It is important to note here that it is theoretically possible to run a three phase motor using single phase power source. This is applied by connecting the single phase voltage to any two of the three wiring terminals of the three phase AC motor, leaving one wiring terminal open. However, this type of application would require helping the motor to start by hand, this is to help the motor rotate until it reaches enough momentum to run on its own with a single phase voltage supplied to only two of the three motor terminals. But the absence of voltage to one winding will result in only 2/3 of the original intended power output of the motor.
To improve the torque capacity of the three phase motor when supplied with a single phase voltage, a capacitor is normally installed across the open motor terminal and one of any of the two voltage connected terminals. The capacitor acts as an indirect source voltage supplying the third open terminal of the motor. Changing the forward and reverse rotational direction of the motor is achieved by changing the location of the connection of the capacitor on the motor terminals (see drawing).
The electrical diagram below illustrates both a star (wye) connected and a delta connected three phase AC induction motor supplied with a single phase source voltage which shows the connection of a capacitor on different points of the motor terminal to achieve the dual rotational direction of the motor.
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| Forward and Reverse Rotation of a Delta Connected 3 Phase AC Motor Powered by Single Phase Supply Voltage |
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| Forward and Reverse Rotation of a Star Connected 3 Phase AC Motor Powered by Single Phase Supply Voltage |