Auto-transformer starters provide stepped voltage starting for AC motors by using a tapped autotransformer to gradually apply reduced voltage to the motor during its startup. This method is commonly employed to limit the inrush current and reduce the mechanical stress on the motor and connected equipment during the initial moments of operation.
Here's how auto-transformer starters work:
Basic Autotransformer Operation: An autotransformer is a single winding transformer with a tap that allows you to select different voltage ratios. It functions as both a step-up and a step-down transformer, depending on the tap connection. In an auto-transformer starter, the winding has multiple taps to allow for different voltage levels.
Starting Sequence: When an AC motor is started using an auto-transformer starter, the voltage applied to the motor is gradually increased in steps. The starter starts with the highest tap (close to the supply voltage) and then moves to lower taps, reducing the voltage in steps. This reduces the starting current and torque, preventing excessive current draw and mechanical stress on the motor.
Reduced Voltage: By starting the motor at reduced voltage, the inrush current is significantly reduced compared to a direct-on-line start, where the motor is connected directly to the full supply voltage. This reduced current helps in preventing voltage dips and disturbances in the electrical system, and it also prevents excessive heating and potential damage to the motor windings.
Voltage Transition: As the motor gains speed and reaches a stable operating condition, the auto-transformer starter progressively switches to lower taps, increasing the voltage and thus allowing the motor to accelerate further. The transition between voltage steps can be automated based on time or the motor's speed or current.
Advantages: Auto-transformer starters provide smoother motor acceleration, reducing the stress on both the motor and the connected mechanical components. This extended acceleration time is especially useful for large motors, where the high inrush current could cause voltage sags and negatively impact other equipment connected to the same electrical network.
Disadvantages: While auto-transformer starters are effective at reducing inrush current and mechanical stress, they are more complex and expensive than direct-on-line starters. They also result in increased voltage drop during starting due to the reduced applied voltage. Additionally, energy efficiency might be compromised as the motor spends more time operating at reduced voltage.
In summary, auto-transformer starters provide stepped voltage starting for AC motors by using a tapped autotransformer to gradually increase the voltage supplied to the motor during startup. This method helps in reducing inrush current, minimizing mechanical stress, and maintaining stable electrical conditions in the system.