Capacitor size for split-phase motor starting

A split phase motor does not have a capacitor. A capacitor start motor does. All so a capacitor run motor does. Need to to know which it is. Capacitor start will have a mechanical centrifugal switch.

you say in one message 'a split phase motor does not have a capacitor'.. then a few messages down you write 'a cap start motor is a split phase motor'???? a split phase motor relies on the induction, the capacitor just starts the process.

sorry mate didnt realise it was 2 different people

A Capacitor Start motor is a Split Phase motor.

The capacitor is in series with the start winding,

which is nearly identical to a split phase motor

having just a start switch.

The formula to get you started:

Microfarads @ 50 Hz = 3180 X Amperes

Divided by Applied Voltage

So, a 1/4 HP motor drawing approx. 3.2 Amps at 240 Volts

(if the start winding is also operating at 240 Volts),

the required capacitor would be in the neighborhood of 38-42,

or 43-48 Microfarads.

Reducing the Microfarads of the capacitor increases the voltage

across the capacitor.

Using a capacitor that's too large effects the switching torque

of the motor.

Try 3 or more different values rated for at least 230-240 volts,

in the 38... to say 64 microfarads, and see what happens.

John

μ

Thanks John.

It looks as though your formula matches the impedance of the capacitor to the impedance of the motor.

Is this the 'rule-of-thumb' for selecting start capacitors.

I will try out a 45μf capacitor.

But I am still puzzled by 4 terminals in what appears to be a standard arrangement for start capacitors where there are only 2 wires to the coil.

Cliff.

The terminal thing is like someone else said -- "for convenience." Terminal blocks on motors are in cramped spaces, and having a dual terminal on each side of the capacitor is convenient. Also, some canned motor capacitors are actually dual capacitors and may have 3 or 4 single or dual terminal connectors.

Thanks Bill, and others for your help. That's got me sorted out. I'll come back to report anon when I have got the motor up and running.

Cliff.

These types of motors are referred to as "Permanent-Split Capacitor Motors" or PSC. They are similar to capacitor start but the same value of capacitor is used for both start and run. This configuration will not have the large starting torque of a capacitor start motor and are used where high starting torque is not a requirement, i.e. sewing machines, etc. I do not know how to size a capacitor for a motor. I hope this clarify the other responses. Pogo

Hello. I have the same motor as Pogo described. The problem is that I can't find how to choose right capacitor. Google found nothing. Maybe there is formula how to calculate capacitance by motors specifications? Thank you. Fred

Not all capacitors have 4 terminals,i have a single phase dust extraction motor with a start cap having only 2 terminals