Page 309 - Electronic Mechanic - TP - Volume - 2

ELECTRONICS MECHANIC - CITS

EXERCISE 208&210: Construct a Semi automatic and fully

automatic star delta starter, RUN a 3

Phase Induction Motor and Measure
voltage and current in STAR and

DELTA connections

Objectives

At the end of this exercise you shall be able to:
• to demonstrate all parts of the AC motor.

Requirements

Tools/ Equipments/ Instruments Materials/ Components
• Combination Plier 250mm - 1 No. • Induction Motor - 1 No.
• Screwdriver Set 150mm - 1 No. • Circuit Barker - 1 No.
• Wire Stripper - 1 No. • Push Button NO/NC - 1 No.
• Overload Relay - 1 No.
• Contactor - 1 No.

Procedure

The Star Delta starting method is a motor starting mechanism that minimizes the large amount of starting current
that motors drawin. The Star Delta, as the name suggests basically involve seeding the motor with 1/√3 (58%) of
the full load current until it attains speed then applying the full load current.

It is required three contactors i.e., the Star Contactor (K3), the Delta Contactor (K4) and the Main Contactor (K1).
However for the motor to be started in Star Delta, its internal connection at the terminal box has to be wired in
Delta-giving it capability of receiving the full-load current at any instant.
When the power is fed into the circuit, K1 allows current to flow to the motor. Current flows into the motor and out
to the K3 which is the star-connected starter. After a specified period defined by the clock delay (usually 5 sec) the
K4 (Delta) Closes and K3 opens to allow the motor to receive the full load current and run at delta.
Traditionally, in many regions there was a requirement that all motor connections be fitted with a reduced voltage
starter for motors greater than 4KW (5HP).This was to curb the high inrush of starting currents associated with
starting induction motors.
The star and delta contactors are mechanically interlocked i.e., if one of them is closed the other cannot close.
This is done to avoid dead short circuit in case both the contactors closing simultaneously. Electrical interlocking
has also been provided, by using contactors control contacts.
Instruments & Equipments

Name Figure

Three phase induction motor

292