Autotransformer starters are a type of reduced voltage starter used for induction motors, particularly for larger motors. They work by using a single winding autotransformer to reduce the voltage applied to the motor during starting.
Here's a breakdown of how it works:
1. The Setup:
* Autotransformer: A single winding transformer with a tap that allows for varying the output voltage.
* Motor: The induction motor to be started.
* Start and Run Contactors: These control the switching sequence.
* Overload Relay: Protects the motor from overcurrent.
2. Starting Sequence:
* Starting:
* The start contactor closes, connecting the autotransformer to the line voltage.
* The tap on the autotransformer is set to provide a reduced voltage (typically 65% to 80% of the full voltage) to the motor.
* The motor starts under reduced voltage, resulting in lower starting current.
* Running:
* After the motor reaches a predetermined speed (usually around 75% of full speed), the start contactor opens, and the run contactor closes.
* This directly connects the motor to the full line voltage, allowing it to operate at full speed.
3. How it Works (Simplified):
* The autotransformer acts as a voltage divider, reducing the voltage applied to the motor during startup.
* This lower voltage results in lower starting current, minimizing the strain on the motor and power system.
* As the motor accelerates, the autotransformer is bypassed, and the motor runs at full voltage.
Advantages of Autotransformer Starters:
* Lower starting current: Reduces strain on the motor and power system.
* Improved starting torque: Provides better starting torque compared to other reduced voltage starters.
* Cost-effective: Generally cheaper than other types of reduced voltage starters.
Disadvantages of Autotransformer Starters:
* Limited voltage reduction: Can't achieve as much voltage reduction as other methods.
* Not suitable for all motors: Not suitable for motors with high starting torque requirements.
* Safety concerns: Can be hazardous if not properly installed and maintained.
Overall, autotransformer starters are a viable option for starting larger motors, especially when low starting current and cost-effectiveness are priorities. However, it's important to carefully consider their limitations and ensure proper installation and maintenance for safe and reliable operation.
