Poly-phase induction motors are widely used in various industrial and commercial applications due to their simplicity, reliability, and efficiency. Squirrel cage induction motors, in particular, are the most common type of induction motors used due to their rugged construction and low maintenance requirements. The term "squirrel cage" refers to the rotor construction, which resembles a cage made of conductive bars.
Starting a squirrel cage induction motor involves overcoming the initial inertia and creating a rotating magnetic field in the stator that induces a current in the rotor bars, thus initiating rotation. There are several methods to start squirrel cage motors:
Direct On-Line (DOL) Starting: This is the simplest method where the motor is directly connected to the power supply. However, the sudden current surge during starting can be quite high and cause voltage drops in the power supply system, affecting other connected equipment.
Star-Delta (Wye-Delta) Starting: In this method, the motor is initially connected in a "star" configuration during starting. This reduces the voltage applied to each winding and subsequently the current drawn. After a period of time, the motor is switched to the "delta" configuration for normal operation. This method reduces the starting current but provides lower starting torque.
Auto-Transformer Starting: An auto-transformer is used to reduce the voltage applied to the motor during starting. This method reduces the starting current and provides intermediate starting torque between DOL and star-delta starting.
Soft Starters: These are electronic devices that gradually ramp up the voltage and current supplied to the motor during starting. This reduces the starting current and torque shock, minimizing mechanical stress on the motor and connected equipment.
Variable Frequency Drives (VFDs): VFDs are advanced devices that allow precise control of the motor's speed by varying the frequency of the supplied power. During starting, the VFD can gradually increase the frequency and voltage, achieving a controlled and smooth acceleration.
Rotor Resistance Starting: By inserting external resistors in the rotor circuit during starting, the starting current can be limited, and the starting torque can be adjusted. As the motor gains speed, the resistors are gradually bypassed.
Each starting method has its advantages and disadvantages, and