The Star-Delta 3-phase Motor Starting Method

The Star Delta starting method is a motor starting mechanism that minimizes the large amount of starting current that motors draw in. The Star Delta, as the name suggests basically involves feeding the motor with 1/sq.root3 (58%) of the full load current until it attains speed then applying the full load current. This method is commonly referred to as "Soft Starting" the motor, For this to work the whole set-up requires 3 contactor i.e The Star Contactor, The Delta Contactor & The Main Contactor.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.

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.
Advantages of this method is low or reduced cost as compared to other methods...


The Concept
For me to be able to explain this method properly we shall need to digrams  for the Star Delta:
            (i)Power Circuit Diagram
            (ii)Control Circuit Diagram

      Power Circuit Diagram

The Power Circuit enables us to see the flow of power and interconnection of phases to and fro the motor.

Initial States:

KM3 (Main Contactor)-NC
KM2 (Delta Contactor)-NO
KM1(Star Contactor)-NC

When the power is fed into the circuit, KM3 allows current to flow to the motor since it is NC (Normally Closed).Current flows into the motor and out to the KM1 which is the star-connected starter.After a specified period defined by the clock delay (Usually 5sec) the KM2 (Delta) Closes and KM1 opens to allow the motor to receive the full load current and run at delta.

      Control Circuit Diagram

 The Control Circuit enables us to see the connection of the starters that are going to start the motors.For small power motors, the starters are fed with Single-Phase (120 or 250V) but as the power rating of the motors increases, 3phase Starters are required.

By using the control circuit diagram one can easily connect the terminals of the contactor easily.

An Example

I will take a motor with some characteristics then we shall examine its values of current & voltage in star and Delta.
Initial Conditions:
         Power Rating: 120kW
         No.of Poles: 4
         Voltage: 380V
         Connection(Terminal Box): Delta
         3phase 50Hz

Running Conditions (Delta):

         Power Intake: 120kW
         Voltage: 380V
         Current: 210A
         Efficiency: 93.9
         p.f: 0.87
         Speed: 1491rpm
         Torque: 769Nm

Starting Conditions (Star):

         Power Intake: 120kW
         Voltage: 380V
         Current: 212A
         Efficiency: 93.9
         p.f: 0.86
         Speed: 1469rpm
         Torque: 780Nm