Tuesday, September 8, 2015
Induction Motor Construction:
Induction motor is the most commonly used machine in domestic purpose and industrial purpose. Induction motor consists essentially of two main parts :
a) stator and b) rotor.
Induction Motor Stator:-
1.The stator of an induction motor is almost same as that of a synchronous motor or generator. The main function of the stator core is to carry the alternating flux.
2.In order to reduce the eddy current loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. It is made up of a number of stampings, which are slotted to receive the windings.
3.The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor. The stator of induction motor carries a 3-phase winding and that is fed from a 3-phase supply.
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| Stator core with winding |
4.It is wound for a definite number of poles, the exact number of poles being detemined by the requirements of speed. Greater the number of poles, lesser the speed and vice versa.
5.When supplied to 3-phase windings, produce a magnetic flux, which is of constant magnitude but which revolves (or rotates) at synchronous speed (given by Ns=120f/P). This revolving magnetic flux also called as rotating magnetic field which induces an e.m.f. in the rotor by mutual induction.
Induction Motor Rotor:
Induction motors are available with two types of rotors basically.
i) Squirrel-cage rotor : Motors employing this type of rotor are known as squirrel-cage induction motors.
ii)Phase-wound or wound rotor : Motors employing this type of rotor are variously known as 'phase-wound' motors or 'wound' motors or as 'slip-ring' motors.
Squirrel-cage rotors :
Almost 90 percent of induction motors arc squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consist of heavy bars of copper, aluminium or alloys. One bar is placed in each slot, rather the bars are inserted from the end when semi-closed slots are used. The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us. what is so picturesquely called, a squirrel-case construction (Fig.).
It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes.
The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew.This is useful in two ways :
i) it helps to make the motor run quietly by reducing the magnetic hum and
ii)it helps in reducing the locking tendency of the rotor i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.
In small motors, another method of construction is used. It consists of placing the entire rotor core in a mould and casting all the bars and end-rings in one piece. The metal commonly used is an aluminium alloy.
Another form of rotor consists of a solid cylinder of steel without any conductors or slots at all. The .motor operation depends upon the production of eddy currents in the steel rotor.
Advantages of squirrel cage induction rotor
- Its construction is very simple and rugged.
- As there are no brushes and slip ring, these motors requires less maintenance.
Phase-wound or wound rotor :
This type of rotor is provided with 3-phase, double-layer, distributed winding consisting of coils as used in alternators. The rotor is wound for as many poles as the number of stator poles and is always wound 3-phase even when the stator is wound two-phase.
The three phases are starred internally. The other three winding terminal!: arc brought out and connected to three insulated slip-rings mounted on the shaft with brushes resting on them.
These three brushes arc further externally connected to a 3-phase star-connected rheostat. This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor, as shown in Fig. and for changing its speed-torque/current characteristics.
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| Phase-wound Induction Motor Circuit |
When running under normal conditions, the slip-rings arc automatically short-circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together. Next, the brushes are automatically lifted from the slip-rings to reduce the frictional losses and the wear and tear. Hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor.
Advantages of slip ring induction motor :
- It has high starting torque and low starting current.
- Possibility of adding additional resistance to control speed.
Application induction motor :
Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc.
*here it is the detailed information about induction motor construction and important part are explained with clean images
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