Electrical World

Q1: Where do we require single-phase induction motors? Ans: Single-phase induction motors are required where (i) 3 phase supply is not available  (ii) efficiency is of lesser importance (iii) Rating is less than one H.P.  (iv) Equipment is portable Q2: Why is the power factor of a single-phase induction motor low? Ans: It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature. Q3: What is the function of centrifugal starting switch in a single-phase induction motor? Ans: The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting ...

Q1: Where do we require single-phase induction motors? Ans: Single-phase induction motors are required where (i) 3 phase supply is not available  (ii) efficiency is of lesser importance (iii) Rating is less than one H.P.  (iv) Equipment is portable Q2: Why is the power factor of a single-phase induction motor low? Ans: It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature. Q3: What is the function of centrifugal starting switch in a single-phase induction motor? Ans: The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting ...

For general lighting purpose in shops, offices, houses, schools etc. Single phase a.c. supply is commonly used. Hence instead of d.c. motors, the motors which work on single phase a.c. supply are very popularly in use. These a.c. motors are called single phase induction motors. The numerous domestic applications use single phase motors. The power rating of such motors is very small. Some of them are even fractional horse power motors, which are used in applications like small toys, small fans, hair dryers etc. This chapter explains the construction, working principle and applications of various types of single phase induction motors.  Construction of Single Phase Induction Motors        Similar to a d.c. motor, single phase induction motor has basically two main parts.one rotating and other stationary. The stationary part in single phase induction motors is called stator while the rotating part is called rotor.    ...

For the motoring action, there must exist two fluxes which interact with each other to produce the torque. In d.c. motors, field winding produces the main flux while d.c. supply given to armature is responsible to produce armature flux. The main flux and armature flux interact to produce the torque.        In the single phase induction motor, single phase a.c. supply is given to the stator winding. The stator winding carries an alternating current which produces the flux which is also alternating in nature. This flux is called main flux. This flux links with the rotor conductors and due to transformer action e.m.f. gets induced in the rotor. The induced e.m.f. drives current through the rotor as rotor circuit is closed circuit. This rotor current produces another flux called rotor flux required for the motoring action. Thus second flux is produced according to induction principle due to induced e.m.f. hence the motor is called induction motor...

According to this theory, any alternating quantity can be resolved into two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity.        In case of single phase induction motors, the stator winding produces an alternating magnetic field having maximum magnitude of Φ 1m .        According to double revolving field theory, consider the two components of the stator flux, each having magnitude half of maximum magnitude of stator flux i.e. (Φ 1m /2). Both these components are rotating in opposite directions at the synchronous speed N s which is dependent on frequency and stator poles.        Let Φ f  is forward component rotating in anticlockwise direction while Φ b  is the backward component rotating in clockwise direction. The resultant of these two components at any instant gives the instant...

Consider a single phase induction motor with standstill rotor as shown in the Fig. 1. The stator winding is excited by the single phase a.c. supply. This supply produces an alternating flux Φ s which acts along the axis of the stator winding. Due to this flux, e.m.f., gets induced in the rotor conductors due to transformer action. As rotor is closed one, this e.m.f. circulates current through the rotor conductors. The direction of the rotor current is as shown in the Fig. 1. The direction of rotor current is so as to oppose the cause producing it, which is stator flux Φ s . Fig.  1        Now Fleming's left hand rule can be used to find the direction of the force experienced by the rotor conductors. It can be seen that when Φ s acts in upward direction and increasing positively, the conductors on left experience force from left to right while conductors on right experience force from right to left. Thus overall, the force experienced by th...

In practice some arrangement is provided in the single phase induction motors so as that the stator flux produced becomes rotating type rather than the alternating type, which rotates in particular direction only. So torque produced due to such rotating magnetic field is unidirectional as there is no oppositely directed torque present. Hence under the influence of rotating magnetic field in one direction, the induction motor becomes self starting. It rotates in same direction as that of rotating magnetic field. Thus depending upon the methods of producing rotating stator magnetic flux, the single phase induction motors are classified as, Split phase induction motor Capacitor start induction motor Capacitor start capacitor run induction motor Shaded pole induction motor         To produce rotating magnetic field, it is necessary to have minimum two alternating fluxes having a phase difference between the two. The interaction of such two fl...

This type of motor has single phase stator winding called main winding. In addition to this, stator carries one more winding called auxiliary winding or starting winding. The auxiliary winding carries a series resistance such that its impedance is highly resistive in nature. The main winding is inductive in nature.                    Let                  I m = Current through main winding                     and                  Ist = Current through auxiliary winding        As main winding is inductive, current I m lags voltage by V by a large angle Φ m while Ist is almost in phase in V as auxiliary winding is...

The construction of this type of motors is similar to the resistance split phase type. The difference is that in series with the auxiliary winding the capacitor is connected. The capacitive circuit draws a leading current, this feature used in this type to increase the split phase angle α between the two currents I m and Ist.        Depending upon whether capacitor remains in the circuit permanently or is disconnected from the circuit using centrifugal switch, these motors are classified as,  1. Capacitor start motor and       2. Capacitor start capacitor run motors         The connection of capacitor start motor is shown in the Fig. 1(a). The current I m lags the voltage by angle Φ m while due to capacitor the current Ist leads the voltage by angleΦ st . Hence there exists a large phase difference between the two currents which is almost 90 o , which is an ideal case. The phas...

This type of motor consists of a squirrel cage rotor and stator consisting of salient poles i.e. projected poles. The poles are shaded i.e. each pole carries a copper band on one of its unequally divided part called shading ban Fig.1(a) shows 4 pole shaded pole construction while Fig. 1(b) shows a single pole consisting of copper shading band. Fig 1 Key point : When single phase a.c. supply is given to the stator winding, due to shading provided to the poles, a rotating magnetic field is generated.        The production of rotating magnetic field can be explained as below :         The current carried by the stator winding is alternating and produces alternating flux. The waveform of the flux is shown in the Fig. 2(a). The distribution of this flux in the pole area is greatly influenced by the role of copper shading band. Consider the three instants say t 1 , t 2 and  t 3 during first half cycle of the fl...

The double revolving field theory can be effectively used to obtain the equivalent circuit of a single phase induction motor. The method consists of determining the values of both the fields clockwise and anticlockwise at any given slip. When the two fields are known, the torque produced by each can be obtained. The difference between these two torques is the net torque acting on the rotor.        Imagine the single phase induction motor is made up of one stator winding and two imaginary rotor windings. One rotor is rotating in forward direction i.e. in the direction of rotating magnetic field with slip s while other is rotating in backward direction i.e. in direction of oppositely directed rotating magnetic field with slip 2 - s.        To develop the equivalent circuit, let us assume initially that the core loss is absent. 1. Without core loss Let the stator impedance be Z Ω       ...