If gradually increased from 0 to 100 duty cycle, does PWM on its own take care of inrush current in a motor drive?


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If gradually increased from 0 to 100 duty cycle, does PWM on it's own take care of inrush current in a motor drive?

Yes, if it is applied sensibly. When the motor is stationary it looks like an inductor in series with a small resistance, so current initially rises linearly at a rate of dI/dt = V/L. At low PWM ratio the current waveform will be a series of sawtooth waves with a high peak to average ratio, but if the PWM pulse is short enough then both the average and peak inrush current will be less than it would be with full DC voltage applied.

As motor speed increases it generates a back-emf which subtracts from the supply voltage, which also reduces current. If the PWM ratio is gradually increased the motor will continue to draw less current than it would on full DC voltage, but of course will take longer to get up to speed because the lower current produces less torque.

Depending on motor construction, the winding inductance could be in the μH or mH range. Iron cored motors generally have relatively high inductance so PWM frequencies of a few kHz are usually enough to limit peak current. Coreless motors have lower inductance so they may need a PWM frequency of 30 kHz or higher.

  • \$\begingroup\$ Well this is absolutely contrary to what the other answer + comments are saying. I don't feel I'm in a position to determine which answer is correct until I test it which is not going to be any time soon. I'll leave it to upvotes and/or comments to decide that. For my case I'm using a brushed iron core motor and my planned frequency is above 20kHz in order to not produce sound, so I'm guessing I'll be alright. \$\endgroup\$ – php_nub_qq Jul 1 '20 at 9:31

No, it doesn't.

However, between PWM pulses, the energy stored in your motor will not completely decay, usually (otherwise you'd have a stuttering instead of a turning motor).

So, you're only seeing the full inrush current for the first couple of PWM periods.

  • \$\begingroup\$ Well, the inductance of the motor does help limit current, itself. Nicht war? \$\endgroup\$ – jonk Jul 1 '20 at 5:29
  • \$\begingroup\$ exactly, but that's orthogonal to PWM, innit? \$\endgroup\$ – Marcus Müller Jul 1 '20 at 5:30
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    \$\begingroup\$ Yeah. Not really related. I was just thinking that the OP is wondering about inrush, which is usually more of a problem with capacitance. When you have a nice fat inductor, it makes me wonder what gave the OP the idea to worry. \$\endgroup\$ – jonk Jul 1 '20 at 5:32
  • \$\begingroup\$ @jonk I just got my motor, and of course the first thing I do is just put the leads on a battery. What the motor does? It jumps on one leg before it starts to rotate, which I assumed is inrush current, didn't really measure if I have tbh but it's quite heavy so you'd need quite some power to do that. I know there are drivers that probably handle this but I'd like to "roll my own" and thus I'm concerned, I'd like to avoid blowing a bunch of MOSFETs if possible, since I'm going to have about 10 in parallel, lol. \$\endgroup\$ – php_nub_qq Jul 1 '20 at 5:43
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    \$\begingroup\$ @php_nub_qq depends on the type of motor, you should ask this as a new question! \$\endgroup\$ – Marcus Müller Jul 1 '20 at 5:48

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