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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.
What else can I help you with?
What will happen if induction motor is in star-star connected?
If a motor is connected in a star configuration it will operate at a slower speed. This is the working process that a star delta starter operates on. At first through the starter the motor starts in the star connection. When the motor gets up to its highest speed in that connection it times a timer out , the star starter drops out and the delta starter pulls in and then connects the motor in the delta connection. This allows more voltage to be applied to the motor and it then speeds up to its nameplate rated RPM.
Are contactor points and bimetallic strip connected to the star delta starter of 3 phase induction motor?
yes
How do star and delta configured three phase induction motors perform differently?
Star Delta starter control wiring ensures that initially the Motor winding is connected in Star and after set timer (few seconds) the winding gets connected in Delta automatically. There three power contactors and timers to make it happen.
What is a manual star-delta starter?
In manual star delta starter, the change over from star to delta does not happen automatically. The operator or user puts on the starter, and after few seconds he pushes a lever or handle to change over to Delta. In automatic method, change over happens from star to delta using an automatic preset timer.
Is a three phase high leg delta better than a wye for a motor?
There is really no advantage, the only purpose of having the variety is for motor manufacturer companies to be able to sell their product to customers if the building's electrical service is of a Delta of Wye configuration.
Where do you find the starter selenoid in a 85 delta 88?
connected to the starter
What will happen if induction motor is in star-star connected?
If a motor is connected in a star configuration it will operate at a slower speed. This is the working process that a star delta starter operates on. At first through the starter the motor starts in the star connection. When the motor gets up to its highest speed in that connection it times a timer out , the star starter drops out and the delta starter pulls in and then connects the motor in the delta connection. This allows more voltage to be applied to the motor and it then speeds up to its nameplate rated RPM.
Are contactor points and bimetallic strip connected to the star delta starter of 3 phase induction motor?
yes
How do star and delta configured three phase induction motors perform differently?
Star Delta starter control wiring ensures that initially the Motor winding is connected in Star and after set timer (few seconds) the winding gets connected in Delta automatically. There three power contactors and timers to make it happen.
What is the difference between a Y - delta starter and a star - delta starter?
No difference. Y is used as a symbol to indicate the star connection.
What is a star delta starter if a pump is connected to the motor should the outlet valve close before starting?
yes should to close few second due to air.
What is a manual star-delta starter?
In manual star delta starter, the change over from star to delta does not happen automatically. The operator or user puts on the starter, and after few seconds he pushes a lever or handle to change over to Delta. In automatic method, change over happens from star to delta using an automatic preset timer.
Is a three phase high leg delta better than a wye for a motor?
There is really no advantage, the only purpose of having the variety is for motor manufacturer companies to be able to sell their product to customers if the building's electrical service is of a Delta of Wye configuration.
What are the different types of starter?
DOL, Star Delta, Soft starter, VFD
Why star delta starter is more efficient?
The star-delta starter allows the motor to start with one third its normal power, minimizing inrush current and heating, thus extending the life of the motor and its control center. When a motor is connected in star the total output torque is only a third of the total torque that could be produced running in delta. Please see related questions and links.
What is the difference of a star or delta connection on motor terminals?
230/400 V These motors can be connected in star to a mains of 400 V between the phases (see figure 1). figure 1 Star connection 400/695 V These motors should be connected in delta to a mains of 400 V between the phases (see figure 2).
What is a Manual starter?
In manual star delta starter, the change over from star to delta does not happen automatically. The operator or user puts on the starter, and after few seconds he pushes a lever or handle to change over to Delta. In automatic method, change over happens from star to delta using an automatic preset timer.
