Page 107 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

When the handle is thrown to delta position, the larger end of the baffles connect the main supply line L1, L2 and
L3 to the winding terminals W1U2, V1W2 and U1V2 respectively to form the delta connection. (Fig 1c)
The overload relay current setting could be adjusted by the worm gear mechanism of the insulated rod. When the
load current exceeds a stipulated value, the heat developed in the relay heater element pushes the rod to open
the hold-on coil circuit, and thereby the coil is de-energised, and the handle returns to the off position due to the
spring tension.
The motor also could be stopped by operating the stop button which in turn de-energises the hold-on coil.
Back-up fuse protection: Fuse protection is necessary in the star-delta started motor circuit against short circuits.
In general, as a thumb rule for 415V, 3-phase squirrel cage motors, the full load current can be taken as 1.5 times
the H.P. rating. For example, a 10 HP 3-phase 415V motor will have approximately 15 amps as its full load current.
To avoid frequent blowing of the fuse and at the same time for proper protection, the fuse wire rating should be 1.5
times the full load current rating of the motor. Hence for 10 HP, 15 amps motor, the fuse rating will be 23 amps,
or say 25 amps.

Construction and operation
A push button type direct on line starter which is in common use. It is a simple starter which is expensive and easy
to install and maintain.

It consists of a contractor, in most cases the no volt coil is rated for 415v and is to be connected across two
phases. Further overloads relay can be situated between ICTP switch and contactor or between the contactor and
motor, depending upon the starter design

Specification of D.O.L. starters
Phases –single or three
Voltage- 230or415
Current rating 10,16,32,40,63,125, or 300amps.

No of main contacts 2, 3, 1NC+1NO or 2NO+2NC
Over load from setting –amp-to-amp. Enclosure –metal sheet or PVC
Applications
In an induction motor with a DOL starter the starting current will be about 6 to 7 times the full load current. These
are recommended to be used only up to 3 HP squirrel cage induction motors and up to 1.5 kw double cage rotor
motors.
Semi-automatic star delta starter

The squirrel cage induction motors with both ends of each of the three windings brought out are known as star
delta motors. The use of manual star delta starter demands a special skill in handling the starter.
Figure shows the wiring diagram of Semi-automatic star delta starter

94

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