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What impact does a PWM signal of 500Hz and 20KHz have on a DC motor, especially in terms of performance, life-span of the motor, etc?

So far I have seen posts that only talk about the buzzing sound that occurs at low PWM frequencies.

Shravan Shravan
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  • at what mechanical load? none? rated load ? or 50%? This affects X(f) / R , Q and winding resonance currents and eddy current losses. vs f https://electronics.stackexchange.com/questions/242293/is-there-an-ideal-pwm-frequency-for-dc-brush-motors – Tony Stewart EE75 Apr 12 '19 at 06:49
  • If the motor is big enough (high inductance) it won't matter. – Marko Buršič Apr 12 '19 at 09:22

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In general, you want to use the right frequency. High enough to get the motor current reasonably constant, low enough to avoid eddy current losses and PWM driver switching losses.

'Reasonably' varies between people and applications, but 10% is probably quite reasonable, <1% is unreasonably good, >50% is unreasonably bad. The frequency needed will vary with the inductance of the motor.

The two main reasons for keeping the current ripple down are (a) to keep torque variations, aka vibration, down, and (b) to keep the efficiency up. As the torque is proportional to the current, but the motor heating (losses) is proportional to current squared, any variation in current will increase the losses and so reduce the efficiency.

You don't want to use too high a frequency either, especially on a motor that has not been specifically designed for PWM control. Motors armatures are usually built from laminations, to reduce the eddy current losses due to current commutation. However, these laminations will usually be designed to a price, to be as thick as they can be for reasonable losses at the rotation speed of the motor. If you impress a varying voltage from PWM at a much higher frequency than this, the eddy current losses will be severe, and will again reduce your efficiency. A motor designed for PWM may have thinner rotor laminations, and a specification for the range of reasonable PWM frequencies.

Neil_UK
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there is no rule that applies to all motors, but in general if the frequency is high enough the inductance of it becomes more relevant and it tends to heat up more easily. An advantage is that most humans cannot hear those frequencies so it would make less sound that you can hear at least. Increased heat can damage winding and other parts easily so you can have that in mind.

At low frequencies, the ripple of the current is more pronounced, this makes the motor less efficient in general, but I doubt it causes any issues due to stress or anything else, it just runs less efficiently if the frequency is too low.

Juan
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  • Where you got this info: "this makes the motor less efficient in general, but I doubt it causes any issues due to stress or anything else, it just runs less efficiently if the frequency is too low." – Marko Buršič Apr 12 '19 at 09:24
  • sorry I do not have a direct source for that, it is something I said from experience. Could be mistaken. – Juan Apr 15 '19 at 07:34