Page 104 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

connected to the same line. The initial current is controlled by applying a reduced voltage to the stator winding
during the starting winding during starting period and then the full normal voltage is applied when the motor has
run up to speed. For small capacity motors up to 3 hp full normal voltage can be applied at the start.
Compare the characteristics of a squirrel cage induction motor

S.No Characteristics Squirrel cage induction motor Slip ring induction motor
1 Speed Nearly constant but slightly falls More speed falls with load than Squirrel
with load cage induction motor
2 Torque Starting torque is not so good Starting torque higher than Squirrel cage
Running torque is good induction motor and is three times full load
torque. Running torque is good
3 Current Starting current is high, about five Starting current is low, about two times of full
times that full load current load current
4 Speed control Speed can be varied by pole It can be slightly changing the rotor
changing method resistance
5 Power factor Low power factor 0.7-.08 High power factor 0.8-.09

6 Cost Low High
7 Uses Factories for lathe , drill, Used for high starting torque is wanted e.g. :
blower,etc. lift, crane,etc.
8 Rotor construction Bars are used in rotor. Squirrel Winding wire is used. Wound rotor requires
cage rotor is very simple, rugged attention. Slip ring and brush fear need
and long lasting. No slipring. frequent maintenance.
9 Starting Cam ne started by DOL ,star Rotor resistance starter is required.
delta, auto transformer starters.
10 Starting torque Low Very high
11 Starting current High Low

12 Speed variation Not easy but could be varied in Easy to vary speed but speed change
larger steps by pole changing through pole changing is not possible.
or smaller incremental steps Speed change possible by insertion of rotor
through thyristors or by frequency resistance –using thyristors –using frequency
variation. variation –injecting emf in the rotor circuit
-cascading

13 Acceleration on Just satisfactory Very good
load

14 Maintenance Almost nil Requires frequent maintenance

Types of starters
a Direct on line starter
b Star delta starter
c Step down transformer starter

d Auto transformer
Selection of starter
Many factors must be considered when selecting starting equipment. These factors include starting current the
full load current voltage rating of motor, voltage drop, cycle of operation, type of load, motor protection and safety
of operation
Types of over load relay
a Magnetic overload relay

b Thermal (bimetallic) overload relay

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CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75