Polyphase Induction Motor (Construction, Types and Principle of operation)

OBJECTIVES :

The aim of this chapter is to gather knowledge about construction, types and principle of operation of 3-phase induction motors. Introduction is dealt before, you can also get here.

CONSTRUCTION :

A typical motor consists of two parts namely stator and rotor like other type of motors.
1. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field,
2. An inside rotor attached to the output shaft that is given a torque by the rotating field.

Stator construction

The stator of an induction motor is laminated iron core with slots similar to a stator of a synchronous machine. Coils are placed in the slots to form a three or single phase winding.

Type of rotors 

Rotor is of two different types.
1. Squirrel cage rotor
2. Wound rotor

Squirrel-Cage Rotor :

In the squirrel-cage rotor, the rotor winding consists of single copper or aluminium bars placed in the slots and short-circuited by end-rings on both sides of the rotor. Most of single phase induction motors have Squirrel-Cage rotor. One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.

Wound Rotor :

In the wound rotor, an insulated 3-phase winding similar to the stator winding wound for the same number of poles as stator, is placed in the rotor slots. The ends of the star-connected rotor winding are brought to three slip rings on the shaft so that a connection can be made to it for starting or speed control.
● It is usually for large 3 phase induction motors.
● Rotor has a winding the same as stator and the end of each phase is connected to a slip ring.
● Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, so it is not so common in industry applications.

PRINCIPLE OF OPERATION

An AC current is applied in the stator armature which generates a flux in the
stator magnetic circuit.
                                This flux induces an emf in the conducting bars of rotor as they are “cut” by the flux while the magnet is being moved (E = BVL (Faraday’s Law)). A current flows in the rotor circuit due to the induced emf, which in term produces a force, (F = BIL) can be changed to the torque as the output.
             In a 3-phase induction motor, the three-phase currents ia, ib and ic, each of equal magnitude, but differing in phase by 120°.
            Each phase current produces a magnetic flux and there is physical
120° shift between each flux. The total flux in the machine is the sum of the three fluxes. The summation of the three ac fluxes results in a rotating flux, which turns with constant speed and has constant amplitude. Such a magnetic flux produced by balanced three phase currents flowing in thee-phase windings is called a rotating magnetic flux or rotating magnetic field (RMF). RMF rotates with a constant speed (Synchronous Speed).
                           Existence of a RFM is an essential condition for the operation of an induction motor. If stator is energized by an ac current, RMF is generated due to the applied current to the stator winding. This flux produces magnetic field and the field revolves in the air gap between stator and rotor. So, the magnetic field induces a voltage in the short-circuited bars of the rotor. This voltage drives current through the bars. The interaction of the rotating flux and the rotor current generates a force that drives the motor and a torque is developed consequently. The torque is proportional with the flux density and the rotor bar current (F=BLI).
              The motor speed is less than the synchronous speed. The direction of the rotation of the rotor is the same as the direction of the rotation of the revolving magnetic field in the air gap. However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced.
                                    If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor the (slip speed) to the speed of the rotating stator field.
            Due to this an induction motor is
sometimes referred to as an asynchronous machine.

SLIP

The relationship between the supply frequency, f, the number of poles, p, and the synchronous speed (speed of rotating field), Ns is given by 120f/p

The stator magnetic field (rotating magnetic field) rotates at a speed, ns, the synchronous speed. If, Nr= speed of the rotor, the slip, S for an induction motor is defined
as S=(Ns-Nr)/ Ns
At stand still, rotor does not rotate ,
 Nr = 0, so S = 1.

At synchronous speed,
Nr = NS, S= 0
The mechanical speed of the rotor, in terms of slip and synchronous speed is given by,
Nr=(1-S)Ns
 Frequency of Rotor Current and Voltage With the rotor at stand-still, the frequency of the induced voltages and currents is the same as that of the stator (supply) frequency, fe.
 If the rotor rotates at speed of Nr, then the relative speed is the slip speed:
Nslip=Ns-Nr
Nslip is responsible for induction.

Phase swinging or Hunting(Synchronous Motor)

Topic is Hunting in synchronous motor.

The phenomenon of oscillation of the rotor about its final equilibrium position is called Hunting.

On the sudden application of load, the rotor search for its new equilibrium position and this process is known as Hunting.
The Hunting process occurs in a synchronous motor as well as in synchronous generators if an abrupt change in load occurs.

The steady state or stable operation of a synchronous motor is a condition of equilibrium. In it, the load torque is equal as well as opposite to the electromagnetic torque. The rotor of the motor runs at synchronous speed in the steady state condition, maintain a constant value of the torque angle δ. The equilibrium gets disturbed if a sudden change occurs in the load torque. Thus, a resulting torque takes place which changes the speed of the motor. It is given by the equation shown below.

Where J is the moment of inertia ,ωM is the angular velocity of the rotor in mechanical units.

The speed of the motor slows down temporarily, and the torque angle δ is sufficiently increased. This is done to restore the torque equilibrium and the synchronous speed when there is a sudden increase if the load torque.

The electromagnetic torque is given by the equation shown below

If the value of δ is increased, the electromagnetic torque is also increased. As a result, the motor is accelerated. As the rotor reaches the synchronous speed, the torque angle δ is larger than the required value. Here the rotor speed continues to increase beyond the synchronous speed.

As the rotor accelerates above synchronous speed, the torque angle δ decreases. The point where the motor torque becomes equal to the load torque, the equilibrium is not restored because now the rotor speed is greater than the synchronous speed. Therefore, the rotor continues to swing backwards and as a result, the torque angle goes on decreasing.

When the load angle δ becomes less than the required value, the mechanical load becomes greater than the developed power. Therefore, the motor starts to slow down. The load angle starts increasing again. Thus, the rotor starts to swing or oscillates around the synchronous speed.

The motor responds to a decreasing load torque by a temporary increase in speed and a reduction of the torque angle δ. Thus, the rotor swings and rotate around the synchronous speed. Thus, this process of rotation of the rotor speed equal or around the synchronous speed is known as Hunting. Since, during the rotor oscillation, the phase of the phasor Ef changes about phasor V. Thus, hunting is known as Phase Swinging.

Causes of hunting-
a. Sudden changes of load.
b. Faults were occurring in the    
    system which the generator
    supplies.
c. Sudden change in the field
    current.
d. Cyclic variations of the load
     torque.

Effects of Hunting-
a. It can lead to loss of synchronism.
b. It can cause variations of the    
    supply voltage producing
    undesirable lamp flicker.
c. The possibility of Resonance
    condition increases. If the
    frequency of the torque
    component becomes equal to that       of the transient oscillations of the
    synchronous machine, resonance       may take place.
d. Large mechanical stresses may  
    develop in the rotor shaft.
e. The machine losses increases and
    the temperature of the machine  
    rises.

Reduction of Hunting-
a. Use of damper windings
b. Uses of flywheels
c. The prime mover is provided with     a large and heavy flywheel. This         increases the inertia of the prime       mover and helps in maintaining         the rotor speed constant.
d. By designing synchronous
     machines with suitable
     synchronising power coefficients.