Page 111 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The position of the handle clearly indicates that at the start position, the contact ‘A’ is in the closed position, and at
the run position, contact ‘B’ is in the closed position, but both cannot close at the same time. The ‘ON’ push-button
needs to be held in the pushed-position till the handle is brought to the run-position. During the run-position, the
handle contact ‘B’ closes the no-volt coil circuit, and the pressure on the ‘ON’ button can be released.
In general, for small machines, the rotor resistance is air-cooled to dissipate the heat developed during starting.
For larger machines, the rotor resistance is kept in an insulating oil tank for cooling. The starter shown is intended
to start the motor only. As speed regulation through the rotor resistance needs intermediate positions, they are
specially designed and always oil-cooled.

Auto-transformer starter

Objectives: At the end of this lesson you shall be able to:
• explain the construction and operation of auto-transformer starter
• explain power circuit and control circuit of auto-transformer starter

Auto-transformer starter
By connecting series resistances reduced voltage is obtained at the motor leads. It is simple and cheap, but more
power is wasted in the external series resistances.
In auto transformer starting method the reduced voltage is obtained by taking tappings at suitable points from
a three phase auto -transformer as shown in Fig 1. The auto transformers are generally tapped at 55, 65, 75
percent points. So that the adjustment at these voltages may be made for proper starting torque requirements.
Since the contacts frequently break, large value of current acting some time quenched effectively by having the
auto-transformer coils immersed in the oil bath.
The power circuit of the auto-transformer is shown in
Fig 2a and control circuit of auto-transformer is shown in Fig 2b.
Auto-transformer starter - operation
In this type of starter reduced voltage for starting the motor is obtained from a three-phase star connected auto-
transformer. While starting, the voltage is reduced by selecting suitable tappings from the auto-transformer. Once
the motor starts rotating 75% of its synchronous speed, full line voltage is applied across the motor and the auto-
transformer is cut off from the motor circuit.

98

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