Digital Excitation Optimizes Synchronous Motor Controls


To take full advantage of the synchronous motor, it is necessary to have an excitation system that maintains constant power factor or var regulation regardless of load and ac supply voltage variations to the excitation controller.

This Application Note discusses how today's excitation systems are designed with features that help improve the quality of machine control in an industrial facility.

In the past, synchronous motor excitation systems used a fixed ac to dc rectifier bridge control or an analog control unit that was subject to ac input voltage variations that could cause lagging power factor on the motor if ac input voltage dropped significantly. Because of this, a pole slip could occur, resulting in loss of synchronization of the motor. New digital excitation systems offer enhance control features and provide reliable feedback to the system.

Today, the digital controller includes:

  • Power Factor Control
  • Underexcitation Limiting
  • Overexcitation Limiting
  • Logic Control
 Motor Protection
 Gas Turbine Generator
 Hydroelectric Generator

The digital control also includes functions that provide added value to the plant and maintenance personnel who are responsible for the system, including:

  • Loss of Voltage Sensing and automatic transfer to manual control to prevent a machine trip caused by loss of PT fuse at the controller input.
  • Autotracking between power factor mode and manual control to avoid bumps and a system disturbance during a unit transfer.
  • Oscillography, a diagnostic tool to evaluate plant issues such as a machine trip to help determine and locate the problem.
  • Sequence of Events that tabulates all events occuring from the time the motor is started to the time the machine is shut down.

Today's digital excitation controllers also include features that might otherwise have been neglected or provided by other discreet devices.

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