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
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