Tuesday, April 28, 2009

Star/Delta Starter

The Star/Delta starter is probably the most commonly used reduced voltage starter, but in a large number of applications, the performance achieved is less than ideal, and in some cases, the damage and interference is much worse than that caused by a Direct On Line starter.

The Star/Delta starter requires a six terminal motor that is delta connected at the supply voltage. The Star Delta starter employs three contactors to initially start the motor in a star connection, then after a period of time, to reconnect the motor to the supply in a delta connection. While in the star connection, the voltage across each winding is reduced by a factor of the square root of 3. This results in a start current reduction to one third of the DOL start current and a start torque reduction to one third of the DOL start torque. If there is insufficient torque available while connected in star, the motor can only accelerate to partial speed. When the timer operates, the motor is disconnected from the supply and then reconnected in Delta resulting in full voltage start currents and torque.





The transition from star connection to Delta connection requires that the current flow through the motor is interrupted. This is termed "Open Transition Switching" and with an induction motor operating at partial speed (or Full load speed), there is a large current and torque transient produced at the point of reconnection. This transient is far worse than any produced by the DOL starter and causes sever damage to equipment and the supply.

If there is insufficient torque produced by the motor in star, there is no way to accelerate the load to full speed without switching to delta and causing severe current and torque transients.

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