[amsat-bb] Re: Motor for Yaesu G5500 Azimuth Rotator?

[Phil Karn]

On 2/25/12 6:46 AM, Joe Fitzgerald wrote:
>
> controller, but it was not particularly sophisticated.  They simply
> pulsed 60Hz AC from a 24V transformer when they wanted to accelerate or
> slow down.  I am not exactly sure how they generated torque since there
> was no evidence that they did anything to shift the phase of the three
> windings.

If it really was a 3-phase induction motor then there *had* to be some
sort of phase shift at least to start it. An induction motor connected
to a single-phase supply has no starting torque, so it'll just sit there
and burn out unless some sort of protection circuit kicks in first. If
you give it a twist in one direction or the other, it'll start and
continue to run in that direction.

This suggests a possible failure mode for these Yaesu/Kenpro antenna
rotors: if the run capacitor fails or the connection to one winding opens.

Most induction motors designed for single-phase supplies (like these
rotor motors) are actually 2-phase motors. The second winding connects
to the supply through a phase shift capacitor either permanently
("capacitor run") or disconnected with a centrifugal switch once the
motor starts ("capacitor start").

People usually think of 3-phase induction motors as industrial-sized
behemoths, but one especially common application is in the so-called
"brushless DC motor". They're actually AC induction or permanent magnet
motors with built in inverters producing 3-phase AC at a frequency set
by a sensor (usually Hall effect) on the rotor.

One could argue that there really is no such thing as a "DC motor"
except for Faraday's homopolar motor that's still not much more than a
curiosity. So-called "DC" motors are really AC motors with built-in
DC-AC conversion.