On Wed, Jun 15, 2016 at 11:52 AM, Peter Hiscocks via talk <talk@gtalug.org> wrote:

> Most people think 60Hz when they think AC. But I would guess that in
> a motor AC is timed to the rotation so that commutators (I think
> that's what they are called) can be eliminated.
>
> Would that AC be 60Hz or would it be variable, matching some function
> of the motor's current speed?

Variable speed AC motors use a variable frequency alternating current.
Sometimes you can hear the frequency as it increases when the vehicle is
starting. The alternating current creates a rotating magnetic field which
'carries along' the rotor.

(I'm sure Lennart has been waiting for me to jump in with this one)

My personal favourite synchronous motor - the one used to spin records. Phonograph as opposed to pantograph... In a typical belt-driven turntable, the motor has two sets of windings, one needing to be driven at 90 degrees phase ahead of the other. In this way, 60Hz AC can be applied directly to one, and through a cap to the other, and start and maintain torque and speed.

if you have 60Hz, the motor spins at 300RPM - 50Hz: 250RPM. The ratio of the motor spindle vs. platter is established to give you 33 1/3 RPM. Cheap & cheerful.

Synthesizing and refining these quadrature signals has become fertile ground for audiophile/hacker types. From my own experience, simply driving the motor on my old table at ~40V instead of 120V allows it to stay locked in sync, but audibly decreases the humming noise of the turntable itself (not via the pickup) and playback sounds audibly more realistic (less timebase jitter?).

Cheers,

Mike