What is a split phase induction motor?
The split-phase control induction motor is a two phase motor with two sets of windings which finds the major application in control systems. Typically there are very small in the range of 1HP and they are used to move something back and forth such as opening and closing the control valve in control systems. Inching operation of control valves also can be done by using connecting this motor to valve actuator.
- Futures of Split phase motor:
- They are self starting motors
- They can be easily controlled to turn in either direction.
Now the problem of using the split phase motors in all control applications is that it requires two phase power supply which is not readily available. All the power system has either single phase or three phases. So the best way is to convert the single phase supply to two phase supply. This adds some complexity in the circuit, but the application which it provides is high compared to circuit complexity. This process of converting single phase supply in to two phase supply is called as phase splitting.
Split phase induction motor operation:
It consists of two windings start winding and run winding. The start winding is used for starting with high resistance and run winding has low resistance. The start winding is connected directly to the line voltage and the run winding is connected through a capacitor. When the power supply is applied to the motor both the windings are energized, but due to the capacitance in series with the run winding causes its current lags by nearly 90 degrees (ideally) to the line current in start winding. The two windings produce the magnetic fields which are out of phase which creates the rotating magnetic field, and the rotor starts rotating. When the rotor attains the speed then the start winding is removed out off the circuit by a centrifugal switch which is operated by the rotor movement force. i.e. once the rotor starts rotating, the force will operate the centrifugal switch.
The direction of the split phase motor can be altered by changing the power supply to the start winding. The circuit connection shows the split phase induction motor winding arrangement in normal direction. When the switch is below position then the line supply is connected directly and the magnetic field is normal direction which allows the motor to rotate in normal direction. When the switch is up, the line supply is connected and directly to run winding and start winding is connected through a capacitor which the current lags by 90 degrees. In this condition the magnetic field direction is reversed and hence the motor changes its rotating direction.
The speed of the split phase induction motor is essentially depends on input AC power supply frequency and number of poles on stator winding. Only the direction is controlled by altering the switch position.
Important notes on split phase AC induction motor:
- The split phase induction motor starting and running windings draws 4 to 5 times of normal full load current during starting. So the motor heats up quickly with this high current.
- The no-load current is 60% to 80% of full current typically which is very high compared to three phase induction motors.
- Power factor is more due to high magnetizing current even under full load current.
The vibration of split phase induction is twice compared to single phase induction motors with power supply line frequency.
- Used mostly control applications like door opening and closing, valve closing and opening.