Starting single phase motors

I often see two types of motors used in heavy appliances. One with a capacitor and one with the starting relay. The relay has a very thick winding connected in series with the main winding that holds the phase winding in the circuit until the motor comes up to speed and the phase winding is switched out of the circuit. (In the application with the relay, I'm not sure if the term *phase winding* is corect.) I believe the capacitor types are called PSC (permanent split capacitor). In either case I see a field wound with finer wire that is offset (angular displacement) from the main field coil. This phase winding is used to produce starting torque. In either case, removing the capacitor or starting relay, the motor will hum and not spin (draws heavy current and can overheat). Manually spinning the shaft in either direction gets it started in the direction turned.

On PSC motors, removing the capacitor while it is running does not change the speed, but current draw is increased by several percent.

My question is: Can the motor that starts with the current relay be adapted to use a capacitor or is the angular displacement if the phase/starting winding different from the PSC motors?

Thanks, John

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It can be done, sort-of...

You have outlined two types of single phase motor starting. Both of these techniques are suitable for light-duty starts. The fundamental problem with changing a split-phase to a PSC is that the starting winding in the split-phase is not designed for continuous duty. It is intentionally wound with small wire to achieve a high resistance. This high resistance causes a lot of heating, but only for the few seconds when it is actually used.You could select a capacitor small enough that it permits only a small current to flow, one which would not overheat the winding, but the starting torque would be lower than intended. The extra phase shift due to the introduction of the capacitor will tend to increase the starting torque somewhat. It may be satisfactory for your application.

What are you trying to do? Eliminate the relay? What is your application? If you don't need high starting torque you can probably pull it off.

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Motors that do not use a capacitor are usually termed "split phase" and get the reqired phase shift for rotation from inductance and resistance instead of capacitance. Placing a capacitace in the inductance circuit (start winding) would defeat the original design of the motor and might even cancel the starting torque if the capacitace was a certain value. However, there would be a certain vector sum of capacitace, inductance and resistance that could allow the motor to operate. One would have to do calculations and experiments on a specific motor to determine this operating point. It may be possible to get a higher running torque without excessive overheating by doing this. Is that your goal? On the other hand it might burn up a given motor if the dissipation and heating are not considered. In otherwords, it takes some measurements and mathematics to answer your question. Bob

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Bob Eldred

This motor is scavenged from an old washing machine. I was experimenting with it - taking stalled and no load current measurements. I was curious to see what a capacitor would do for starting (haven't tried yet). Starting, it draws around 20 amps, running it draws 5. AFAICT, the starting windings looks to be at wound 45 Deg from the main windings. John

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-------------------- The winding of the motor with the starter are designed for that use. The windings of a split capacitor motor are also designed for that use. There are motors with a single capacitor and a centrifugal switch (this appears to be your starting relay) and again they are designed for the purpose. While it is possible to use a capacitor, you still might not be able to eliminate the switch.

Bob Eldred is right on. -- Don Kelly remove the urine to answer

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Don Kelly

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