Step 1 – Start (Reduced Voltage)
• The Main Contactor and the Start Contactor close.
• The motor receives reduced voltage via the autotransformer taps, limiting the current and gently starting rotation.
An Auto Transformer Starter is a type of reduced-voltage starter used to limit the high inrush current drawn by large $\mathbf{AC}$ squirrel-cage induction motors during startup. It is one of the most effective and popular methods for "soft starting" powerful motors in industrial applications.
The starter uses an autotransformer—a single-winding transformer—to temporarily lower the voltage applied to the motor's stator windings during the initial acceleration phase.
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The primary goal of the starter is to reduce the starting current, which can be 5 to 7 times the motor's full-load current (FLC). High inrush current can cause large voltage dips in the electrical system, potentially affecting other connected equipment.
The key component is the autotransformer, which has taps (usually at 50%, 65%, or 80%) that allow the user to select a reduced voltage.
When the voltage applied to the motor is reduced by a factor of S× (the tap setting), the current drawn from the main supply is reduced by a factor of S².
If you use a 65% tap (S=0.65), the starting torque is 42.25% of the full-voltage torque, and the line current drawn from the supply is also reduced to approximately 42.25% of the full-voltage starting current.
This square relationship between voltage reduction and line current reduction is the main reason the Auto Transformer Starter is more efficient than a simple Series Reactor or Resistor Starter.
The starting sequence is a time-based, two-step operation managed by a timer and three main contactors to bring the motor smoothly from standstill to full speed.
• The Main Contactor and the Start Contactor close.
• The motor receives reduced voltage via the autotransformer taps, limiting the current and gently starting rotation.
• After a set time delay (usually about 5–10 seconds) and once the motor has reached roughly 80% of its full speed, the timer trips.
• The Start Contactor opens, disconnecting the autotransformer.
• The Run Contactor closes, connecting the motor directly to the full line voltage so it quickly accelerates to nominal speed.
The autotransformer is only energized during the brief starting period and is then completely switched out of the circuit. This makes the system very energy efficient during normal operation.
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