also I put a photocoupler to isolate the drive side and the logic. its working as expected.

I need to know, in long term usage, will 10KHz will damage the motor windings, because I have heard that high frequencies are likely to damage the insulation of the windings

thanx
Nimesh

 

[saeed_pk]

Motor Windings core of Brushed DC Motor is not designed for high frequency PWM better you run in below 1 KHZ
for a 220V Motor just change to FET and Include a driver rest of the system remains the same
Look for
IRF840 MOSFET

 

[FvM]

I don't see an actual problem with 10 kHz PWM. Generally, high frequency AC may bring up a problem of partial discharge,
but unlikely with 12 V supply voltage. It would be rather a problem with mains voltage VFD, but they are working with typically
4 to 16 kHz PWM frequency without special design.

If have operated BDC motors with PWM in various designs, as others do. No failure related to windings short or similar have
been yet reported. Instrument EMC can be an issue, however. So grounding and filtering schemes have to be well considered.

High frequency current ripple causes additional losses due to skin effect (windings) and eddy currents (armature). But
the ripple current reduces by a factor of 10 at 10 compared to 1 kHz, so the losses most likely don't increase.

If you are really concerned about PWM compatibilty of the motor, supply an external inductor and a filter cap.

 

[nimeshasilva]

Thanx guys,

and FvM, pls tell me some more about filtering the power supply (Inductor and filter cap) and eddy currents, I know there are something called "Eddy currents" but don't know how to prevent them and all.

(BTW, I'm talking about the 220V DC 8.3A Shunt motor, and I have selected the MOSFET 2SK725 Rated, 500V 15A 0.38 Ohm RDS(on) is it suitable for this application??)

Thanx

 

[bobcat1]

Hi

I can give you 2 advices from my experience on using motor:

1) If you can increase the motor PWM frequency to over 20KHz - this will lose the audio able sound

2) Use sinus table to generate PWM signal - this will let your motor live longer

The 2 issue above are microcontroller dependent since not every micro is capable of generating 24KHz sinus modulated PWM signal

All the best

Bobi

The microcontroller specialist