On 2/25/12 9:24 PM, Joe Fitzgerald wrote:
On 2/25/2012 4:25 PM, Phil Karn wrote:
If it really was a 3-phase induction motor then there *had* to be some sort of phase shift at least to start it.
Of course you are right. I found a schematic of their controller
http://www.ecse.rpi.edu/courses/CStudio/ham_radio_docs/Prosistel61C.pdf
The phase shift is done with C7 and C8, but I have to spend some time with the schematic to figure out exactly what they are doing. I expected to see a microcontroller/DSP individually controlling each of the 3 legs.
They do not show the internal wiring of the motor, but the controller strongly suggests this is simply a 2-phase capacitor-run motor just like the Yaesu/Kenpro models, except operating on 48V AC rather than 24V AC. The run capacitor is formed by C7 and C8 in series, and pin F1 on the motor is the common return (neutral) for the two windings. Switching is done with electrically isolated triacs (AC switches) rather than relays.
BTW, this shows some of the reasons 2-phase AC eventually fell out of favor in the power grid. Both require 3 wires but they don't use them as efficiently. In the 2-phase system, two wires are hot phases and the third is a (usually grounded) neutral wire. If the two phases each carry 1 A, then the neutral returns 1.4A. (Sum currents 90 degrees out of phase and the total is the square root of the sum of the squares.) If each hot phase is 1V to ground, then the total power transferred over the 3 wires is 2 watts.
A true 3-phase motor has 3 windings connected in wye or delta to the three terminals in a symmetrical way. You'd need a 4th wire to provide a neutral, but because the phase currents are balanced they cancel in the neutral and you can do without it. If each wire draws 1A and is 1V from ground, then together they carry 3W -- one more watt than the 2-phase system even though the 2-phase system's third wire has to be heavier to carry 1.4A.
This is why 3-phase power became *the* grid standard. But for some reason, the guy who originally invented 2-phase AC is the one we generally credit for our use today of 3-phase. Maybe that's because most people wouldn't want to get into a 1.5 Dolivo-Dobrovolsky MRI machine at the local radiology clinic. "1.5 Tesla MRI machine" sounds a lot cooler.
Efficient use of wire isn't as important in a rotator system as in a power grid, so a 2-phase motor is easier to run as a reversible motor on a single-phase supply than a 3-phase motor would be. But if I were to design a rotor system completely from scratch I'd probably use a 3-phase motor driven by a 3-phase motor variable inverter/controller.
--Phil