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Electrical Motors - Starting Devices

Direct-on-line starters, star-delta starters, frequency drives and soft starters

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Commonly used starting methods for squirrel cage motors are

  • direct-on-line starters
  • star-delta starters
  • frequency drives
  • soft starters

Direct-on-Line Starters

The simplest and most common starting device is the direct-on-line starter where the equipment consists of a main contactor and a thermal or electronic overload relay.

The disadvantage of the direct-on-line method is very high starting current (6 to 10 times the rated motor currents) and high starting torque, causing

  • slipping belts, heavy wear on bearings and gear boxes
  • damaged products in the process
  • water hammers in piping systems

Star-Delta Starters

A star-delta starting device consists normally of three contactors, an overload relay and a timer for setting the time in the star-position (starting position).

The starting current is about 30 % of the direct-on-line starting device. The starting torque is about 25 % of the direct-on-line starting torque.

The stress on an application is reduced compared to the direct-on-line starting method.

Frequency Drives

With a variable frequency drive the electrical frequency to the motor is modulated between typical 0-250 Hz.

  • the rated motor torque is available at lower speed
  • the starting current is low - ranging 0.5 - 1 times the rated motor current

The frequency drive can also be used for soft stops.

Soft Starters

With soft starters thyristors are used to reduce starting voltage. With lower motor voltage

  • the starting current and starting moment can be very low compared to other methods

Stress on the application can be close to minimum compared to other methods.

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