Introduction of Three Phase Induction Motor
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Induction motor consist mainly two parts
The stator holds the windings which receive energy from the supply mains and the rotor also carries windings in which the current is induced.
The laminated stator core, made of thin sheet steel is slotted on its inner cylindrical surface and the winding consists of embedded coils disposed in exactly the same manner as those of a revolving field synchronous alternator designed for the same number of poles. It is wound for a definite number of poles, the exact number of poles being determined by the requirements of speed. Greater the number of poles, lesser the speed and vice-versa when the stator winding is supplied with current, produces a magnetic flux which is of constant value but revolves at synchronous speed.
P=no. of poles
= synchronous speed
Synchronous speed minus the actual speed gives the slip in the motor.
The revolving magnetic flux induces an EMF in the rotor by mutual induction.
Rotor winding is generally of the squirrel cage type, in machines of large size consists of copper bars occupying nearly closed slots on the outer periphery of the rotor core: these bars are solidly brazed to end rings, In early motors of this type, the bars were bolted to the end rings, but this construction was found to be faulty because the expansion and contraction caused by temperature changes under load condition to loosen the bolts and thereby alter the contact resistance, which though small in itself, was considerably in comparison with the small resistance of the winding as a whole. This consideration led to the construction, which is now in motors of small and moderate size, where the entire cage winding, bars and end rings together is made of aluminum cast in place of an integral unit. 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.
The difference in speed for any given load between synchronous speed and load speed is called the slip of the motor. Slip is usually expressed as the percentage of the synchronous speed. Since the amount of slip is dependent on the load, the greater the load, the slip will be, greater that is the motor will run slower.
The slip of an induction motor may be expressed in the following manner:
1.) As a percentage of synchronous speed.
2.) As a decimal fraction of synchronous speed.
3.) Directly in revolution per minute.
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