Advantage of having a star - delta connected motor starter?

Whichever way it is connected, no matter whether it is in a star configuration or in a delta configuration, a 3-phase motor's start-up current can be more than 4 times its normal running current.

If the star configuration is used when first switching-on power to a 3-phase motor, a much smaller "start-up surge" is forced onto the power lines than if it were switched-on directly in the delta configuration.

So "using star for start-up" achieves very worthwhile purchase cost savings because smaller circuit breakers and thinner 3-phase line wire sizes can be installed to supply power to the motor.

However, leaving it running in star has a major disadvantage: the motor can never deliver as much power and torque as when it is running in delta.

For that reason a 3-phase motor was usually started in star mode and then - after reaching a steady speed - switched over to run in delta mode to achieve its maximum power output.

The explanation for this is easier to understand if you draw a sketch of the wirings and their connections, but unfortunately we cannot use diagrams when giving an answer here! Anyway, if you draw the circuit diagram for the windings connected in a "star" or "Y" configuration, it should look like a three-pointed star, with a phase input power line attached to each point of the star.

Thus, when a 3-phase motor's three windings are connected in a star configuration, the current from each individual phase power input line goes directly into one winding and is then series-connected to both of the other two windings via the star's "center-point".

If you draw the circuit diagram for a delta configuration, it should look like a triangle with a phase power input line attached to each point of the triangle.

Thus, when a 3-phase motor's three windings are connected in a delta configuration, each winding is effectively connected directly to two phase supply lines. The third phase supply line is also connected to that winding, but indirectly via the other two windings. They are connected in series to one another, and that series pair is connected in parallel across the first winding, to form the "delta".

The much lower starting current is the main reason why a three-phase motor was usually started in star mode and then - after gaining a steady speed - was switched over to run in delta mode to achieve its maximum power output.

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.