Complete solution required for assignment based on Motor protection Describe the similarities and differences between the three different types of polyphase AC motors. (10 marks) Describe five (5)...

Solution 1:
The three motor which we compare here are
1. Squi
el cage induction motor
2. Wound rotor induction motor
3. Synchronous motor
Similarities:
a) They work at three phase Ac power
) There are three alternating cu
ent at same frequency and amplitude but at a phase shift of
120◦
c) Torque is produced by interaction of rotor magnetic field with stator magnetic field
Difference:
a) Synchronous machine has a stationary magnetic field with respect to rotor. This field is
generated by permanent magnets or field winding powered through slip ring. Whereas, in
Induction motor the magnetic field is rotating with respect to the rotor.
a. In Squi
el cage motor field is created by motion stator field relative to short circuit cage
. In wound rotor there are rotor winding similar to synchronous machine in which cu
ent
is induced by rotating stator field.
) Synchronous motor always rotates at synchronous speed. Whereas, Induction motor rotates at
a speed less than synchronous i.e. at slip.
c) Synchronous motor cannot start by itself it requires external variable frequency AC source or
auxiliary motor to speed up the motor to synchronous speed. Induction motor is self starting as
it is able to produce a small torque at zero speed.
d) Synchronous machine requires a control to keep rotor speed locked at line frequency. Induction
motor does not require control.
e) Synchronous motor is more efficient than induction motor.
f) It is more expensive that squi
el cage induction motor.
Solution 2:
Advantage of 3 phase motor over single phase
a) For a given frame size the output of a poly phase motor is higher than single phase motor. For
example 3 phase motor has output 1.5 times that of single phase motor
) 3 Phase motor has uniform torque where as torque of a single phase motor is pulsating.
c) Power factor of 3 phase motor is higher than single phase motor of same rating
d) 3 phase motor is a 3 phase 4 wire connection where as if we apply 3 single phase motor for
same operation then we require 6 conductors. Hence increasing the cost
e) 3 phase motor are self starting where as to start a single phase motor we need to put a special
starting winding.
Solution 3:
Starting Torque: It is minimum torque that is developed when motor is at rest, with rated voltage and
frequency.
Pull Up Torque: It is the minimum torque that is produced during the acceleration period of motor from
est to the speed at which
eak down occur.
Breakdown Torque: It is the maximum torque that is developed by motor at rated voltage without
a
upt fall in speed.
Rated load torque: It is the torque of motor necessary to produce rated horse power at rated load
speed.
Solution 4:
The three major components of DC motor are:
i) Motor Housing Assembly
ii) Armature Assembly
iii) Endcap Assembly
Motor Housing Assembly: It is stator of the DC motor.
a) Metal housing: It is used to place magnets, bearing and fixed end caps. It is also
esponsible for producing magnetic field using magnets.
) Bearing: It is used to support the shaft. This shaft is a part of armature assembly. Bearing
also provides lu
ication for movement of shaft which very less friction.
c) Magnet: It is the main component to produce magnetic field which produces...