2. Three phase induction motor
Week 4
2.2 The principle of operation of the Induction Motor:
The three-phase current with which the motor is supplied establishes a rotating magnetic
field in the stator. This rotating magnetic field cuts the conductors in the rotor inducing
voltages and causing currents to flow. These currents set up an opposite polarity field in the
rotor(Lenz's law). The attraction between these opposite stator and rotor fields produces the
torque which causes the rotor to rotate. This simply is how the squirrel-cage motor works
Figure2.8:The magnetic field created in the stator and the rooted in the squirrel cage induction motor
2.3 The Slip:
There must be a relative difference in speed between the rotor and the rotating magnetic
field. If the rotor and the rotating magnetic field were turning at the same speed no relative
motion would exist between the two, therefore no lines of flux would be cut, and no voltage
would be induced in the rotor. The difference in speed is called slip. Slip is necessary to
produce torque. Slip is dependent on load. An increase in load will cause the rotor to slow
down or increase slip. A decrease in load will cause the rotor to speed up or decrease slip.
Slip is expressed as a percentage and can be determined with the following formula.
Slip (%) 
( Ns  Nr ) 100
Ns
1
2. Three phase induction motor
Week 4
Where Nr is the actual speed of the rotor
For Example, a four-pole motor operated at 60Hz has a synchronous speed (Ns) of
1800 RPM. If the rotor speed at full load is 1765 RPM (Nr), then the slip will be
calculated
Slip (%) 
(1800  1765) 100
 1.9%
1800
2.4 Name Plate:
It is essential that all motors have nameplates with certain information useful in the
identification of the type of motor, The following Table explain the indication of each code
used on the shown nameplate
Name Of Manufacturer
ORD. No.
IN123456789
HIGH EFFICIENCY
FRAME
286T
H.P.
42
SERVICE
FACTOR
1.10
3PH
AMPS
40
VOLTS
415
Y
R.P.M
1790
HERTZ
60
4POLE
DUTY
CONT
DATE
01/15/2003
TYPE
CLASS INSUL.
F
NEMA
Design
B
NEMA
NOM. EFF.
95
Address Of Manufacturer
Figure2.9:Typical Nameplate of an AC induction motor
Term
Description
Volts
Rated Supply voltage
HP
Rated motor output
Amps
Rated full load current
RPM
Rated full load speed of the motor
2
2. Three phase induction motor
Week 4
Hertz
Rated supply frequency
Frame
External dimensions based on NEMA Regulations
Duty
Motor load condition ,either its continuities load, short time,etc
Date
Date of manufacturing
Class Insulation
Specifies the max. limit temperature of the winding
NEMA Design
Types of NEMA design, A,B,C etc
Service factor
Factor by which the motor can be overloaded beyond the full load.
NEMA Nom
Efficiency
Motor efficiency at rated load
PH
Number of phases
Pole
Number of poles
Motor safety standard
Y
The connection either star or delta
Table 2.1:Explanation of the codes used on AC motor nameplates
3