Induction motors are the most commonly used electrical machines. They are cheaper, more rugged and easier to maintain compared to other alternatives. In this video we will learn the working of a 3 phase squirrel-cage induction motor.
The following article gives a detailed description of the video lecture.
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.
|Fig.1 Stator and Rotor of an induction motor|
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.
|Fig.2 Construction details of 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. We will see how this is produced in the next section.
|Fig.3 Rotating magnetic field produced in an induction motor|
The Concept of a Rotating Magnetic Field
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|
|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.|
|Fig.8 RMF produces a torque on rotor as in the simple winding case|
|Fig.9 Thin layers of iron lamina which are packed in rotor|
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|
|Fig.11 A cooling fan is used to remove heat liberated by motor|