Robert Bruninga via EV wrote:
3 phase 60 Hz AC synchronous motors seem to start and run just fine on fixed 60 Hz frequency, why does one need to ramp up and have precise feedback of rotor position therefore frequency and phase to make a DC controller?
Synchronous motors are rare on fixed 60Hz AC because they are hard to start; and draw excessive current while trying to start.
When you try to start it, the rotor tries to *instantly* move to the shaft angle that corresponds to the phase of the applied AC at that instant. To jump instantly from 0 to synchronous speed would require *infinite* torque (0 to 60 in 0 seconds)! And that would require infinite current! Not gonna happen.
Of course, it can't draw infinite current; but starting current will be very high. The torque will be correspondingly high; but is often still not enough to jump the rotor from a dead stop up to synchronous speed in a single cycle. The motor might lurch forward; or even *backward* (if that happened to be the closest direction to make shaft position and AC waveforms match). Or just sit there and vibrate, trying in vain to "catch" one of the cars in that train of rapidly passing AC sinewaves.
Also, once running, the synchronous motor needs to be wound so its back EMF at synchronous RPM just happens to be the AC supply voltage. If the supply voltage is high, the motor still runs but has a lagging power factor. If the supply voltage is low, it has a leading power factor. Both of these lead to increased I2R losses and thus poorer efficiency.
Most fixed 60Hz AC synchronous motors are modified to reduce these problem. Synchronous-hysteresis motors have no permanent magnets; but use a "hard" magnetic rotor material that self-magnetizes once it's running (and de-magnetizes when it stops). Synchronous-induction motors "bury" the magnets inside a conventional induction rotor that is used to start the motor. Or a wound-rotor synchronous motor, with slip rings so the field can be controlled externally.
That said... I do think there is hope for driving a true synchronous motor with a simple square-wave DC inverter within limits. It will mainly be hard to start, and you won't get peak efficiency.
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