Page 109 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Automatic star-delta star delta starter
Applications: The primary applications of star-delta motors are for driving centrifugal chillers of large central air
conditioning units for loads such as fans, blower, pumps, and for situations where a reduced starting toque is
necessary. A star delta motor is also used where a reduced starting torque is necessary. These are widely used
on loads having high inertia and a long acceleration period. Three overload relays are provided on star delta
starters. These relays are use so that they carry the motor winding current. This means that the relay units must
be selected on the basis of the winding current, and not the delta connected full load current.
Operation: Pressing the start button S-energises the star contactor K3.once K3 energises the K3 NO contact
closes and provide path for the current to close the contactor K1. The closing of contactor K1 establish parallel
path to start button via K1 NO terminals 23 & 24. The time delay contact changes opening star contact. Co

Three phase slip induction motor - wound rotor

The slip ring induction motor could be used for industrial drives where variable speed and high starting torque are
prime requirements. The construction of rotor is very much different.
Working: When the stator winding of the slip ring motor is connected to the 3-phase supply it produces a rotating
magnetic field in the same way as squirrel cage motor. This rotating magnetic field induces voltages in the rotor
windings and a rotor current will flow through the closed circuit. The rotor current will flow through the closed
circuit formed by the rotor winding, the slip-rings, the brushes and the star connected external resistors. At the
time of starting the external resistors are set for their maximum value. As such the rotor resistance is high enabling
the starting current to be low. At the same time, the high resistance rotor circuit increases the rotor power factor
and thereby the toque developed at the start becomes much higher than the toque developed in squirrel cage
motors. As the speed up the external resistance is slowly reduced, rotor resistance the motor operates with low
slip and high operating efficiency. The motor could be started for heavy loads with higher resistance or vice versa.
The resistance in the external circuit could be designed and varied to change speed of the slip ring motor between
50to 100 percentage of the rated speed.

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