In which case we should use dol starter&star delta strter?

Question

In which case we should use dol starter&star delta strter?

Answer 1

A direct on line starter, often abbreviated DOL starter, is a widely-used starting method of electric motors. The term is used in electrical engineering and associated with electric motors. There are many types of motor starters, the simplest of which is the DOL starter.

A DOL starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. This type of starting is suitable for small motors below 5 hp (3.75 kW). Reduced-voltage starters are employed with motors above 5 hp.

Star/delta starter :

The Star Delta starter can only be used with a motor which is rated for connection in delta operation at the required line voltage, and has both ends each of the three windings available individually. At Start, the line voltage is applied to one end of each of the three windings, with the other end bridged together, effectively connecting the windings in a star connection. Under this connection, the voltage across each winding is 1/(rt 3) of line voltage and so the current flowing in each winding is also reduced by this amount. The resultant current flowing from the supply is reduced by a factor of 1/3 as is the torque. i.e. A motor which exhibits a LRC of 600% and an LRT of 180% will exhibit characteristics of: LRCstar of 200% and LRTstar of 60%. In some cases, this may be enough to get the motor up to full speed, but most, as this is a constant voltage starter, the transition to full voltage will occur at part speed resulting in a virtual DOL type start. To step to full voltage, the star connection is opened, effectively open circuiting the motor, and the ends of the windings are then connected to the three phase supply in a fashion to create a delta connection. This type of starter is an open transition starter and so the switch to delta is accompanied by a very high torque and current transient. In most situations, there would be less damage to the equipment and less interference to the supply if a DOL starter was employed.

The star delta is not easily converted to a closed transition starter, and even the closed transition (Wanchop) star delta starter still has the problem that the start voltage can not be altered. If there is insufficient torque available in star, then it will go DOL. The star delta starter does get around the regulations in some countries where there is a requirement for a reduced voltage starter, but in reality, in many situations results in more severe transients than DOL. The main benefits of the star delta starter are that it puts more money in the pockets of the switchgear supplier, and it is politically correct.