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

[WA6FWF]

Sorry my mistake, been looking at shaded pole motors for a SB-200 amp 
and it stuck...

    Yes you are correct AC induction with a capacitor providing the 
phase shift, when
I was making a board to connect to a LVB controller to totally do away 
with the G-5500
control box I measured the current while running and then measured it 
locked and to
my surprise the current was actually less.

   I have had other people tell me that this happens because the cap is 
too small or
inefficiencies in the motor itself and it would not act this way in a 
larger motor.

   Based on the number of burned out motors and transformers and perfectly
good fuses left over, there is something going on.

   I was originally measuring it to find the values so I could put a PTC 
in to protect it,
the current dropping instead of going up ruled that out, so I went with 
the thermal
approach instead.

73
Kevin WA6FWF


On 2/23/2012 12:23 PM, Phil Karn wrote:
> On 2/23/12 8:29 AM, WA6FWF wrote:
>> Hi David,
>>      As you found out shaded pole motors do not pull more current when
>> they stall, they
>> actually pull less and so they wont blow the fuse, they just sit and cook.
> Are you sure these are shaded-pole motors? They're designed for a
> single-phase supply and are not normally reversible. Nor do they provide
> much starting torque, which you need in a rotor. They're common in small
> appliances, especially fans.
>
> The motors in these Kenpro/Yaesu rotors are 2-phase (presumably
> quadrature), reversible, capacitor-run, AC induction motors. You control
> direction by applying 24V AC directly to one stator winding and to the
> other through a phase shift capacitor. In the older Kenpro design that I
> have, the capacitor is in the control box. In the newer Yaesu models,
> it's in the rotor. I'm not sure but I think this may have been to
> provide limit switches to protect the motor.
>
> The running capacitor advances the phase of the current in the second
> stator winding to establish the rotating magnetic field that drags the
> rotor in the desired direction. The phase shift isn't quite 90 degrees,
> nor are the two phase currents the same, so the motor doesn't run as
> efficiently as it would on an ideal 2-phase supply. Like most induction
> motors it should draw quite a bit of current when stalled so Dave's
> fuses were probably just too large.
>
> I'm working on a variable frequency, variable voltage drive for these
> rotors that changes the frequency and voltage together to vary speed.
> The main thing I'm after is the ability to run the rotor continuously at
> whatever speed is required to avoid constant starts and stops that
> stress the motors, shake the antenna hardware loose, and increase the
> average pointing error.
>
> The torque curve of a classic induction motor has zero torque at
> synchronous speed, increasing as the rotor slips under load below
> synchronous speed and eventually reaching a breakaway peak. When this
> happens with most loads, the result is a motor stall. Lowering the drive
> frequency reduces the breakaway speed and increasing stall torque, so
> starting at low frequency will greatly increase starting torque at the
> same time greatly lowering the motor drive current.
>
> These variable frequency/variable voltage AC motor drives have long been
> common in industry, and they've become the standard in hybrid and newer
> electric cars. Only a few cars like the Tesla actually use induction
> motors; most now use permanent magnet rotors but the principles are much
> the same except that a permanent magnet motor has no slip. This would
> make it easier to keep track of position in an antenna rotor, but
> there's still the potentiometer, assuming it's calibrated.
>
> --Phil
>
>
>