Construction Of Induction Motor:
Stator:
As the name suggests, it is a stationary part of the motor. This part is made of silicon steel stampings (i.e. thin sheets). The stampings are slotted.
When the complete Stator is assembled, the through slots are formed on the inner side of the stator. The slots may be open type, semi-open type or closed type.
In these slots, a 3-phase winding (made of super enameled copper) is accommodated. This winding may be star or delta connected.
The three ends of these windings are brought out into the terminal box where a 3-phase A.C. supply can be connected.
The stator windings, stator and the A.C. supply connected to the stator winding is shown in the following simple Fig.
2. Rotor:
There are two types of induction motors depending upon the construction of the rotor.
1. Squirrel cage type
2. Wound rotor type
Squirrel cage type rotor:
This is the simplest and most rugged construction. The rotor consists of a cylindrical laminated core with skewed rotor slots.
The rotor conductors which are thick copper bars, are placed in these slots and are brazed or welded to and Tings. Thus, the rotor conductors are permanently short-circuited.
Therefore it is not possible to add any external resistance in the rotor circuit. The rotor body is made of silicon steel stampings. The construction of the rotor looks like a cage of "squirrel" and hence called as "squirrel cage type motor".
Wound type rotor with slip rings:
In this type of induction motor, the rotor is wound for the same number of poles as that on the stator. The rotor is made up of laminations with slots on the outer periphery in which a 3-phase rotor winding is placed. The three phases are starred internally.
The remaining three terminals are brought out and connected to the slip-rings mounted on the shaft.
The slip-rings are made up of copper or phosphor bronze and there are three brushes resting on them. External connections to additional resistances are done at the brushes.
When running under normal conditions, the slip-rings are short-circuited by a metal collar that is pushed along the shaft and the brushes are lifted from the slip-rings to reduce the frictional losses and wear.
As a regular 3-phase winding is used for the rotor, this type of is called as “phase wound rotor type”. It is also called as "slip-ring type” because slip-rings are used.
A section view of a "slip-ring" type 3-phase induction motor is shown in the following Fig.
Working of 3-Phase Induction Motor:
When a 3-phase stator winding is fed from a 3-phase A.C. supply, a magnetic flux of constant magnitude but rotating at synchronous speed is set up.
The magnetic flux cuts the rotor conductors which are stationary. Due to the relative speed between the rotating flux and the stationary conductors, an emf is induced in the conductors.
The frequency of this induced emf is same as the supply frequency, its magnitude is proportional to the relative velocity and its direction is given by Flemming's right-hand rule.
Since the conductors form a complete closed circuit, the rotor currents are produced. And according to Lenz's law, this current tries to oppose the relative speed. Hence the rotor starts rotating in the same direction as that of the flux and tries to catch up with it.
The same is explained as under:
Fig.(a) shows the stator magnetic field at one instant and it is acting downwards and also rotating in a clockwise direction.
This field is cut by the stationary rotor conductor. The E.M.F. is produced in the rotor conductor.
The direction of this E.M.F. is found by Flemming's right hand rule. (Remember that the relative direction of rotation of rotor conductor is to be taken opposite to rotating field direction).
This E.M.F. is shown in the rotor conductor by (.) dot. The rotor current is circulated and this current will produce its own flux. This is shown in Fig. (b).
The resultant flux is shown in Fig. (c). The force is produced on the rotor conductor and rotor starts rotating in the same direction as the rotating magnetic field.
This is shown in Fig. (c).
In practice, the rotor never catches up with the stator field due to friction and inertia of the rotor and the induction motor always runs at a speed slightly less than the synchronous speed.
The difference between the two speeds is termed as slip-speed. (Ns - N)
The direction of rotation of the induction motor can be changed by changing the phase sequence of the supply (by interchanging any two supply leads).
Table of Parts and their Functions:
| Sr. No. | Part | Material | Function |
|---|---|---|---|
| 1 | Stator frame | cast iron | supports the core, protect inner parts |
| 2 | stator core | Silicon Steel | Houses the stator winding |
| 3 | Stator winding | Copper and Insulated | produced rotating magnetic field |
| 4 | Rotor core | Silicon Steel | Houses rotor winding |
| 5 | Rotor Winding | Copper and insulated | To produce rotor current |
| 6 | Air Gap | - | To provide a gap in rotor and stator |
| 7 | Air inlet-outlet | - | For air circulation |
| 8 | Cooling fan | Aluminium | For air Circulation |
| 9 | Slip Rings | Phosphorus Bronze | Connects Resistance to rotor Circuit Via brushes |
| 10 | Brushes | Carbon | To provide a connection between resistance and slip rings |
| 11 | Shaft | M.S | supports rotor |
| Sr. No. | Item | Squirrel Cage Motor | Slip ring Motor |
|---|---|---|---|
| 1 | Torque | Starting torque is poor but running torque is better | Starting torque is higher but running torque is also good |
| 2 | Slip | Less | More |
| 3 | Speed Control | Nearly Constant but decreases with load | More decreases in speed with load |
| 4 | Speed Control | Speed can be varied by pole changing method | More decrease in speed with load |
| 5 | Current | High starting current (5 to 6 times FL current) | Starting current is about twice the FL Current |
| 6 | P.F | Low p.f 0.6 to 0.8 | High P.f (between 0.8 to 0.9) |
| 7 | Capacity | Low | High |
| 8 | Maintanace | Very Low | High because of brushes, slip rings, brush gears, etc. |
| 9 | Uses | Used in the workshop for lathe machines, drills, grinding machines, printing machines etc. | Used in cranes, lifts, and locomotives where high starting torque is required |