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We use brushless DC motors (three phase, star wound, 540 size)in RC car applications, these are controlled by electronic speed controllers (ESC's) which will obviously vary the voltage etc supplied to the motors. They use hall effect sensors on the phases. We can vary the motors timing on both the motor itself as well as in software in the ESC, we use a combination to acheive the desired power characteristics depending on track size, grip level etc. We can also set the 'drive frequency' which I believe is the PWM frequency. The available settings are 1KHz, 2KHz, 4KHz, 8KHz and 16KHz. Of those available settings, which should give more torque and which one should give higher rpm (if that's even correct)

I understand that the answer is never quite that simple, but this is the only setting I don't properly understand.. The following page gives info on the motors I use, there is a chart at the bottom and I'm interested in settings for the 4.5turn and 13.5 turn motors

http://www.gforce-hobby.jp/products-en/G0005.html

Skiddins
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  • Surely at *max* torque and RPM, the PWM duty cycle is "on all the time", making the frequency irrelevant? – pjc50 Jan 21 '16 at 17:22
  • (Also, I love the engrish typo on that page for "Blushless motor") – pjc50 Jan 21 '16 at 17:22
  • I would not think it would make a lot of difference, higher frequency gives slightly higher losses and very low frequency may give problems due to ripple, but I think some experiments with the motors you have would give the best answer. – user1582568 Jan 21 '16 at 17:49

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PWM frequency has no effect on maximum torque and rpm, because at full power the PWM ratio is 100% (full on) so there is no frequency. At lower PWM ratios the frequency has an effect on motor efficiency and throttle linearity - the lower the frequency the worse it gets.

PWM controls motor speed by switching the power on and off rapidly with a varying ratio, so the motor is getting getting a series of full power pulses. At low PWM frequency this causes extra loss due to vibration and current surges, but the increased rms current and voltage at mid throttle causes the motor to produce more torque and rpm than it would with the equivalent DC voltage. So you might think that it's more powerful, but that's only because the throttle response is non-linear. In reality more power is lost due to the higher rms current heating up the windings more, and the vibration wasting energy in the gears etc.

As PWM frequency is increased the motor's winding inductance begins to smooth out current flow, so the motor runs more efficiently due to lower rms current and torque ripple. On the down side, higher PWM frequency causes higher loss in the controller. So you should choose a frequency high enough to make the motor run smoothly without making the controller get too hot. Inductance is proportional to number of turns, so the 13.5T motor could get away with a lower PWM frequency than the 4.5T. However if the controller can handle it I would just choose 16KHz for both.

Bruce Abbott
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The frequency is related to the motor coil inductance and associated resonant frequency, which can be further tuned if you have external caps stabilizing the motors.

What you have is essentially a LC tank circuit: https://en.wikipedia.org/wiki/LC_circuit

It's not so much having the highest frequency, as matching the right frequency.

Speed (RPM) is related to load and applied voltage and calculated by a motors speed constant.

Torque is related to power so the more voltage for a higher duty cycle, the more power. With a poorly matched frequency, your efficiency will drop and it will reduce your torque.

Also, some apply higher than spec'd voltage with shorter duty cycles to squeeze out excess speed without burning up the motors, but that is an application specific conversation.

This link here is a similar article with further details. Calculate DC motor frequency

Community
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SpaceCowboyMDK
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  • I'm most familiar with Maxon's motors which have a very different spec sheet with more common units... You might want to review one of their spec sheets for a better perspective of correlations to your specific motor. http://www.maxonmotorusa.com/medias/sys_master/root/8817905696798/15-34-EN.pdf – SpaceCowboyMDK Jan 21 '16 at 18:14
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Neither torque or rpm will depend on PWM frequency. Motors with high inductivity can work on lower frequency. From industry: a big induction motor can run at 2kHz, default value for VFD are typicaly 4kHz, synchronous servo motor driver has a typical default frequency of 8kHz, while a very fast dynamic control uses 16kHz frequency.
As for your motor you should try to use lowest possible frequency, because the heat loss in ESC is minimal, but I'm afraid that small BLDC just needs the max. PWM frequency, you should try it.

Marko Buršič
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