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I can clock the PWM to drive motors at most any reasonable frequency, but is there a "best" PWM frequency for motor control?

  • What must be considered when choosing a PWM Hz?
  • Is there a most efficient Hz per motor?
    • If so, is there a way to calculate that based on measured motor characteristics such as L, C, and R?

(This is the Lego motor we are using to drive an antenna rotor.)

KJ7LNW
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2 Answers2

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As already mentioned, you want the PWM period to be much shorter than the L/R time constant of your DC motor to better approximate constant current for a given setpoint. I'll just add that the other side of the trade is that as you go to higher and higher PWM switching frequencies, your controller FETs (in your H-bridge) start dissipating more power because their ~fixed switching time is larger as a percentage of the PWM period.

gcr
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For a small motor my rule of thumb is to make the maximum pulse width of a given f and d.c. 10% of the L/DCR = Tau time constant. This puts it in continuous conduction mode (CCM) using the motor L as a RL low pass filter.

However aliasing noises in the magnetics on different RPM frequencies of brush noise and switching frequency and if too low fSW then motor jitter occurs. Also sometimes e-caps are used to smoothen ripple and noise as long as it is bipolar. Then the Q of SRF must be examined.... To keep it well damped.

Going too fast at high currents starts to encroach on the SOA limits so this must be reduced as the current is still flowing when the switch voltage increases. Often in the audio band such as for cordless drills.

So you can measure or choose just under or about 20kHz for now or measure L/R and compute Pd in the switch from the turn off decay current pulse.

With BLDC motors PWM might interfere with the Hall sensors electronics such as in variable speed fans, so it gets more complex and this why PWM fans are designed to support this rather than treat all CPU fans as PWM controllable via power.

Tony Stewart EE75
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  • so My L=7.8mL and R=30 Ohms, so f should be about 10*R/L = ~38kHz? – KJ7LNW Mar 26 '21 at 00:11
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    Yes but you also go down to 5R/L with more ripple current 5*R/L Schmitt inverter Osc. Can do with a pot , fixed R and a cap. – Tony Stewart EE75 Mar 26 '21 at 00:13
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    As a matter of 2nd opinion @Big6 said $$f_s\geq \frac{5}{2\pi\tau}$$. For $$\tau=L/R$$and 5x to settle a pulse enough so he said essentially f > 0.8 R/L which makes it whine at a high pitch if = – Tony Stewart EE75 Mar 26 '21 at 00:31