8/13/2019 Star Delta Starter.docx

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explanation for this is easier to understand if you draw a sketch of the wirings and theirconnections, 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 isthen 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 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 outputtorque is only 1/3 of the total torque it can produce when running in DELTA. For more

information please see produce when running in DELTA. For more information please see the

answers to the Related Questions shown below.

it is connected directly across two incoming power service lines

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