Star-delta motor starter explained in details

Star-delta motor starter explained in details
Introduction to Star-Delta motor starter

Most induction motors ar started directly online, however once terribly giant motors ar started that manner, they cause a disturbance of voltage on the availability lines thanks to giant starting current surges.

To limit the beginning current surge, giant induction motors ar started at reduced voltage and so have fulls 2 strategies used for the reduction of beginning voltage are: Star delta

starting and Autotransformer starting.

supply voltage reconnected after they run up to close turned speed.

Two strategies used for the reduction of beginning voltage are Star delta beginning and automotive vehicle electrical device beginning.

Working Principle of Star-Delta Starter

This is the reduced voltage starting method.

Voltage reduction throughout the star-delta beginning is achieved by physically reconfiguring the motor windings as illustrated within the figure below.

During the beginning, the motor windings ar connected in a star configuration, and this reduces the voltage across every winding three.

This additionally reduces the torsion by an element of 3.

After an amount of your time, the winding is reconfigured as delta and also the motor runs unremarkably.

Star/Delta starters are most likely the foremost common reduced voltage starters.

They are employed in a shot to scale back begin the beginning} current applied to the motor throughout start as a way of reducing the disturbances and interference on the electrical provide.

Traditionally in many supply regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW).

The Star/Delta (or Wye/Delta) starter is one in all rock bottom price mechanical device reduced voltage starters that may be applied.


The Star/Delta starter is factory-made from 3 contactors, a timer, and a thermal overload.

The contactors are smaller than solely the one} contactor employed in an immediate on Line starter as they’re dominant winding currents only.

The currents through the winding are 1/root three (58%) of the present within the line.

There are two contactors that are close during a run, often referred to as the main contractor and the delta contactor.

These area unit AC3 rated at fifty-eight of this rating of the motor.

The third contractor is that the star contactor which solely carries star current whereas the motor is connected in star.

The current in the star is one-third of this in the delta, so this contactor can be AC3 rated at one third (33%) of the motor rating.

Star-delta Starter Consists following units

1. Contactors (Main, star, and delta contactors) 3 No’s (For Open State Starter) or 4 No’s (Close Transient Starter).
2. Time relay (pull-in delayed) 1 No.
3. Three-pole thermal overcurrent release 1 No.
4. Fuse elements or automatic cut-outs for the main circuit 3 Nos.
5. Fuse element or automatic cut-out for the control circuit 1No.


Power Circuit of Star Delta Starter

The main fuse is the most power provide a switch that provides electricity to the facility circuit.

The main contactor connects the reference supply voltage R, Y, B to the primary terminal of the motor U1, V1, W1.

In operation, the Main Contactor (KM3) and the Star Contactor (KM1) are closed initially, and then after a period of time, the star contactor is opened, and then the delta

the contactor (KM2) is closed.

The management of the contactors is by the timer (K1T) designed into the starter.

The Star and Delta area unit electrically interlocked and ideally automatically interlocked moreover.

Advantages of Star-Delta starter

• The operation of the star-delta method is simple and rugged
• It is relatively cheap compared to other reduced voltage methods.
• Good Torque/Current Performance.
• It draws 2 times starting current of the full load ampere of the motor connected

Disadvantages of Star-Delta starter

1. Low Starting Torque (Torque = (Square of Voltage) is also reduced).
2. Break-In Supply – Possible Transients
3. Six Terminal Motor Required (Delta Connected).
4. It requires 2 sets of cables from starter to motor.
5. It provides only 33% starting torque and if the load connected to the subject motor requires higher starting torque at the time of starting than very heavy transients and stresses square measure created whereas dynamic from star to delta connections, and because of these transients and stresses many electrical and mechanical break-down occurs.

6. In this method of starting initially motor is connected in star and then after the change over the motor is connected in delta. the delta of the motor is created in the starter and not on motor terminals.

7. High transmission and current peaks: When starting up pumps and fans, for example, the load torque is low at the beginning of the start and increases with the square of the speed. When reaching approx.80-85 you look after the motor rated speed the load force is adequate the motor force and also the acceleration ceases.

   To reach the rated speed, a switch over to delta position is necessary, and this will very often result in high transmission and current peaks.

   In some cases, the present peak will reach a worth that’s even larger than for a D.O.L start.

8. Applications with a load torque higher than 50 % of the motor rated torque will not be able to start using the start-            delta starter.

9. Low Starting Torque: The star-delta (wye-delta) starting method controls whether the lead connections from the motor are configured in a star or delta electrical connection. The initial connection should be in the star pattern that results in a reduction of the line voltage by a factor of 1/√3 (57.7%) to the motor and the current is reduced to 1/3 of the present at full voltage, but the starting torque is also reduced 1/3 to 1/5 of the DOL starting torque.

10. The transition from star to delta transition usually occurs once nominal speed is reached, but is sometimes performed as low as 50% of nominal speed which makes transient Sparks.

Features of star-delta starting

1. For low- to high-power three-phase motors.
2. Reduced starting current
3. Six connection cables
4. Reduced starting torque
5. Current peak on the changeover from star to delta
6. Mechanical load on the changeover from star to delta

Application of Star-Delta Starter

The star-delta methodology is sometimes solely applied to low to medium voltage and lightweight beginning force motors.

The received starting current is about 30 % of the starting current during direct on line start and the starting torque is reduced to about 25 % of the torque available at a D.O.L start.

This beginning methodology solely works once the applying is light-weight loaded throughout the beginning.

If the motor is simply too heavily loaded, there’ll not be enough force to accelerate the motor up to hurry before switch over to the delta position.


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