Induction Motors: Submitted BY P.Durga Prasad 09A25A0205
Induction Motors: Submitted BY P.Durga Prasad 09A25A0205
Induction Motors: Submitted BY P.Durga Prasad 09A25A0205
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 , having a number of evenly spaced slots, providing the space for the stator winding
Stator of IM
Construction
a revolving rotor
composed of punched laminations, stacked to create a series of rotor slots, providing space for the rotor winding one of two types of rotor windings conventional 3-phase windings made of insulated wire (wound-rotor) similar to the winding on the stator aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage)
Construction
Squirrel cage rotor
Wound rotor
Construction
Slip rings
Cutaway in a typical woundrotor IM. Notice the brushes and the slip rings
Brushes
nsync
120 f e P
rpm
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
ind kBR Bs
Where ind is the induced torque and BR and BS are the magnetic flux densities of the rotor and the stator respectively
nslip nsync nm
Where nslip= slip speed nsync= speed of the magnetic field nm = mechanical shaft speed of the motor
The Slip
s
Where s is the slip Notice that : if the rotor runs at synchronous speed s=0 if the rotor is stationary s=1 Slip may be expressed as a percentage by multiplying the above eq. by 100, notice that the slip is a ratio and doesnt have units
nsync nm nsync
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
load
Pout
N .m
and
2 nm m 60
rad / s
Torque-speed characteristics
THAN Q