Motor Noise Reduction + PWM

Question

I'm trying to suppress EMF coming from a brushed DC motor that I am running. I've tried using 0.1uF capacitors across the motor leads as discussed here: https://www.pololu.com/docs/0J15/9.

It works great for reducing noise, but it makes the motor respond very differently to the same PWM power supply. Specifically, I'm running an L298 at 30kHz.

Before, I was getting my intended minimum RPM at around 50% duty cycle. The motor was also running very steadily with constant power.

With the capacitors, I am getting a much higher RPM at a lower duty cycle and I'm having trouble running the motor at a sufficiently RPM. Also, at a constant PWM, the motor will start up and gradually slow down a little bit.

Is there a better way to control noise? I can understand vaguely why a high pass filter might have issues with a high frequency PWM - but the motor lead capacitor approach and PWM both appear to be the standard in implementing small, brushed DC motors.

Answer 1

You plainly shouldn't be using a capacitor of that size at the motor with PWM. The capacitor is literally meant to smooth away current changes, and your PWM is trying to rapidly change the current. Using both makes no sense.

You'll want a flyback diode, and input-side capacitors to your switching transistors.

Also, you really want something less bad than the L298: it's really an obsolete part; it's high on-resistance means you're wasting a lot of power in the IC that would otherwise be available to the motor, and that you have to get rid of as heat. Atop of that, it's slow and doesn't integrate any good protection circuitry.

Answer 2

I agree that you should try smaller caps from drain to source of the FET's. Use the smallest ones that give the desired reduction in noise.

You can also use shielded cable for the phase wires. Terminate the shield to DC bus ground on the PCB and stator or motor housing on the motor. If you can't manage a fully shielded cable, you can add a ground wire running from PCB to motor housing or stator assembly (terminate the ground wire the same way as the shield).

You can also use a ferrite on the phase wires. All the conductors, including shield and ground wire if any should go inside the ferrite.

Answer 3

I would never use a cap directly across hard switched PWM. You could blow up the caps or the PWM switch. Now that you have done this and found that your noise has gone down it would be good to keep the cap. You can do this by placing inductance between the PWM drive and the motor with the 100n. The inductance does not need to be large because it only needs to divorce the 100n. Remember that the motor inductance is in mH and this smoothes the current. This added inductance can be in uH. Try 47 uH with a current rating suitable for the motor.

Answer 4

30 kHz is too high for L298, I would say 5 kHz is the sweet spot.