working of single phase induction motor

Single-phase induction motors main winding is supplied with a single -phase A.C. current. This produces fluctuating magnetic flux around the rotor. This means as the direction of the A.C. current changes, the direction of the generated magnetic field changes. This is not enough condition to cause rotation of the rotor. Here the principle of double revolving field theory is applied.

According to the double revolving field theory, a single alternating filed is due to the combination of two fields of equal magnitude but revolving in the opposite direction. The magnitude of these two fields is equal to the half the magnitude of the alternating field. This means that when A.C. is applied, two half magnitude fields are produced with equal magnitudes but revolving in opposite directions.So, now there is a current flowing in the stator and magnetic field revolving on the rotor, thus Faraday’s law of electromagnetic induction acts on the rotor. 

According to this law, the revolving magnetic fields produce electricity in the rotor which generates force ‘F’ that can rotate the rotor.Why Single Phase Induction Motor is Not Self Starting?When faradays electromagnetic induction law is applied to the rotor, electricity is induced and force is generated on the rotor bars. But according to Double Revolving Field theory, there are two magnetic fields with the same magnitude but revolving in the opposite direction. Thus, two force vectors are produced with equal magnitude but opposite in direction.Thus, these force vectors, as they are of the same magnitude but opposite in direction, doesn’t cause the rotor to rotate. So, single-phase induction motors are not self-starting. The motor simply buzzes in this condition. To prevent this situation and rotate the rotor, the starting force has to be applied for a single -phase motor. As the force in one direction, becomes greater than the force the other direction, the rotor starts rotating. In single -phase induction motors, Auxiliary windings are used for this purpose.Starting Methods of Single Phase Induction MotorSingle -phase induction motor doesn’t have starting torque, so external circuitry is needed to provide this starting torque. 

The stator of these motors contains Auxiliary winding for this purpose. The Auxiliary winding is connected in parallel to a capacitor. When the capacitor is turned on, similar to main winding, revolving two magnetic fields of the same magnitude but opposite direction are observed on Auxiliary winding.From these two magnetic fields of Auxiliary winding, one cancel outs one of the magnetic fields of main winding whereas the other adds up with another magnetic field of main winding. Thus, resulting in a single revolving magnetic field with high magnitude. This produces force in one direction, hence rotating the rotor. Once the rotor starts rotating it rotates even if the capacitor is turned off.