[GreenKeys] GreenKeys] Single phase induction and synchorous motors.

[Bruce Gentry]

All self starting single phase induction motors have some sort of start 
winding. Their purpose is to act with the main winding to create a 
rotating magnetic field, this whirling field induces currents in the 
rotor which produce magnetic fields in the rotor that interact with the 
whirling field and push the rotor along. Think of a magnetic wave 
sweeping over the surface of the rotor and pushing it along.

    The simplest way to start  a single phase motor is to use a shading 
coil, a single (usually) turn of heavy wire located on the edge of the 
stator pole piece. It causes the magnetism within itself to decay 
slowly, so combined with the main field it creates a rotating field to 
push the rotor to turn. It is rather inefficient and can't provide much 
starting torque.

    The next way to start the motor (called a split phase) is with a 
start winding switched off by a centrifugal switch or starting relay. 
The design and placement of the start winding causes it's magnetism to 
persist after the main winding has declined, again causing a rotating 
field. It has better starting torque than a shaded pole, but not as much 
as a capacitor start, and the motor is fairly efficient.

    The next way is to use a start winding designed much like the split 
phase motor, but power it through one or more  condensor(s). Many 
capacitor start motors  use centrifugal switches, others use starting 
relays. In either case, the start winding is disconnected when the motor 
is up to speed. Some motors will have a smaller (running) condensor that 
provides power to the start winding all the time, assisted by the start 
condensor until the motor is up to speed.  Capacitor start motors have 
excellent starting torque and good efficiency, especially if a running 
condensor is used.

    There is one more design feature of the motor in the Teletype- it is 
synchronous. Instead of the rotor having a laminated iron core with a 
squirrel cage winding, the outer part is made of hard steel. The fact 
there are no laminations causes the start and run windings to induce 
eddy currents into the steel, and this pushes it along. As it approaches 
synchronous speed, the hard steel becomes magnetized and locks into 
synchronism with the whirling stator field.  These motors run at 
constant speed governed by the power frequency, very important for a 
Teletype. Their starting torque  and efficiency are fair. That's why 
they run  hot. They rotate at the correct speed overall, but are always 
trying to turn a bit faster and then slower during each rotation, much 
like a race car going fast on the straightaways,  slowing in the 
corners, yet completing  each lap at a very precise overall time. That's 
why the synchronous Teletype motor has such a large fan. It's as much 
for a flywheel as for cooling. Don't try to run the machine without it, 
it will make a lot of noise and is very hard on the gears.

      Bruce Gentry,  KA2IVY

On 10/25/16 4:52 PM, Dave Horsfall wrote:
> On Tue, 25 Oct 2016, teletypeparts at comcast.net wrote:
>
>> Not sure but dont think so.  Good question tho.
> Perhaps it's my limited understanding, but isn't a starting capacitor always
> required to provide the phase shift, and then gets switched out with e.g.
> a centrifugal switch?
>
>
>
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