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.

SCR [V-I Characteristics]

A detailed study of the characteristics reveal that the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. Which are discussed in details below :

Reverse Blocking Mode of Thyristor

Cathode is made positive with respect to anode. Junctions J1 and J3 are reverse biased whereas the junction J2 is forward biased. The behavior of the thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across them. As a result only a small leakage current of the order of a few μAmps flows.

This is the reverse blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular voltage, known as the critical breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. 

A large current associated with VBR gives rise to more losses in the SCR, which results in heating. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as open circuit.

Forward Blocking Mode

Now considering the anode is positive with respect to the cathode, with gate kept in open condition. The thyristor is now said to be forward biased as shown the figure below.

As we can see the junctions J1 and J3arenow forward biased but junction J2 goes into reverse biased condition. In this particular mode, a small current, called forward leakage current is allowed to flow initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the forward biased anode to cathode voltage.

In this particular mode, the thyristor conducts currents from anode to cathode with a very small drop of potential across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J2 will have an avalanche breakdown at a voltage called forward break over voltage VB0 of the thyristor. But, if we keep the forward voltage less than VBO, we can see from the characteristics of thyristor, that the device offers a high impedance. Thus even here the thyristor operates as an open switch during the forward blocking mode.

Forward Conduction Mode

When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J2 will have an avalanche breakdown at forward break over voltage VBO leading to thyristor turn on.
Once the thyristor is on we can see from the diagram for characteristics of thyristor. 

In this mode of operation, the thyristor conducts maximum current with minimum voltage drop, this is known as the forward conduction forward conduction or the turn on mode of the thyristor.

SCR (Silicon Controlled Rectifier) {Introduction, Symbol & why Silicon is used?}

The idea for the thyristor is not new. The idea for the device was first put forward in 1950 by William Shockley, one of the inventors of the transistor.

What is a thyristor?

The thyristor has a p-n-p-n structure with the outer layers with their electrodes referred to as the anode (n-type) and the cathode (p-type). The control terminal of the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer.

Why Silicon in a thyristor?

Silicon is the ideal choice because of its overall properties. It is able to handle the voltage and currents required for high power applications. Additionally it has good thermal properties. The second major reason is that silicon technology is well established and it is widely used for a variety of semiconductor electronics components. As a result it is very cheap and easy for semiconductor manufacturers to use.

Thyristor symbols & basics

The thyristor or silicon controlled rectifier, SCR is a device that has a number of unusual characteristics. It has three terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology. As might be expected the gate is the control terminal while the main current flows between the anode and cathode.

As can be imagined from its circuit symbol shown below, the device is a "one way device" giving rise to the GE name for it the silicon controlled rectifier. Therefore when the device is used with AC, it will only conduct for a maximum of half the cycle.

In operation, the thyristor or SCR will not conduct initially. It requires a certain level of current to flow in the gate to "fire" it. Once fired, the thyristor will remain in conduction until the voltage across the anode and cathode is removed - this obviously happens at the end of the half cycle over which the thyristor conducts. The next half cycle will be blocked as a result of the rectifier action. It will then require current in the gate circuit to fire the SCR again.

The silicon controlled rectifier, SCR or thyristor symbol used for circuit diagrams or circuit seeks to emphasis its rectifier characteristics while also showing the control gate. As a result the thyristor symbol shows the traditional diode symbol with a control gate entering near the junction.