Replacing Constant Speed With Variable Speed

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In the past, applications operating at less than 300rpm, and rated from 500 kW to 5000 kW, have been controlledusing a medium-voltage constant speed synchronous motors. Commonapplications include crushers and various mills used inmetals, cement or mining.Replacing a constant speed with variable speed drive control brings substantial operating and maintenanceimprovements. Variable-speed drives provide soft starting, precise speed andtorque control and low stress on the electrical network.Soft starting lets the motor get up to speed gradually, therebyavoiding high inrush currents that can prevail when direct-onlinestarting is used. As such, the stress on the motor and allcomponents in the transmission train are much reduced. Thisavoids failure of key components while reducing maintenancecosts and prolonging motor and transmission life time.

ABB industrial drives feature the motor control platform, and direct torque control (DTC). DTC provides the highly accuratespeed and torque control often needed in slow speedapplications.With a power factor approaching unity, the variable-speeddrive has low reactive power consumption; thereby loweringvoltage variations on the supply network. The variable speed drive and motorcan be connected to weaker supply networks without theneed for power factor correction equipment. An advantage of using new synchronous motors with ABB industrial drives is that, for most cases, the existingfoundations can be re-used.A key benefit of variable-speed control is that the new motorhas fewer pole pairs and a smaller shaft height. This is becausea constant speed synchronous motor needs to be designed forits maximum speed. So a 2500 kW motor requiring 150 rpmhas 40 poles and typically a shaft height of 2000 mm. However,using variable-speed drive, the motor has between 8 to 24poles and a typical shaft height between 1250 to 1600 mm.This substantially lowers the motor costs and even when thedrive and transformer are considered, the investment will beless expensive than a 40-pole motor.

Benefits Of Replacing Constant Speed With Variable Speed:

  • Soft starting capability reduces stress on motor andtransmission train
  • Optimum speed according to process requirements
  • Lower maintenance costs and prolonged motor life
  • Low stress on the electrical network through unity powerfactor and low reactive power consumption
  • Choice of smaller traditional synchronous motor
  • No need to alter foundations thereby avoiding any loss ofproduction and costly civil works
  • Typically lower total cost than a new constant speedsynchronous motor

To learn more about replacing constant speed with variable speed, or forvariable speed drive repair and replacement quotes, contact Precision Electric, Inc.