SIRs,
SEASONAL GREETINGS
Can the Auto Transformer Starter (ATS starters), and the Star Delta Startes be used to as variable speed drive reduce the speed of the AC induction Squirrel cage motors?
Thank you in advance

Hello MUTHU

Welcome to the forum.

The induction motor is a pseudo synchronous device, that is it tries to run at the speed of the supply. If you reduce the voltage to the motor, you will increase the slip for an given load and this will cause the motor to slow, and increase the losses in the motor. There is a slip where maximum torque is developed and below this speed, the motor will stall.
In most induction motors, this maximum torque speed is close to full speed so the speed control that can be achieved by variable voltage is very small. Additionally, if the load on the motor is high, reducing the voltage will increase the current and damage the motor.

There are some motor designs that use a high resistance rotor. These can operate at slips of up to 100% and these can be controlled by a variable voltage. These are not standard induction motors, but are typically special motors designed into fan assemblies.

The only way to control the speed of a standard induction motor, is to use a VFD which controlls both frequency and voltage.

Best regards,