Page 101 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

When the Megger handle is rotated at its rated speed, the reading should be more than one megohm. A lower
reading than one megohm shows weak insulation between the windings, and needs to be improved. Likewise the
insulation resistance between the other windings is tested.
Insulation resistance between windings and frame: As shown in Fig 3, one terminal of the Megger is connected
to one of the phase windings, and the other terminal of the Megger is connected to the earthing terminal of the
frame. When the Megger handle is rotated at the rated speed, the reading obtained should be more than one
megohm. A lower reading than one megohm indicates poor insulation between the winding and the frame and
needs to be improved by drying and varnishing the windings.

Starter for 3-phase induction motor - power control

circuits - D.O.L starter

Objectives: At the end of this lesson you shall be able to:
• state the necessity of starters for a 3-phase induction motor and name the types of starters
• explain the basic contactor circuit with a single push-button station for start and stop.

Necessity of starter: normal voltage is applied to the stationary motor, then, a very large initial current, to the
tune of 5 to 6 times the normal current, will be drawn by the motor from the mains. This initial excessive current
is objectionable, because it will produce large line voltage drop, which in turn will affect the operation of other
electrical equipment and lights connected to the same line.
The initial rush of current is controlled by applying a reduced voltage to the stator winding during the starting
period, and then the full normal voltage is applied when the motor has run up to speed. For small capacity motors,
say up to 3 Hp, full normal voltage can be applied at the start. However, to start and stop the motor, and to protect
the motor from overload currents and low voltages, a starter is required in the motor circuit. In addition to this, the
starter may also reduce the applied voltage to the motor at the time of starting.
Types of starters: Following are the different types of starters used for starting squirrel cage induction motors.
• Direct on-line starter

• Star-delta starter
• Auto-transformer starter
In the above starters, except for the direct on-line starter, reduced voltage is applied to the stator winding of the
squirrel cage induction motor at the time of starting, and regular voltage is applied once the motor picks up the
speed.
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 (line) drop, cycle of operation, type of load,
motor protection and safety of the operator.

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