The block diagram of a star delta starter?

Answer 1

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 (1 /.'/'3) [1 divided by root three]. This results in a start-current reduction to (1 /.'/'3) [1 divided by root three] 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 a partial speed compared to the full speed it would reach if connected in delta. When the timer operates (set normally from 5-10 seconds), the motor is disconnected from the supply and then reconnected in delta, resulting in full line voltage running 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 a partial speed compared to full load speed, there is a large current and torque transient produced at the poi, unless proper protection methods are used, can cause severe damage to the supply service's infrastructire and to other connected equipment.

If there is insufficient torque produced by the motor when running in star, there is no way to accelerate the load to full speed without switching to delta and causing those severe current and torque transients. These must be allowed-for in the design of the motor and its starting system if they are to have an economic useful life.

Update: Electronic motor-control systems, which offer soft-starts in DELTA configuration, are now replacing the use of manual or semi-automatic star-delta starters.

Technical explanation

When the windings of a 3-phase motor are connected in STAR:

• the voltage applied to each winding is reduced to only (1 /.'/'3) [1 divided by root three] of the voltage applied to the winding when it is connected directly across two incoming power service lines in DELTA.
• the current per winding is reduced to only (1 /.'/'3) [1 divided by root three] of the normal running current taken when it is connected in DELTA.
• so, because of the Power Law V [in volts] x I [in amps] = P [in watts],

  the total output power when the motor is connected in STAR is:

  PS = [VL x (1/.'/'3)] x [ID x (1/.'/'3)] = PD x (1/3) [one third of the power in DELTA]

  where:

  VL is the line-to-line voltage of the incoming 3-phase power service

  ID is the line current drawn in DELTA

  PS is the total power the motor can produce when running in STAR

  PD is the total power it can produce when running in DELTA.

• a further disadvantage when the motor is connected in STAR is that its total output torque is only 1/3 of the total torque it can produce when running in DELTA.

For more information please see the answers to the Related Questions shown below.

Answer 2

input--> star configuration -->delta configuration. both configurtaion is connected to 3 phase motor coil. mail contactor is connected to both star and delta configuration. timer is connected to main contector....

Answer 3

The components consist of two three phase contactors joined on a common back plate with a mechanical interlock between them so as to prevent both contactors being able to pull in at the same time.

Answer 4

Star Delta starter is used to start the AC motors with rating generally higher than 11 kw, depending upon the type of load.