Types of Three Phase Induction Motor

Types of Induction Motor

 A- SINGLE PHASE INDUCTION MOTOR

       1-  Split phase induction motor

       2- Capacitor start induction motor

       3- Capacitor start capacitor run induction motor

       4- Shaded pole induction motor

 B-  THREE PHASE INDUCTION MOTOR

       1- Squirrel cage induction motor

       2- Slip ring induction motor

 B-  THREE PHASE INDUCTION MOTOR

Construction Of Squirrel Cage Induction Motor

Any Induction Motor has a Stator and a Rotor. The construction of Stator for any induction motor is almost the same. But the rotor construction differs with respect to the type which is specified above.

STATOR:

The stator is the outer most component in the motor which can be seen. It may be constructed for single phase, three phase or even poly phase motors. But basically only the windings on the stator vary, not the basic layout of the stator. It is almost same for any given synchronous motor or a generator. It is made up of number of stampings, which are slotted to receive the windings. Lets see the construction of a three phase stator. The three phase windings are placed on the slots of laminated core and these windings are electrically spaced 120 degrees apart. These windings are connected as either star or delta depending upon the requirement. The leads are taken out usually three in number, brought out to the terminal box mounted on the motor frame. The insulation between the windings are generally varnish or oxide coated.

 Squirrel Cage Rotor:

This kind of rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors, which are not wires, as we think, but thick, heavy bars of copper or aluminium (aluminium) or its alloys. The conductor bars are inserted from one end of the rotor and as one bar in each slot. There are end rings which are welded or electrically braced or even bolted at both ends of the rotor, thus maintaining electrical continuity. These end rings are short-circuited, after which they give a beautiful look similar to a squirrel thus the name.

One important point to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, thus it is not possible to add any external resistance in series with the rotor circuit for starting purpose. The rotor conducting bars are usually not parallel to the shaft, but are purposely given slight skew. In small motors, the rotor is fabricated in a different way. The entire rotor core is placed in a mould and the rotor bars & end-rings are cast into one piece. The metal commonly used is aluminium alloy. Some very small rotors which operate on the basis of eddy current, have their rotor as solid steel without any conductors.

Reasons for Having Skewed Rotor

1. It helps in reduction of magnetic hum, thus keeping the motor quiet,

2. It also helps to avoid “Cogging", i.e. locking tendency of the rotor. The tendency of rotor   teeth remaining under the stator teeth due to the direct magnetic attraction between the two.

3. Increase in effective ratio of transformation between stator & rotor,

4. Increased rotor resistance due to comparatively lengthier rotor conductor bars, &

5. Increased slip for a given torque.

As 95% of the motors used are of Squirrel cage type, they have been classed under 6 different Categories. These categorization is based on the electrical characteristics of the rotor and its construction. Know these different classes and appreciate each and every individual characteristics of these classes. Also check out what came up to over come the poor starting torque of squirrel cage induction motor.Know about the construction of Double Squirrel cage motor with a graphical comparison with single cage.

Double Squirrel Cage Motor

We have already discussed that the rotor which is of squirrel cage type has two end rings, which are permanently shorted with the rotor conductors. Thus external resistance could not be added to the rotor circuit. Since the rotor has a very low resistance, the starting torque was very poor. The starting torque could be increased by having a cage of high resistance, but the motor will have a very poor efficiency during normal running conditions, as there will be more Cu losses. So Mr. Boucheort, came up with a motor, which had two independent cages on the same rotor. The outer cage consists of bars of high resistance metal, whereas the inner cage has low-resistance copper bars. Thus the outer cage has a high resistance with low reactance to resistance ratio and the inner cage has low resistance but high reactance to resistance ratio. Thus the outer cage develops a maximum torque at starting, while the inner cage does so at about 15% slip.

Slip-Ring Induction Motor

Stator:

The construction of a stator is the same for both the squirrel cage and slip ring induction motor. The main difference in a slip ring induction motor is on the rotor construction and usage. Some changes in the stator may be encountered when a slip ring motor is used in a cascaded system, as the supply for the slave motor is controlled by the supply from the rotor of the other slip ring motor with external resistance mounted on its rotor.

Rotor:

he slip ring induction motors usually have a “Phase-Wound" rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which have slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart.

The rotor is wound for as many poles as the number in the stator and is always 3-phase, even though the stator is wound for 2-phase. These three windings are “starred" internally and the other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of a spring assembly.

What happens when external resistance is added?

In the case of a squirrel cage induction motor, the rotor resistance is very low so that the current in the rotor is high, which makes its starting torque poor. But adding external resistance, as in the case of a slip ring induction motor, makes the rotor resistance high when starting, thus the rotor current is low and the starting torque is maximum. Also the slip necessary to generate maximum torque is directly proportional to the rotor resistance. In slip ring motors, the rotor resistance is increased by adding external resistance, so the slip is increased. Since the rotor resistance is high, the slip is more, thus it's possible to achieve “pull-out" torque even at low speeds. As the motor reaches its base speed (full rated speed), after the removal of external resistance and under normal running conditions, it behaves in the same way as a squirrel cage induction motor.

Thus these motors are best suited for very high inertia loads, which requires a pull-out torque at almost zero speed and acceleration to full speed with minimum current drawn in a very short time period.

Advantages of slip ring induction motors

The main advantage of a slip ring induction motor is that its speed can be controlled easily.

"Pull-out torque" can be achieved even from zero RPM.

It has a high starting torque when compared to squirrel cage induction motor. Approximately 200 - 250% of its full-load torque.

A squirrel cage induction motor takes 600% to 700% of the full load current, but a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current.