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.

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.

4.Excitation input when motor starts in direct on line

The process is in following figure:

When motor starts in direct on line, medium voltage switch closes first. Deexcitation circuit of rotating rectifier connects deexcitation resistor to excitation winding of motor according to inducted voltage of excitation winding. Motor accelerates gradually.

After closing medium voltage switch, excitation device triggers thrysistor, inputs some exciting current to stator excitation winding of generator. With motor speed rising, rotor armature winding voltage of generator rises gradually. When it is higher than minimum voltage of rectifier, rectifier controller is power on. Rectifier monitors inducted voltage of excitation winding. When its period is more than preset value and reaching reverse zero crossing point, rectifier triggers thrysistor, closes deexcitation thrysistor. After being rectified, rotor armature voltage of generator forces on excitation winding of motor. excitation input finishes.

After short synchronizing, motor enters stable operation state. Start-up finishes.

5.Stop process when motor runs in direct on line

Open medium voltage breaker. Meanwhile, excitation device adjusts trigger angle of thrysistor to active inverting area, reduce stator exciting current of generator to zero quickly. Rotor armature winding voltage of generator lowers quickly. When it is less than minimum work voltage of rectifier, rectifier controller is power off. Thrysistor is blockage. Freewheel diode connects deexcitation resistor to excitation winding of motor. Exciting current of motor lowers to zero quickly. Motor stops stably under action of load and resistance torque.

II.Variable frequency operation of brushless synchronous motor

1.Characters of excitation generator during variable frequency operation

Differing from direct slip ring excitation of brush synchronous motor, exciting current of brushless synchronous motor is from excitation generator. Because voltage from generator is in direct rate to product of motor speed and stator current of generator, when motor speed is much less than its rated speed, generator outputs low voltage. At the moment, despite excitation device outputs max exciting current to generator, exciting current of motor will be less than its rated value. In early starting of low speed, motor can not get exciting current.

When motor without exciting current starts in variable frequency, its stator armature winding will absorb big reactive current from VFD (typical value is 2~3 times of motor rated current). the current just flows between VFD and motor, not to power grid. But it will cause short time heat of VFD and stator armature winding of motor. Therefore, when motor speed is low, force current as big as possible on stator excitation winding of generator to lower start current of motor farthest.

2. Process of excitation input and synchronizing

Please see following figure:

After closing medium voltage breaker, VFD powers on. After receiving command of “start”, VFD begins to output voltage to stator armature winding from 0.5Hz, and raises frequency according to preset accelerating time and V/F curve. Motor starts with idle load.

Meanwhile, VFD informs excitation device to output strong exciting current to stator excitation winding of generator. The current is more than rated exciting current of generator, less than max instant exciting current. At the moment, rotor armature winding current is near to zero.

After VFD starts, by salient torque and rotor residual magnetism, motor enters synchronous operation after synchronizing (about 1~2s). at the moment, motor has no exciting current, it runs depending on salient torque and rotor residual magnetism. So current of stator side is big, about 2~3 times of motor rated current.

With motor speed accelerating, inducted voltage of generator rotor armature winding rises gradually. When it is higher than minimum work voltage of rectifier, rectifier controller powers on. Because motor works in synchronous operation at the moment, its rotor angle swing inducts voltage of low frequency on excitation winding. After rectifier monitoring this voltage, rectifier triggers thrysistor instantly, inputs exciting current to rotor excitation winding of motor. Motor speed is still low at the moment, armature voltage of generator is low, so exciting current to motor is also low. After excitation input, motor gets increasing exciting current. stator armature current of motor lowers to rated current down.

After motor accelerates to minimum operation speed, VFD informs excitation device to adjust stator current to max continuous work exciting current (or rated exciting current). start-up of motor finishes. Motor can drive load in this speed, or accelerates to expected speed according to technics need.

Because above start-up process is quite short (about 30s), temperature of generator and motor winding is usually not high before starting, the process would not lead to over heat.

3. Speed adjustment scope

For brushless synchronous motor, because voltage from generator is low during low speed operation. Motor gets small exciting current. its max output torque is small. So it needs to confirm minimum operation frequency according to excitation-torque curve of motor.

Generally, in order to heighten speed adjustment scope, during low speed operation, excitation device outputs max continuous exciting current to stator winding of generator. At the moment, rotor armature winding of generator will output max inducted voltage ( still less than its rated armature voltage of rated speed) of the speed. Motor will get max exciting current of this speed.

According to output voltage of generator in its max continuous exciting current, rotor excitation winding resistor of motor and excitation-torque curve of motor, we can calculate motor max output torque of each speed. Motor minimum operation speed (60~70% of rated speed) is usually decided according to tenet that max output torque of motor is not less than 1.3 times of load torque peak of this speed.

4. Excitation adjustment

a. Low speed full excitation operation

When motor runs in minimum operation speed, it absorbs some reactive current from VFD. Power factor lags. With speed rising, its power factor rises, till unit power factor (PF=1). in order to lower loss and heighten efficiency, excitation device should output its max continuous exciting current to stator winding of generator before power factore reaches to unit power factor.

b. Excitation adjustment during high speed operation

When motor speed rises further, it sends reactive current to VFD. Power factor is ahead. At the moment, in order to reduce loss and heighten system efficiency, VFD will communicate with excitation device according to its output power factor, lower exciting current to generator, make motor run in unit power factor.

4.Stop process in variable frequency

After receiving command of “stop”, VFD stops outputing voltage to stator armature winding of motor. Meanwhile, informs deexcitation. Exciting current of generator decreases quickly. Exciting current of motor decreases quickly through deexcitation resistor. Motor stops stably under action of load and resistance torque.

6.Loss synchronism of protection

When VFD monitors loss synchronism, it stop outputing voltage to stator armature winding of motor immediately. Meanwhile, VFD informs deexcitation, reports fault.

III.Concluding

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.