Re: Starting a Teletype sync motor question
> My teletype motor uses a cap marked "88-108 MFD, 125VAC 60 cycles"
This is a "Motor Start Capacitor". This kind of cap is used on motors with
"high starting torque'. The motor will have either a centrifugal switch or a
potential relay. The cap is connected between common and the start winding
by the switch or relay only when the motor reaches a certain RPM. Then the
switch opens due to the weights flying out, or if it has a potential relay
instead of a centrifugal sw., the back emf (start winding is starting to act
like a generator) is high enough that the potential across the relay coil is
low enough to reach the dropout point. Larger motors may have both a start
cap and a run cap. This type of motor is called "Split Phase".
Some motors do not use a start cap at all. They just use a run cap. There is
no switch or potential relay and the cap is connected between start and
common permanantly. This type of cap is usually in a metal case and is
always marked in a specific MFD rating. A run cap will have a higher voltage
rating than the motor runs on. For example a fan motor rated 1/4 HP 208/230V
60Hz 1ph would use a 4 or 5 MFD run capacitor rated 370WV. This type of
motor is called " Permanant Split Capacitor". They are used for fans and
blowers, etc. where starting torque is not too high.