Download Variable frequency operation of brushless synchronous motor

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.
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.