Brushless Excitation

One of the most significant developments in recent years is brushless excitation for synchronous machines. This development became possible with the availability of reliable, long-life, solid-state control and power devices (diodes, transistors, SCR's, etc.). As the name indicates, there are no brushes, collector rings, or commutators on the motor or exciter. This eliminates brush, collector ring and commutator maintenance and permits the use of synchronous motors in many hazardous (Class 1, Group D, Division 2) and corrosive areas where conventional motors could not be used without extensive additional protection. All the advantages of conventional synchronous motors are retained: constant speed, high efficiency, power factor correction, and varied performance capability. High precision, fast-acting, solid-state field application control is rotor-mounted and provides the same full complement of functions as a conventional synchronizing panel. Brushless excitation is presently almost universally used.

The exciter is an AC generator with a stator-mounted field. Direct current for the exciter field is provided from an external source, typically a small variable voltage rectifier mounted at the motor starter. Exciter output is converted to DC through a three-phase, full-wave, silicon-diode bridge rectifier. Thyristors (silicon-controlled rectifiers) switch the current to the motor field and the motor-starting, field-discharge resistors These semiconductor elements are mounted on heat sinks and assembled on a drum bolted to the rotor or shaft.

Semiconductor control modules gate the thyristors, which switch current to the motor field at the optimum motor speed and precise phase angle. This assures synchronizing with minimum system disturbance. On pull-out, the discharge resistor is reapplied and excitation is removed to provide protection to the rotor winding, shaft, and external electrical system. The control resynchronizes the motor after the cause of pull-out is removed, if sufficient torque is available. The field is automaticall> applied if the motor synchronizes on reluctance torque. The control is calibrated at the factory and no field adjustment is required. The opti mum slip frequency at pull-in is based on total motor and load inertia. All control parts are interchangeable and can be replaced without affecting starting or running operation.

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