Page 123 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Repulsion start motor
In this motor the brushes are advanced by 20 degrees from the direction of the main field. When 230V AC is
applied to the stator then 25-30V are reduced in the rotor windings. The brushes are short circuited so that a
current may flow in the rotor windings which may energise the rotor poles since the stator and rotor poles have
the same polarities, therefore the rotor will start to rotate. The wire used for short circuiting brushes should have
a low resistance. As the motor speed up the torque will increase. Initially the motor has a low torque and draws
small current in the start. Different speeds can be obtained by advancing or retarding the brushes. If the brushes
are advanced further, then the motor speed will increase and vice versa. The speed will be zero if the brushes are
adjusted in the neutral axis. For an advance angle of 20 degrees the motor speed will be maximum but in opposite
direction.The motor is suitable for lifting heavy loads crane, host etc

Repulsion start-run motor
The motor start in the same manner as a repulsion start motor starts. But as the rotor attains nearly 70% speed
of its full speed value a copper ring short circuits all the segments of the commutator. The copper ring is thrown
over the commutator by the centrifugal mechanism. Also the brushes are lifted above the commutator due to
same mechanism, otherwise the brushes will continue to cause sparking. After short circuiting the commutator
segment the will start to work as an induction motor. In start the motor having high torque with low speed but now
it has low torque and high speed.

Repulsion induction motor
In this motor the stator winding is similar to that of an induction motor of a repulsion start motor, but the rotor
has two separate windings. The inner winding of squirrel cage type motor winding while the outer winding is of
repulsion start rotor type, which is connected to the commutator. In start the motor works as an ordinary repulsion
motor and cage winding has no effect owing its high inductance.
The motor a high efficiency high starting torque and power factor up to 0.9. As a disadvantage there is more
sparking at the commutator and the motor will require more maintenance. It is suitable for driving workshop
machinery.
Stepper Motors
Industrial motors convert electric energy into mechanical energy. They cannot be used for precision positioning of
an object or precision control of speed without using closed-loop feedback. Stepping motors are ideally suited for
situations where either precise positioning or precise speed control or both are required in automation systems.
This motor rotates through a fixed angular step in response to each input currentpulse received by its controller.
They can be controlled directly by computers, microprocessors and programmable controllers.
They can be controlled directly by computers, microprocessors and programmable controllers.They can be
controlled directly by computers, microprocessors and programmable controllers.

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