Construction of Three Phase Induction Motor

Construction

Basically, the induction motor consists of two main parts, namely

1. The part i.e. three phase windings, which is stationary called stator.

2. The part which rotates and is connected to the mechanical load through shaft called rotor.

The conversion of electrical power to mechanical power takes place in a rotor. Hence rotor develops a driving torque and rotates.

1. Stator

The stator has a laminated type of construction made up of stampings which are 0.4 to 0.5 mm thick. The stampings are slotted on its periphery to carry the stator winding. The stampings are insulated from each other. Such a construction essentially keeps the iron losses to a minimum value. The number of stampings are stamped together to build the stator core. The built up core is then fitted in a casted or fabricated steel frame. The choice of material for the stampings is generally silicon steel, which minimizes the hysteresis loss. The slots on the periphery of the stator core carries a three phase winding, connected either in star or delta. This three phase winding is called stator winding. It is wound for definite number of poles. This winding when excited by a three phase supply produces a rotating magnetic field as discussed earlier. The choice of number of poles depends on the speed of the rotating magnetic field required. The radial ducts are provided for the cooling purpose. In some cases, all the six terminals of three phase stator winding are brought out which gives flexibility to the user to connect them either in star or delta.

The Fig. 5.4.1 shows a stator lamination. 

2. Rotor

The rotor is placed inside the stator. The rotor core is also laminated in construction and uses cast iron. It is cylindrical, with slots on its periphery. The rotor conductors or winding is placed in the rotor slots. The two types of rotor constructions which are used for induction motors are,

1. Squirrel cage rotor and 2. Slip ring or wound rotor

1. Squirrel Cage Rotor

The rotor core is cylindrical and slotted on its periphery. The rotor consists of uninsulated copper or aluminium bars called rotor conductors. The bars are placed in the slots. These bars are permanently shorted at each end with the help of conducting copper ring called end ring. The bars are usually brazed to the end rings to provide good mechanical strength. The entire structure looks like a cage, forming a closed electrical circuit. So the rotor is called squirrel cage rotor. The construction is shown in the Fig. 5.4.2.

As the bars are permanently shorted to each other through end ring, the entire rotor resistance is very very small. Hence this rotor is also called short circuited rotor. As rotor itself is short circuited, no external resistance can have any effect on the rotor resistance. Hence no external resistance can be introduced in the rotor circuit. So slip ring and brush assembly is not required for this rotor. Hence the construction of this rotor is very simple. 

Fan blades are generally provided at the ends of the rotor core. This circulates the air through the machine while operation, providing the necessary cooling. The air gap between stator and rotor is kept uniform and as small as possible.

In this type of rotor, the slots are not arranged parallel to the shaft axis but are skewed as shown in the Fig. 5.4.3.

The advantages of skewing are,

1. A magnetic hum i.e. noise gets reduced due to skewing hence skewing makes the motor operation quieter.

2. It makes the motor operation smooth.

3. The stator and rotor teeth may get magnetically locked. Such a tendency of magnetic locking gets reduced due to skewing.

4. It increases the effective transformation ratio between stator and rotor.

2. Slip Ring Rotor or Wound Rotor

In this type of construction, rotor winding is exactly similar to the stator. The rotor carries a three phase star or delta connected, distributed winding, wound for same number of poles as that of stator. The rotor construction is laminated and slotted. The slots contain the rotor winding. The three ends of three phase winding, available after connecting the winding in star or delta, are permanently connected to the slip rings. The slip rings are mounted on the same shaft. We have seen that slip rings are used to connect external stationary circuit to the internal rotating circuit. So in this type of rotor, the external resistances can be added with the help of brushes and slip ring arrangement, in series with each phase of the rotor winding. This arrangement is shown in the Fig. 5.4.4.

Key Point This way the value of rotor resistance per phase can be controlled. This helps us to control some of the important characteristics of the motor like starting torque, speed etc.

In the running condition, the slip rings are shorted. This is possible by connecting a metal collar which gets pushed and connects all the slip rings together, shorting them. At the same time brushes are also lifted from the slip rings. This avoids wear and tear of the brushes due to friction. The possibility of addition of an external resistance in series with the rotor, with the help of slip rings is the main feature of this type of rotor.

3. Comparison of Squirrel Cage and Wound Rotor

Wound or slip ring rotor      

1. Rotor consists of a three phase winding similar to the stator winding.

2. Construction is complicated.

3. Resistance can be added externally.       

4. Slip rings and brushes are present to add external resistance.

5. The construction is delicate and due to brushes, frequent maintenance is necessary.         

6. The rotors are very costly.

7. Only 5 % of induction motors in industry use slip ring rotor.     

8. High starting torque can be obtained.    

9. Rotor resistance starter can be used.     

10. Rotor must be wound for the same number of poles as that of stator.

11. Speed control by rotor resistance is possible.         

12. Rotor copper losses are high hence efficiency is less.   

13. Used for lifts, hoists, cranes, elevators, compressors etc.

Squirrel cage rotor      

1. Rotor consists of bars which are shorted at the ends with the help of end rings.

2. Construction is very simple.

3. As permanently shorted, external resistance cannot be added.

4. Slip rings and brushes are absent.

5. The construction is robust and maintenance free.

6. Due to simple construction, the rotors are cheap.

7. Very common and almost 95 % induction motors use this type of rotor.

8. Moderate starting torque which cannot be controlled.

9. Rotor resistance starter cannot be used.

10. The rotor automatically adjusts itself for the same number of poles as that of stator.

11. Speed control by rotor resistance is not possible.

12. Rotor copper losses are less hence have higher efficiency.

13. Used for lathes, drilling machines, fans, blowers, water pumps, grinders, printing machines etc.

Review Questions

1. Describe the constructional features of squirrel cage and slip ring induction motors. Discuss the merits of one over other.

2. Compare squirrel cage and wound rotor constructions of induction motor.