Page 106 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Manual star-delta starter

Objectives: At the end of this lesson you shall be able to:
• state the necessity of a star-delta starter for a 3-phase squirrel cage induction motor
• explain the construction, connection and working of a star-delta switch and starter
• specify the back-up rating of the fuse in the motor circuit.

Necessity of star-delta starter for 3-phase squirrel cage motor: If a 3-phase squirrel cage motor is started
directly, it takes about 5-6 times the full load current for a few seconds, and then the current reduces to normal
value once the speed accelerates to its rated value. As the motor is of rugged construction and the starting current
remains for a few seconds, the squirrel cage induction motor will not get damaged by this high starting current.
However with large capacity motors, the starting current will cause too much voltage fluctuations in the power lines
and disturb the other loads. On the other hand, if all the squirrel cage motors connected to the power lines are
started at the same time, they may momentarily overload the power lines, transformers and even the alternators.
Because of these reasons, the applied voltage to the squirrel cage motor needs to be reduced during the starting
periods, and regular supply could be given when the motor picks up its speed.
Following are the methods of reducing the applied voltage to the squirrel cage motor at the start.
• Star-delta switch or starter
• Auto-transformer starter

• Step-down transformer starter
Star-delta starter: A star-delta switch is a simple arrangement of a cam switch which does not have any additional
protective devices like overload or under-voltage relay except fuse protection through circuit fuses, whereas the
star-delta starter may have overload relay and under voltage protection in addition to fuse protection. In a star-
delta switch/starter, at the time of starting, the squirrel cage motor is connected in star so that the phase voltage is
reduced to times the line voltage, and then when the motor picks up its speed, the windings are connected in
1
3
delta so that the phase voltage is the same as the line voltage. To connect a star-delta switch/starter to a 3-phase
squirrel cage motor, all the six terminals of the three-phase winding must be available.
As shown in Fig 1a, the star-delta switch connection enables the 3 windings of the squirrel cage motor to be
connected in star, and then in delta. In star position, the line supply L1, L2 and L3 are connected to the beginning
of windings U1, W1 and V1 respectively by the larger links, whereas the short links, which connect V2 U2 and W2,
are shorted by the shorting cable to form the star point. This connection is shown as a schematic diagram. (Fig 1b)

When the switch handle is changed over to delta position, the line supply L1, L2 and L3 are connected to
terminals U1 V2, W1 U2 and V1 W2 respectively by the extra large links to form a delta connection. (Fig 1c)
Manual star-delta starter: Fig 2a shows the conventional manual star-delta starter. As the insulated handle is
spring-loaded, it will come back to OFF position from any position unless and until the no-volt (hold-on ) coil is
energised. When the hold-on coil circuit is closed through the supply taken from U2 and W2, the coil is energised
and it holds the plunger, and thereby the handle is held in delta position against the spring tension by the lever
plate mechanism. When the hold-on coil is de-energised the plunger falls and operates the lever plate mechanism
so as to make the handle to be thrown to the off position due to spring tension. The handle also has a mechanism
(not shown in Fig) which makes it impossible for the operator to put the handle in delta position in the first moment.
It is only when the handle is brought to star position first, and then when the motor picks up speed, the handle is
pushed to delta position.

The handle has a set of baffles insulated from each other and also from the handle. When the handle is thrown to
star position, the baffles connect the supply lines L1, L2 and L3 to beginning of the 3-phase winding W1, V1 and
U1 respectively. At the same time the small baffles connect V2, W2 and U2 through the shorting cable to form the
star point. (Fig 1b)

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