AC INDUCTION MOTOR

Induction motors are the most commonly used electrical machines. They are cheaper, more rugged and easier to maintain compared to other alternatives

Parts of an Induction Motor

                              An induction motor has 2 main parts; the Stator and Rotor. The Stator is the stationary part and the rotor is the rotating part. The Rotor sits inside the Stator. There will be a small gap between rotor and stator, known as air-gap. The value of the radial air-gap may vary from 0.5 to 2 mm.

Construction details of a Stator

A Stator is made by stacking thin-slotted highly permeable steel laminations inside a steel or cast iron frame. The way the steel laminations are arranged inside the frame is shown in the following figure. Here only few of the steel laminations are shown. Winding passes through slots of the stator.

Effect of 3 Phase Current Passing Through a Stator Winding

When a 3 phase AC current passes through the winding something very interesting happens. It produces a rotating magnetic field (RMF). As shown in the figure below a magnetic field is produced which is rotating in nature. RMF is an important concept in electrical machines

To understand the phenomenon of a rotating magnetic field, it is much better to consider a simplified 3 phase winding with just 3 coils. A wire carrying current produces a magnetic field around it. Now for this special arrangement, the magnetic field produced by 3 phase A.C current will be as shown at a particular instant.

Fig.4 Magnetic field produced around the simplified winding and a single wire

The components of A.C current will vary with time. Two more instances are shown in the following figure, where due to the variation in the A.C current, the magnetic field also varies. It is clear that the magnetic field just takes a different orientation, but its magnitude remains the same. From these 3 positions it’s clear that it is like a magnetic field of uniform strength rotating. The speed of rotation of the magnetic field is known as synchronous speed.

Fig.5 Rotating magnetic field produced over simplified winding

The Effect of RMF on a Closed Conductor

Assume you are putting a closed conductor inside such a rotating magnetic field. Since the magnetic field is fluctuating an E.M.F will be induced in the loop according to Faraday’s law. The E.M.F will produce a current through the loop. So the situation has become as if a current carrying loop is situated in a magnetic field. This will produce a magnetic force in the loop according to Lorentz law, So the loop will start to rotate.

Fig.6 Effect of RMF on a closed conductor

The Working of an Induction Motor

A similar phenomenon also happens inside an induction motor. Here instead of a simple loop, something very similar to a squirrel cage is used. A squirrel cage has got bars which are shorted by end rings.

Fig.7 Squirrel cage rotor which is the most commonly used one in induction motors.

A 3 phase AC current passing through a Stator winding produces a rotating magnetic field. So as in the previous case, current will be induced in the bars of the squirrel cage and it will start to rotate. You can note variation of the induced current in squirrel cage bars. This is due to the rate of change of magnetic flux in one squirrel bar pair which is different from another, due to its different orientation. This variation of current in the bar will change over time.

Fig.8 RMF produces a torque on rotor as in the simple winding case

That's why the name induction motor is used, electricity is induced in rotor by magnetic induction rather than direct electric connection. To aid such electromagnetic induction, insulated iron core lamina are packed inside the rotor.

Fig.9 Thin layers of iron lamina which are packed in rotor

Such small slices of iron layers make sure that eddy current losses are at a minimum. You can note one big advantage of 3 phase induction motors, as it is inherently self starting.

You can also note that the bars of a squirrel cage are inclined to the axis of rotation, or it has got a skew. This is to prevent torque fluctuation. If the bars were straight there would have been a small time gap for the torque in the rotor bar pair to get transferred to the next pair. This will cause torque fluctuation and vibration in the rotor. By providing a skew in the rotor bars, before the torque in one bar pair dies out, the next pair comes into action. Thus it avoids torque fluctuation.

The Speed of Rotation of a Rotor & the Concept of Slip

You can notice here that the both the magnetic field and rotor are rotating. But at what speed will the rotor rotate?.To obtain an answer for this let's consider different cases.

Consider a case where the rotor speed is same as the magnetic field speed. The rotor experiences a magnetic field in a relative reference frame. Since both the magnetic field and the rotor are rotating at same speed, relative to the rotor, the magnetic field is stationary. The rotor will experience a constant magnetic field, so there won’t be any induced e.m.f and current. This means zero force on the rotor bars, so the rotor will gradually slow down.

But as it slows down, the rotor loops will experience a varying magnetic field, so induced current and force will rise again and the rotor will speed up.

In short, the rotor will never be able to catch up with the speed of the magnetic field. It rotates at a specific speed which is slightly less than synchronous speed. The difference in synchronous and rotor speed is known as slip.

Energy Transfer in the Motor

The rotational mechanical power obtained from the rotor is transferred through a power shaft. In short in an induction motor, electrical energy is enters via the Stator and output from the motor,the mechanical rotation is received from the rotor.

Fig.10 Power transfer in a motor

But between the power input and output, there will be numerous energy losses associated with the motor. Various components of these losses are friction loss, copper loss, eddy current and hysteresis loss. Such energy loss during the motor operation is dissipated as heat, so a fan at the other end helps in cooling down the motor.

Advantages and disadvantages of induction motors

Advantages

The biggest advantage of AC induction motors is their sheer simplicity. They have only one moving part, the rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free. DC motors, by contrast, have a commutator and carbon brushes that wear out and need replacing from time to time. The friction between the brushes and the commutator also makes DC motors relatively noisy (and sometimes even quite smelly).

Disadvantages

Since the speed of an induction motor depends on the frequency of the alternating current that drives it, it turns at a constant speed unless you use a variable-frequency drive; the speed of DC motors is much easier to control simply by turning the supply voltage up or down. Though relatively simple, induction motors can be fairly heavy and bulky because of their coil windings. Unlike DC motors, they can't be driven from batteries or any other source of DC power (solar panels, for example) without using an inverter (a device that turns DC into AC). That's because they need a changing magnetic field to turn the rotor.