What is the Purpose of Reduced Voltage Motor Control Starter?


Electric Motors | Source: Tekwell Services

A squirrel cage motor draws high starting current (inrush) and produces high starting torque when started at full voltage. High current draw during a full voltage start can cause the line voltage to sag which may adversely affect other equipment within the facility. 

The high torque developed during a full voltage start can cause excessive wear on the driven equipment or damage to material in the process. A reduced voltage start reduces the negative effects of both high current inrush and torque.

A typical squirrel cage induction motor started at full voltage will draw current from four to ten times the current required to run the motor at full speed, fully loaded. The current drawn during a full voltage start - or the locked rotor current - can cause negative effects on the distribution system. A motor started across the line can cause voltage drops which may cause lights to dim, or other equipment on the feeder circuit to slow down or drop out. 

When reduced voltage is applied to a motor, the motor will draw current in proportion to the voltage reduction:

  • I RV = I FV (VRV/VFV)

For example, a typical NEMA design B motor, the inrush or locked rotor current will be approximately 600% of the motor full load amperage (FLA) rating. If started with 65% of line voltage, the motor will then draw 0.65 x 600% or 390% FLA. 

Reduced Starting Torque

When started at full voltage, a typical NEMA design B motor will produce approximately 150% of its rated torque. The torque produced by the motor in excess of the torque required by the driven load is used to accelerate the load from rest to full speed. 

Excess acceleration torque can cause mechanical shocks which can result in twisted shafts and couplings, excessive belt wear, or early failure of moving parts in the driven equipment. If the motor accelerates too quickly, the material in the process can also be damaged during a full voltage start. 

When a motor is started at reduced voltage, the torque is reduced by the square of the voltage reduction. 

  • TRV = TFV (VRV/VFV) ^2
For example, if the NEMA B motor described above is started at 65% of line voltage, the torque developed would then be (0.65)2 or 42% of the normal starting torque.

Reduced Voltage Application

Reduced voltage starters are used in a wide variety of applications. It is important to remember that an application that is difficult to start with full voltage will be even more difficult to start with reduced voltage. 

If the motor has high inertia or if the motor rating is marginal for the applied load, reducing the starting torque may prevent the motor from reaching full speed before the thermal overloads trip. Applications that require high starting torque should be reviewed to determine if reduced voltage starting is suitable. 


Autotransformer starters are the most widely used electromechanical reduced voltage starter. An Autotransformer starter may be used on any three-phase AC motor and provides the maximum starting torque with the minimum starting current. Autotransformer starters are ideal for applications where available starting current is limited, but a minimal decrease in starting torque is required. Because the Autotransformer provides the highest torque efficiency, it is also ideal for applications which may be run from generator power. 

Typical applications for autotransformer starters include: 
  • Centrifugal pumps 
  • Fans 
  • Compressors
  • Conveyors

Wye-Delta Starters

Wye Delta starters are recommended for use on applications that require long start-up time or frequent starts when using a wye-delta motor. The Wye-Delta starter is available in both open and closed transition configurations. The open transition starter is recommended for inertial loads while a closed transition is best suited for frictional loading. Wye Delta starters can be applied in any application where the low starting current is and low starting torque are necessary. 

Some typical applications include: 
  • Elevators 
  • Chillers 
  • Large air conditioning units 
  • Centrifuges

Part Winding Starters

Part Winding starters are recommended for use on applications that do not require long start-up time or frequent starts. Part Winding starters are a cost-effective means of providing a reduced voltage start where total line current draw is not of primary concern. 

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