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

[Phil Karn]

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