Originally Posted by

**PaulWay**
For DC, the switching frequency isn't really a problem - you're using PFM or PWM to chop the battery DC up according to how much power (i.e. amps) you want. This then gets converted[1] to DC but at a lower amperage.

For non-induction AC and brushless DC (which are basically the same), switching does matter, as you're switching the DC from positive to negative and back, so the maximum frequency of that oscillation is limited to half your switching frequency (one positive cycle, one negative cycle). That then gets divided by the number of coils on each phase - so if there are 36 coils in total, on three phases, then you have to go through twelve oscillations to move the rotor around one revolution.

So with a 15KHz switching rate on a 36-coil motor, you can only get (15000 / (2 * 12)) = 625 revs per second, or 37500 RPM. OK, so that doesn't sound so bad. In reality, you usually don't want to get anywhere near half the switching frequency.

For AC induction motors, the frequency is more or less irrelevant, as the induced magnetic field in the rotor is itself rotating. It'll never be moving as fast as the stator frequency, so there's always some 'slip' - that's OK, the higher the slip the more torque. And that's about as much as I understand about AC induction motor theory.

Hope this helps,

Paul

[1] I don't know how. I'm assuming big capacitors or something.

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