A simple circuit diagram of the squirrel cage induction motor is shown in the figure. The stator winding can be either delta-connected or star-connected. The stator winding is connected to the three phase supply terminals. The rotor is represented as shown in the figure and no resistance can be inserted in the rotor circuit.
The figure shows the simple circuit diagram of a slip ring induction motor. The stator can be either delta-connected or star-connected. The rotor is always star connected and any resistance can be inserted in the rotor circuit as shown in the figure.
The external resistance is usually known as starting resistance because it is generally inserted in rotor circuit at the start to limit the heavy rush of current and increase the starting torque.
Three Phase Induction Motor Working Principle
When a three-phase stator winding of an induction motor is connected to the AC supply, current starts flowing in the stator conductors. Due to this current, a rotating magnetic field of constant amplitude and a constant speed is set up in the air gap between stator and rotor.
The rotor winding is still stationary. So rotating magnetic field cuts the motionless rotor conductors and induces an EMF in the rotor winding. Voltage induced in the rotor gives rise to rotor currents.
The direction of the rotor current is such that it will oppose the cause that produces the current (Lenz’s law). And the cause behind producing the rotor current is the ‘relative velocity’ between the rotor and rotating magnetic field.
So the rotor current will flow in such a direction that the rotor will experience a force that accelerates it in the same direction as that of the rotating magnetic field. At the no-load rotor rotates at slightly less speed than Ns.
The speed of the rotor (N) is always less than Ns. Because as soon as N = Ns, the relative velocity between rotor and rotating magnetic field becomes zero, and motor slows down. This happens every time when N tends to become equal to Ns. Therefore, the rotor in an induction motor cannot rotate at the synchronous speed.
When we put a load on the motor, its speed (N) decreases to produce the required torque. The reduction in speed of motor (N) will stop as soon as torque produced by the motor is exactly equal to load torque. The stator winding of induction motors can be either delta connected or star connected.
Causes of Low Power Factor of Induction Motor
When a 3-phase supply is given to a 3-phase wound stator of an induction motor, a revolving field is set up in the stator. This field (flux) is also set up in the air gap between stator and rotor. To set up this air gap flux, induction motor draws magnetizing current from the mains which lag behind the voltage vector by 90o.
The magnitude of this current is quite large because of a high reluctance of air gap between stator and rotor.
The power factor of the induction motor is minimum at no load since this magnetizing current has its dominating effect.
However, the power factor increases with the increase in load on the induction motor and is maximum at full load. Therefore, it is advised to operate the induction motor at the full load.
Thanks for reading about “three phase induction motor working principle”.
Electrical Drives – 2 | Objective Type Question Answers
#1 Which of the following motor is usually preferred for kiln drives?
Any of the above
#2 Heat control switches are used in
#3 In which of the following applications the load on motor changes in cyclic order ?
All of the above
#4 Which of the following machines has heavy fluctuation of load ?
#5 In…………. the speed can be varied by changing the position of brushes.
#6 In which of the following applications variable speed operation is preferred ?
#7 Heavy duty cranes are used in
all of the above
#8 The travelling speed of cranes varies from
1 to 2.5 m/s
#9 Besides a constant speed a synchronous motor possesses which of the following advantages?
High power factor
#10 Which of the following motors is preferred when quick speed reversal is the main consideration ?
#11 For a crane travel which of the following motors is normally used ?
A.C. slip ring motor
#12 The capacity of a crane is expressed in terms of
#13 The characteristics of drive for crane hoisting and lowering are which of the following?
All of the above
#14 Which of the following motor is preferred for boom hoist of a travelling crane ?
A.C. slip-ring motor
#15 Which of the following motors has series characteristics ?
#16 For a D.C. shunt motor which of the following is incorrect?
Unsuitable for heavy duty starting
#17 For which of the following applications motor has to start with high acceleration?
Lifts and hoists
#18 Which of the following types of motor enclosure is safest ?
Totally enclosed fan cooled
#19 While selecting motor for an air conditioner which of the following characteristics is of great importance?
#20 The diameter of the rotor shaft for an electric motor depends on which of the following?
Horse power and r.p.m.
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- Three Phase Induction Motor Construction
- Rotating Magnetic Field in Three Phase Induction Motor
- Induction Motor Slip
- Torque Formula for Induction Motor
- Torque Slip Characteristics of Induction Motor
- Losses in Induction Motor
- Induction Motor Tests
- Starting Methods of Induction Motor
- Double Squirrel Cage Induction Motor
- Speed Control of 3 Phase Induction Motor
- What is a variable frequency drive?
- Autotransformer Starter Working Principle
- Thermal Overload Relay Working
- Induction Motor Equivalent Circuit
- Linear Induction Motor Working | Applications