TYPES OF SINGLE PHASE INDUCTION MOTORS

INDUCTION MOTOR

Introduction

The induction motor is one kind of AC motor it is mostly used in industries and home appliances so that’s why induction motor is treated as the king of the industry.

                        Mainly induction motor categorized into three types those are a single phase (1 ), three phase (3 ) and poly phase. Poly phase is nothing but two or more phases having the same voltage and same frequency. As a rough estimate 90% of the world using poly phase induction motors. The reasons are simple in construction, good power factor, speed regulation, low cost.

SINGLE PHASE INDUCTION MOTOR

The main difference between the single phase motor and three phase motor is, three phase motor is self-starting but the single phase motor is not self-starting motor because of rotating magnetic field is not produced in the stator, it producing only alternative magnetic field. Here alternative magnetic field is nothing but the direction of magnetic field is changed two times in one cycle. In other words according to double field revolving theory a flux ( ) is resolved into two types

                               m /2+ m/2

Each of these components rotates in opposite directions. That is one m/2 is works on clockwise direction other one rotates in an anti-clockwise direction. These two fluxes having same magnitudes and opposite direction so they cancel each other then the net torque at the starting is zero.

                                Due to this reasons, torque will not develop in the stator. If the rotor is pushed by the hand or by using the other mechanism the motor will be started.

                            To start the motor without using any external mechanism the single phase motor is fed to the two phase supply but since the supply is a single phase  so, two windings are mounted on the stator  due to this 90  degrees of phase angle created between those two windings. Depending upon this mechanism single phase motors can be classified into two types those are split phase motor and shaded pole motor. Split phase motor classified into two types namely resistance split phase, capacitor split phase again this capacitor split phase further classified as capacitor start –induction run, capacitor start capacitor run induction motors.

RESISTANCE SPLIT PHASE INDUCTION MOTOR

                     Resistance split phase induction motor is also called as split phase motor the name itself indicates that it shifts the one phase into two phases as shown in below.

  Resistance split phase induction motor

The stator having two windings namely main winding, auxiliary winding or starting winding. Auxiliary winding has high resistance and low inductance.  Main winding having low resistance high inductance. These two windings are connected in parallel so 90 phase displacement created between two windings. A centrifugal switch connected in series with the auxiliary winding. The rotor is of squirrel cage type. The reason behind to choose the squirrel cage rotor is we needed constant torque i.e., we can’t vary the torque of the motor and maintenance is also easy. Whenever single phase supply is fed to the stator then rotating magnetic field is produced. Due to Faraday's law of electromagnetic induction principle emf is induced in the rotor conductor then the interaction between the stator flux and rotor currents torque is developed motor will run. To change the direction of rotation interchange the auxiliary winding terminals by this we can say the direction of the motor depends on the auxiliary winding or starting winding.

                                V-I characteristics

Where

           im  is the current flowing through main winding

                            is  is the current flowing through starting winding

           V is the voltage

           I is resultant current             

Due to the resistance and reactance ratios of two windings large phase angle created in between them. im  lags behind the v by a much larger than ia. When the rotor reaches to the 75% of rated torque then the centrifugal switch is opened. By this, we can say starting winding only used for starting purpose but it doesn’t effect the running of the motor. Then the motor will run as a single phase.

             Applications of this motors are centrifugal pumps, blowers etc. These kinds of motors are available in the range of 0.05 to 0.5 HP

CAPACITOR START INDUCTION MOTOR

Capacitor start induction motor is quite similar to the resistance split phase motor except for the capacitor. Here the Capacitor is an electrolytic capacitor. This capacitor is connected in series with the starting winding. Due to this capacitance large angle (in practice 90 ) created in between starting and auxiliary windings.

                                 CAPACITOR START MOTOR

Due to this phase angle more starting torque developed as compared to the split phase motor. Operation of centrifugal switch is same as in the resistance split phase it is also connected in series with the capacitor. When motor reaches to 75% of rated torque then the centrifugal switch open from the circuit.

           These types of motors are used in centrifugal pumps, fans etc. these types are rated in the range of 0.125 to 0.75HP

CAPACITOR START CAPACITOR RUN MOTOR

Capacitor start capacitor run induction motor is also one kind of split phase induction motor. These kind of motors are used when the high starting is needed. This type of motors is having high starting torque than the capacitor start motor.

             These types of motors having two electrolytic capacitors. Those are starting capacitor and running capacitor name itself that starting capacitor is works at starting but, running capacitor will works both starting and running conditions. Which are properly chosen by the ratings. We already discussed in previous, centrifugal switch is opened when the motor reaches to the 75% of rated torque. So the starting capacitor and auxiliary winding is disconnected from the circuit. Running capacitor is useful for the improving the power factor. These kinds of motors having high efficiency than the other motors.

                              Which are used in high starting applications like compressors, conveyors and centrifugal pumps etc.

PERMANENT SPLIT CAPACITOR MOTOR

These types of motors modified from the capacitor start-capacitor run motors. This type of motors is also having starting, running windings and capacitor but there is no centrifugal switch is used. So the capacitor and starting winding is permanently closed. In other words starting winding and capacitor is used at the starting and running conditions.

                     As compared to the other type of motors this permanent split capacitor motor having starting torque is quite low, the motor does not run with the heavy load and it has improved power factor. such motors are available with the range of 0.05 to 0.75HP and which are used in blowers, fans, pumps and many other low torque applications.