07-09-2013, 03:40 PM
Induction Motors
Induction Motors.ppt (Size: 3.32 MB / Downloads: 78)
Introduction
Three-phase induction motors are the most common and frequently encountered machines in industry
simple design, rugged, low-price, easy maintenance
wide range of power ratings: fractional horsepower to 10 MW
run essentially as constant speed from no-load to full load
Its speed depends on the frequency of the power source
not easy to have variable speed control
requires a variable-frequency power-electronic drive for optimal speed control
Construction
An induction motor has two main parts
a stationary stator
consisting of a steel frame that supports a hollow, cylindrical core
core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding
Rotating Magnetic Field
Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source
A rotating magnetic field with constant magnitude is produced, rotating with a speed
Where fe is the supply frequency and
P is the no. of poles and nsync is called the synchronous speed in rpm (revolutions per minute)
Principle of operation
This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings
Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings
The rotor current produces another magnetic field
A torque is produced as a result of the interaction of those two magnetic fields
Where ind is the induced torque and BR and BS are the magnetic flux densities of the rotor and the stator respectively
Induction Motors and Transformers
Both IM and transformer works on the principle of induced voltage
Transformer: voltage applied to the primary windings produce an induced voltage in the secondary windings
Induction motor: voltage applied to the stator windings produce an induced voltage in the rotor windings
The difference is that, in the case of the induction motor, the secondary windings can move
Due to the rotation of the rotor (the secondary winding of the IM), the induced voltage in it does not have the same frequency of the stator (the primary) voltage
Torque
While the input to the induction motor is electrical power, its output is mechanical power and for that we should know some terms and quantities related to mechanical power
Any mechanical load applied to the motor shaft will introduce a Torque on the motor shaft. This torque is related to the motor output power and the rotor speed