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| Fig: Three Phase Motor connection diagram with a star-delta starter. |
What is Star Delta Starter for Motor Starting?
Star-Delta starter is an electrical motor starting device, generally uses in big size motors to overcome some technical limitations. Star and Delta mean here 2 separate states of motor running, first Star connection and then Delta connection.
The starting current of any heavy
electric motor can be more than 4 times the normal load current it draws when
it has gained speed and has reached its normal running condition.
To overcome this initial high current enchanting problem, such arrangement needs Star connection at starting time and if Star connection has sufficient torque to run up to 75% to %80 of full load speed, then the motor can be connected in Delta mode.
To overcome this initial high current enchanting problem, such arrangement needs Star connection at starting time and if Star connection has sufficient torque to run up to 75% to %80 of full load speed, then the motor can be connected in Delta mode.
When the motor is connected to the Delta
configuration the phase voltage increases by 173% and phase currents increase
by the same ratio. The line current increases three times its value in star
connection.
What Problem Starting Motor without Star-Delta?
We already know, if the motor starts
simply when connected in Delta, the starting current would be huge just to
be able to start the motor, not to running condition. To do this would
require -
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v Large capacity circuit breakers to allow the start-up surge
current to pass without immediately shutting the motor off.
v Oversized 3-phase power service cables require just for
starting time, but the normal running time is not necessary.
v Large size of coils and contacts on the relays or contractors need to control the motor, but normal running time needs smaller sizes of them.
Sometimes, utility company also do not
permits the big motor to start without a Star-Delta starter because of system
instability due to switching transients.
How Wiring a Star-Delta Starter for Motor?
Following the figure below, the power connection between magnetic contact for Main, Star, Delta and motor is shown in the schematic line diagram and wiring diagram for easy wiring.
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| Star-Delta Starter Connection with Control wiring |
What is Important During Star-Delta Transition Period?
It is important that the break between the Star contractor switch OFF
and the Delta contactor switch is ON because the Star contactor must be reliably
quenched before the Delta contactor is activated. During the transition period of switch-over, the motor must be free running with little deceleration. It is also
important that the switch over pause is not too long, it may generate a voltage
of its own and this may add to or subtract from the applied line voltage.
That’s all for the Star-Delta starter for today; here in this article, we discussed only motor starting by 3 magnetic contracts Star-Delta, solid-state motor starters from different companies also are available in the market now.
