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.