The induction motor is the most common type of electric motor in the industry. If the induction motor starts in a direct online method, the motor is started with the application of full voltage and the starting current will be 7-10 times of rated current. So large induction motors do not start directly. They need something else. They need a star-delta starter (Wye-Delta Starter). This article describes the fundamentals of a star delta starter. Definition, the working principle, and connection diagrams. All you need to know!
To discover more, keep reading!
What is Star Delta Starter?
Star delta starter is a starting method that reduces the starting current and starting torque of an electrical motor. Star delta starter design normally consists of three contactors, an overload relay or circuit breaker, and a timer for setting the time in the star position.
For the star delta starter, a motor must be in delta connected during a normal run and the main purpose is to be able to use the star-delta starter.
In the star delta starter the received starting current is about 33 % of the starting current during direct online start and the starting torque is reduced to about 33 % of the torque available at a direct online starter.
Star delta starter is used to reduce the starting currents when the application is lightly loaded during the start. If the motor is too heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position.
Star Delta Starter Working Principle
With a star delta motor start, the start-up of the three-phase asynchronous motor is implemented by a changeover of the windings. The jumpers in the motor terminal box are omitted, and all 6 winding connections are connected to the mains supply using a so-called star-delta switch (manually actuated switch or automatic contactor circuit).
During the operating connection, the windings of the motor are connected in the delta. The winding voltage (UW) must therefore be equal to the phase voltage (ULN) of the three-phase system. For example, at a mains supply voltage of 3 AC 400 V, the voltage ratings on the rating plate of the motor must be specified as 400/690 V.
In a star connection, the mains voltage (ULN) on the individual motor windings is reduced by a factor of 0.58. For example, 400 V x 0,58 = 230 V. Starting torque and inrush current is (in the star connection) reduced to about a third of the values for the delta connection.
The available starting current is 33% of the full load current. The peak starting characteristics are about 1.3 to 2.6 full load current. Peak starting torque is about 33% of full load torque.
- The start button is pressed.
- The coils of K1 and K3 contactors and the coil of the T1 time relay are energized.
- Main contacts of K1 and K3 contactors close. And the time relay starts to count. Now the motor works on star mode.
- After a while (1…10 seconds) relay stops counting and changes its T1 auxiliary contacts
- Then K3 contactor opens and the K4 contactor closes.
- The motor starts to work in delta mode.
(Check the diagram below)
The star and delta contactors are mechanically interlocked. If one of them is closed the other cannot close. This is done to avoid a short circuit. Electrical interlocking has also been provided, by using auxiliary contacts of contactors.