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Variable frequency operation of brushless synchronous motor
Abstract: because brushless synchronous motor does not part which may create spark, such as slip ring and carbon brush, it is used widely in site of explosive environment, such as chemical industry and coal mine. With idea of energy saving promoting, brushless synchronous motors need to reconstruct with variable frequency technology. This article states principle, structure and operation mode of brushless synchronous motor, analyzes problems during synchronizing, adjusting and excitation input, puts forward variable frequency operation of brushless synchronous motor.
Key words: brushless synchronous motor, variable frequency, speed adjustment, operation mode

I.Preface
Because brushless synchronous motor does not part which may create spark, such as slip ring and carbon brush. In addition, due to no wearing part such as carbon brush, reliability of motor is heightened. There are lots of applications in site which requires long time continuous reliable operation. With idea of energy saving promoting in recent years, brushless synchronous motors are confronted with variable frequency reconstruction.
Because of inherent characters of brushless motor, during variable frequency operation, problems of weak magnetism excitation input, motor under-excitation operation and VFD-excitation device coordinating control have been restricting variable frequency application of brushless synchronous motor.
In base of principle and structure of brushless synchronous motor, this paper analyzes kinds of problems during variable frequency operation and answers these problems according to theory analysis and simulation test.

II. Structure of motor and its operation in direct on line state
1. Structure of brushless synchronous motor

Please see following figure:


Structure of brushless synchronous motor

In figure,
1: slipping bearing
2: winding
3: cooling device
4: rotating rectifier
5: excitation generator

2. Excitation structure of synchronous motor
Please see following figure. Excitation generator and synchronous motor rotate in same shaft.

Excitation structure of motor

Rotating rectifier is charge of deexcitation and excitation input logic. Its circuit is in following figure.


Circuit of rectifier

3. Line frequency operation of motor
When motor runs in direct on line, excitation device inputs appropriate exciting current to stator winding of excitator generator. Armature winding terminal of rotor of generator inducts three phase AC voltage. The voltage is converted to DC voltage by rotating rectifier. DC voltage forces on rotor excitation winding of motor to supply consecutive exciting current.
According to physical character of excitation generator, excitation device can adjust rotor exciting current of motor by adjusting trigger angle of thrysistor and stator exciting current of excitator generator.

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