Star Delta Starter for Three Phase Induction Motor

 Hello and welcome back to TECKHME.In this article ,we learn the working of  Star Delta Starter for Three Phase Induction Motor.

We all know that three phase induction motors draw a lot of electricity when they first start up,compared to when they are not running at full load. Various methods for beginning these induction motors with low current values have been developed through time to lessen the impacts of high starting current, and the sophisticated starting methods to lower starting current are typically compromised with other factors. unexpected inrush currents caused by the beginning system itself This project uses relays and an electronic timer to provide a low-current start to the motor while also protecting it from inrush currents. In 50 Hz industrial motors, the most popular low voltage starters are star or delta starts. They're utilized to try to cut down on the amount of current used to start the engine. Three contactors, a timer, and a thermal overload are used in the Star/Delta starter to operate a three-phase motor at 440 volts at 50 Hz from an AC mains supply. 

A motor must be in the delta linked position during a normal run to utilize the Star Delta Starter, and the major purpose is to be able to use the starter. Because all the rotor bars are joined together to form a closed channel when the motor is at idle, it acts like a short-circuited transformer on the secondary side. The rotor bars will be subjected to a considerable current flow as a result of this. As a result, when the motor is turned on, the stator draws a large current, 8-10 times the rated current. As a result, before starting the motor, the voltage applied to it must be reduced. The current in a Star connection is split into three phases, yet the line voltage is three times the phase voltage. When a motor is started in a star connection, the voltage is dropped, resulting in a reduction in current. When we operate the motor in a delta connection, the voltage is the same as the phase voltage, hence full voltage is applied. Three contactors, a pneumatic timer, and a thermal overload relay or circuit breaker are used to make a Star/Delta Starter for a three-phase motor with a 440-volt AC main supply at 50 Hz.



• Three Contactors

• Overload Relay

• Timer

• Connectors



This is a low-voltage beginning method. Voltage is reduced during star-delta beginning by physically redesigning the motor windings, as seen in the image below. When the motor is started, the windings are connected in a star arrangement, which reduces the voltage across each winding 3. This reduces torque by a factor of three as well. After some time, the windings are reorganized as delta and the motor operates normally.

Star Connect State

During the start-up of the star delta starter, the main contactor and the star contactor remain closed, completing the power circuit.

The motor is connected in the star state at startup. In the star-connected configuration,the voltage applied to the motor winding is lowered to one-third of the line voltage. When the motor reaches full speed, i.e. 90 percent of full R.P.M, the timer connected in the circuit is engaged. It first disconnects the star contactor and then connects the delta contactor to the circuit, implying that the delta contactor is closed.

Open State

The circuit gets open between switching from star to delta, and the motor does not remain in either star or delta mode. This is referred  to as an open transition stage.

Delta State

When the timer was triggered, the motor shifted from star to delta mode. The phase voltage is equivalent to the line voltage in the delta connection condition. As a result, full line voltage is applied to the motor winding, and the motor operates at maximum speed.


The most frequent method for starting a three-phase induction motor is with a star delta starter. During the starting period of star delta starting, an induction motor is connected in via a star connection. The motor is then hooked in through a delta connection once it has reached the required speed.

The Star/Delta starter is made up of three contactors, a timer, and a thermal overload.Because they only manage winding currents, the contactors are smaller than the single contactor used in a Direct on Line starter. The currents passing through the winding are 13 = 0.58 (58%) of the total current in the line.

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