Poly-phase induction motors are commonly used in various industrial applications due to their simplicity, reliability, and cost-effectiveness. These motors typically operate on three-phase AC power, but they can also work with other poly-phase configurations. Starting a poly-phase induction motor involves providing an initial torque to overcome the inertia of the motor and accelerate it to its operating speed.
There are several methods used to start poly-phase induction motors, each with its own advantages and disadvantages. Some of the common methods for starting induction motors include:
Direct-On-Line (DOL) Starting: In this method, the motor is directly connected to the power supply, applying full voltage to the motor terminals. This results in a high starting current and torque, which can cause mechanical stress and voltage drop in the power supply network. DOL starting is suitable for small motors with low starting torque requirements.
Star-Delta (Wye-Delta) Starting: This method involves starting the motor in a star (wye) connection, which reduces the starting current and torque. Once the motor reaches a certain speed, it is switched to the delta connection for normal operation. Star-delta starting reduces mechanical stress on the motor and the power supply but may not provide enough starting torque for high-inertia loads.
Auto-Transformer Starting: An auto-transformer is used to reduce the voltage supplied to the motor during start-up, which decreases the starting current and torque. This method is more efficient than direct-on-line starting but less complex than star-delta starting.
Soft Starting: Soft starters use electronic devices to gradually increase the voltage supplied to the motor during start-up, resulting in a controlled acceleration and reduced starting current. Soft starters minimize mechanical shock and electrical stress, making them suitable for applications requiring smooth acceleration.
Variable Frequency Drives (VFDs): VFDs are advanced electronic controllers that allow precise control of motor speed and torque by varying the frequency and voltage of the input power. VFDs offer smooth acceleration, reduced starting current, and energy savings, making them suitable for a wide range of applications.
The choice of starting method depends on factors such as the motor size, load type, power supply capacity, and control requirements. It's essential to select the appropriate starting method to ensure reliable motor operation and efficient energy usage while avoiding mechanical and electrical stress.
Note that these methods primarily apply to asynchronous (squirrel-cage) induction motors. More specialized motors, such as wound rotor induction motors, may have different starting methods and characteristics.