Why is the star delta starter used for induction motors?

When a motor is connected in star delta starting point, the phase voltage is 58% of the line voltage. Motor current is decreased as a result of the lower voltage. As a result of the lower current, the motor starts more reliably and without any mechanical or electrical strain. Star Delta Starter is hence recommended.

At startup, the induction motor uses 6 to 7 times its rated current. Since the frequency of rotor current equals the frequency of the stator supply at startup, the rotor impedance is extremely inductive

XL=2 x pi x sfs x L

The motor uses a lot of electricity and produces minimal torque when starting in this condition.

A lower supply voltage must be used to lower the voltage generated in the rotor circuit in order to restrict an induction motor’s beginning current. The supply voltage will drop as the motor current decreases. Small rated motors can be started on the Direct on Line, though when the motor rating increases, a star delta starter must be used to start the motor.

The star-delta starter’s key advantages are:

1). Reduced starting current

Induction motors start with 5-7 times their rated current. High starting current may cause power supply voltage dips and excessive heating & mechanical stress on motor windings. A star-delta starter connects the motor in a star configuration, reducing line current to 1/3 (about 58% of delta connection’s current). Reduced current reduces voltage drop and protects motor & power system components.

2). Voltage Regulation

Star-delta starter gradually increases voltage upon startup. Each motor winding first receives 1/3 of line voltage in a star arrangement. This lower voltage limits inrush current & torque at start, smoothing motor acceleration. The starter shifts to the delta arrangement when the motor achieves a certain speed to run at full voltage.

3). Mechanical Stress Reduction

Direct-on-line (DOL) starting causes mechanical stress & torque spikes in the motor & linked equipment due to its high starting current. Star-delta starters minimise starting torque and current, making start-ups smoother. This reduces stress on the motor and mechanical components, preventing damage (or) premature breakdown.

4). Cost Saving Solution

The star-delta starting is simpler and cheaper than autotransformer starters or VFDs. It is ideal for situations where the motor does not need frequent starts (or) speed control since it has fewer components and a simple control method.

Therefore, The star-delta starter is best for low-starting torque motors. VFDs or soft starts may be better for high-torque (or) highly accurate speed control applications.