What happens if you change over too soon from star to delta?

Answer 1

The Star/Delta starter was 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.

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

The Star/Delta starter requires a six terminal motor that is normally delta-connected direct to the 3-phase service lines. 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.

If there is insufficient torque available while connected in star, the motor can only accelerate to partial speed. 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 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 can cause severe 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 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.

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 line phases 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 = (1 /.'/'3 VL) x (1 /.'/'3 ID) = 1/3 [one third] PD

  where VL is the phase line voltage

  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.