star delta starter.docx
Post on 04-Jun-2018
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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
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