Identifying The Start & Run Windings On Single Phase Motors

Identifying The Windings

Single phase motor

Single Phase Motors have a wide range of applications both domestic and industrial. They have two windings for Starting and Running purposes. Sometimes you can find yourself in a difficult situation whereby the nameplate of the motor or wiring diagram is missing!! Sometimes the motor itself is unaccessible like the case of bore hole pumps.

Usually there are three wires that comes out of the motor casing. The start winding is normally connected in series with a start capacitor. Some Motors have only the start capacitors while others have the start and run capacitors.

Capacitor Start Motor

The start capacitor is only connected when starting the motor and its disconnected automatically by a centrifugal switch when the motor is up to speed.

If you open the single phase motor , you will find out that your motor have two types of winding in which one is made from thick wire gauge and one thin. The thick wire gauge winding is main winding and thin wire gauge winding is starting winding.

Capacitor start and Run motor

But this Identification can only be done by disassembling the motor. This is mostly not necessary because the wiring diagram is printed on the motor nameplate.

The start winding has a high resistance compared to the run winding
Its best to measure the ohms with a analog multimeter
Another way is by measuring the resistance between the three wires respectively.
Example you have: A,B & C Wires.

                       Resistance between A+B=15

                                                    A+C=20

                                                    B+C=25

You add the results of each cable like for A you add 15+20=35, for B 15+25=40 and for C 20+25=45.

So the results are A=35, B=40, and C=45.

The highest reading will be the START winding which is C.
The second highest will be the RUN winding which is B.
The one with the least resistance will be the COMMON winding which is A.
Now you know where to connect your start capacitor.

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