As the power requirements of single load systems are usually small, all our homes, offices are supplied with a single–phase A.C. supply only. To get proper working conditions using this single-phase supply, compatible motors have to be used. Besides being compatible, the motors have to be economical, reliable and easy to repair.
One can find all of these characteristics in a single phase induction motor readily. Similar to three-phase motors but with some modifications, single-phase induction motors are a great choice for domestic appliances. Their simple design and low cost have attracted many applications.
Single Phase Induction Motor Definition
Single-phase induction motors are the simple motors which operate on single -phase A.C. and in which torque is produced due to induction of electricity caused by the alternating magnetic fields. Single phase induction motors are of different types based on their starting conditions and various factors.
The main parts of a single -phase induction motor are the Stator, Rotor, Windings. The stator is the fixed part of the motor to which A.C. is supplied. The stator contains two types of windings. One is the main winding and the other is the Auxiliary winding. These windings are placed perpendicular to each other. A capacitor is attached to Auxiliary winding in parallel.
As A.C. supply is used for working of single -phase induction motor, certain losses should be looked out for such as- Eddy current loss, Hysteresis loss. To remove the eddy current loss the stator is provided with laminated stamping. To reduce the hysteresis losses, these stampings are usually built with silicon steel.
The rotor is the rotating part of the motor. Here the rotor is similar to the squirrel cage rotor. Besides being cylindrical the rotor has slots all over its surface. To get smooth, quite working of the motor, by preventing magnetic locking of the stator and rotor, slots are skewed rather than being parallel.
Rotor conductors are the aluminium or coppers bars, are placed in the slots of the rotor. End rings made up of either aluminium or copper electrically shorts the rotor conductors. In this single-phase induction motor slip rings and commutators are not used, so their construction becomes very simple and easy.
Equivalent Circuit of Single Phase Induction Motor
Based on Double revolving field theory the equivalent circuit of the single -phase induction motor can be drawn. The circuit is drawn at two positions – standstill rotor condition blocked rotor condition. The motor with blocked rotor condition acts as a transformer with its secondary winding short-circuited.
In standstill rotor condition, two rotating magnetic fields are of opposite direction with equally divided magnitudes and appear as connected in series to each other.
Working Principle 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 Motor
Single -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.
There are different stating methods of single-phase induction motors. Usually, these motors are chosen based on their starting methods. These methods can be classified as
Repulsion motor starting
In the split -phase starts, the stator has two types of windings – main winding and Auxiliary winding, connected in parallel. Motors with this type of starting methods are
Resistor split -phase motors.
Capacitor split -phase motors.
Capacitors start and run motors.
Single Phase Induction Capacitor-Start Motor
This is also called a capacitor split -phase motor. Here the number of turns of Auxiliary winding is equal to that of the main winding. The capacitor is connected in series with Auxiliary winding. The Auxiliary winding is disconnected using a centrifugal switch when the rotor attains 75% of synchronous speed. The motor continues to accelerate until it reaches the normal speed. The power ratings of capacitor start motors lie in between 120W to 750W. These motors usually opt for applications such as Refrigerators, Air-conditioners, etc.. because of their high starting torque.
Applications of Single Phase Induction Motor
These motors find use in fans, refrigerators, Air-conditioners, Vacuum cleaners, washing machines, centrifugal pumps, tools, small farming appliances, blowers etc….These are mostly used for low power but constant speed devices such as agricultural tools and machinery where three -phase supply are not available. 1/400 kW to 1/25 kW motors are used in toys, hair dryers, etc…
So, basically, we use single phase induction motors in our daily life frequently. These motors are easy to repair. Yet there are some disadvantages to these motors. Which of the disadvantage of these motors have you come across? Can you name some of them?