02-08-2011, 04:51 PM
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INTRODUCTION
Three-phase induction motors
An induction motor or asynchronous motor is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction. An electric motor converts electrical power to mechanical power in its rotor. In an induction motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator is essentially the primary side of the transformer and the rotor is the secondary side. Unlike the normal transformer which changes the current by using time varying flux, induction motors use rotating magnetic fields to transform the voltage. The primary side's current creates an electromagnetic field which interacts with the secondary side's electromagnetic field to produce a resultant torque, thereby transforming the electrical energy into mechanical energy.
RATINGS OF AN INDUCTION MOTOR
KILOWATT(HP) 5.5(7.5)
VOLTS 600 V
FREQUENCY 50 Hz
CURRENT 40 A
PHASE 3-Ph
RPM 1429
PARTS OF MOTOR
1. Cooling fan
2. End shield
3. Inside and outside grease cap
4. Bearing chamber
5. Rotor
6. Stator and stator winding
7. Terminal box
8. Bearings(ball bearing or roller bearing)
9. Eye ball/lifting hook
10. Winding slots and rotor slots
PRINCIPLE OF OPERATION
A 3-phase power supply provides a rotating magnetic field in an induction motor
The induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energized with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and in effect causes a rotational motion on the rotor.
The speed of the physical rotor must be less than the speed of the rotating magnetic field in the stator, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip.
There are three types of rotor:
• Squirrel-cage rotor
The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper or aluminum that span the length of the rotor, and those solid copper or aluminum strips can be shorted or connected by a ring or some time not. The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise and harmonics.
• Slip ring rotor
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to slip ring. When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor; they can also be connected to resistors to produce a high-resistance rotor circuit, which can be beneficial in starting.
STARTING OF INDUCTION MOTORS
At starting, the voltage induced in the induction motor rotor is maximum. Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current in the stator and consequently the starting torque is low. Because of the short duration this value of large current does not harm the motor if the motor accelerates normally. However this large starting current will produce large line voltage drop. This will adversely affect the operation of other electrical equipment connected to the same line. Therefore it is necessary to reduce the magnitude of stator current at starting and several methods are used. The common methods use to start induction motors are:
1. Direct-on-line starting
2. Star-Delta starting
3. Autotransformer starting
4. Stator resistance starting
5. Rotor resistance starting
