Product Manual

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Reduced Voltage Starters
• If the driven load or the power distribution system cannot accept a
full voltage start, some type of reduced voltage or “soft” starting
scheme must be used.
• Typical reduced voltage starters are: primary resistance starters,
autotransformers, part winding starters, wye-delta and solid state
starters.
• These devices can only be used where low starting torque is acceptable.
Primary Resistance Starters
• Closing the contacts at A connects the motor to the supply via
resistors which provide a voltage drop to reduce the starting volt-
age available to the motor.
• The resistor’s value is chosen to provide adequate starting torque
while minimizing starting current.
• Motor inrush current declines during acceleration, reducing the volt-
age drop across the resistors and providing more motor torque. This
results in smooth acceleration.
• After a set period of time, contacts A open and the resistors are
shorted out by contacts B, applying full voltage to the motor.
Three-Phase
Motor
Figure 4.3
Primary Resistance Starter