Understand Star Delta Connection and Starter

Induction motor stator coils
A three phase induction motor consists of three separate coils wound inside as shown in the image below. Each coil is wound in the stator and has two ends on each side protruding out of motor. Most Induction motors have 3 poles i.e. three coils so for three coils there are 6 ends

Two joining or adjacent coils can be found using a continuity test (the two ends that gives a value shares the same coil).

A motor can be connected to the power using star or “y” connection donated by a (Υ) or by a Delta connection donated by a (Δ)

Star connection

To connect the motor in a star connection, all the one end of the coil(U1,V1,W1) are joined together to form a common while the other ends (U2,V2, W2) are connected to a 3 phase supply respectively without.

The common connection at U1, V1 and W1 can be used as a neutral point. i.e.

Line voltage= √3 Phase voltage

Where

Line voltage is V (u2-v2) = V (u2-w2) =V (v2-w2)

Phase voltage is V (u2-v2) = V (u2-w2) =V (v2-w2)

Line Current= Phase Current

Where

Line current is the current going into I (u2) =I (v2) =I ( w2)

Phase current is the current passing through the coil I (u1-u2) =I (v1-v2) =I (w1- w2)

For low torque motors, usually, motors are given by Direct on line (D.O.L) method while coil is connected in star connection because they provide high starting speed at low currents

Delta connection

To form a delta connection all three connections are joined together in series end to end (u1 is connected with v2, v1 is connected with w2 and w1 is connected with u2) as shown in the figure. The joining ends (U2, V2, and W2) is connected to a 3 phase supply voltage.

In delta connections

Line voltage= Phase voltage

Where

Line voltage is V (u2-v2) = V (u2-w2) =V (v2-w2)

Phase voltage is V (u2-v2) = V (u2-w2) =V (v2-w2)

Line Current= √3 Phase Current

Where

Line current is the current going into I (u2) =I (v2) =I (w2)

Phase current is the current passing through the coil I (u1-u2) =I (v1-v2) =I (w1- w2)

Due to High starting current motors are not connected directly onto Direct on line (D.O.L) connection. Instead they are given power either by a star delta starter (which automatically connects the coil first in start connection then in delta connection) or connected in delta connection and then given power through soft starters or VFDs

Star/Delta motor starter

If the output torque of the motor, when it is wired in star connection is sufficient for the job then it can be wired in star. If the torque is insufficient, then it has to be wired in delta connection. Normally when the output torque required is so great then the startup current causes an overcurrent in such cases we have to use a star/delta starter.
This starter allows the motor to be started in star, and when the motor is running at ideally 75-80% of the delta speed, contactors change the wiring configuration of the motor from star to delta.
Star connection motors takes approximately 1 third the starting current and starting torque as compared to motors connected directly on delta as it reduce the chance of over load ripping of the circuit.

Star delta circuit operates in three stages

Stage 1: (OFF) there is no voltage across the motor.

Stage 2: (STAR) The motor in star connection

Stage 2: (DELTA) The motor is connected in delta connection.

It consist of 3 contactors, 1 timer, Main breaker and overload.

• main breaker and overload for motor and wiring protection
• Contactor (K1) for delivering the main power to one set of coil ends(u2, v2, w2)
• Contactor (K2) for making the star connection on second set of coil ends, (i.e. it connects u1 with v1 and w1)
• contactor(K3) for making the delta connection on second set of coil ends( i.e. it connects u1 with v2, v1 to w2, w1 to u2)
• timer (T1) to control the time after which star will be disconnected and motor will run at delta connection.(time depends on the time required for the motor to reach the speed in star connection to that of 80-90% of the desired speed of motor in delta connection)

When the operator pushes the start button, the contactor (K1) and contactor (K2) turns on and latches, connecting the motor winding with the power in star connection and in the meantime starting off the timer(T1). When the timer’s goes off, it disconnects the star connection and almost immediately connects the wiring in delta connection.