Transcript Discrete Time Wavelet Transforms

Chapter 6
Synchronous Motors
Edit by Chi-Shan Yu
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Synchronous Machine
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Chapter 6 Synchronous Motors
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Basic Principles of Motor Operation
Steady-State Synchronous Motor Operation
Starting Synchronous Motors
Synchronous Generators and Synchronous Motors
Synchronous Motor Rating
Summary
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Basic Principles of Motor Operation
• Two dependent magnetic
fields exist in the machine
• The dc field current IF
produces a rotor field BR
• The three-phase ac current
produce a stator field BS
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The equivalent circuit
• The equivalent circuit of a synchronous motor is the
same as the equivalent circuit of a synchronous
generator. (Besides, the current flow direction is
reverse)
• Since the current flow direction is reverse, the
relation between the terminal and internal voltage is
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The phasor diagram of a synchronous
generator – with lagging power factor load
• The induced torque is opposing the applied torque
from prime mover
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The phasor diagram of a synchronous
motor
• The induced torque drives the motor rotation.
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Steady-state synchronous motor
operation – Torque speed curve
• The steady-state speed is constant from no-load to
full-load.
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The maximum torque or pull-out torque
• While the voltages are constant, the induced torque
of the synchronous motor only depends on the power
angle d.
• Thus, the pull-out torque is maximum induced torque.
– That is, d = 90 degrees.
–
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The effect of load changes on a synchronous
motor (what is the meaning of pull-out torque)
• At steady-state the torque that the synchronous
motor supports is equal to the load torque.
• When the load torque increases ?
– The motor will slow down (rotor mechanical speed)
– The stator rotation flux still remain its constant speed (stator
flux speed)
– Thus, the power angle d increases
– Finally, the synchronous motor’s will generate a large torque
that is equal to the load torque.
• However, if the maximum induced torque is still lower
than the load torque ?
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The phasor diagram of increasing load
torque
• When the load increases, all the armature current IA, power
angle d, and the power factor angle q are increase (from
leading to lagging).
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Example 6-1
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The effect of field current changes on a
synchronous motor
• An increase in field current increase the magnitude of
EA but does not affect the real power supplied by the
motor.
– Why ?
– The power supply by the motor is (Pm = w×tload)
• The terminal voltage Vf supplies to the motor is
constant
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The phasor diagram – increase the field
current
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What is the effect of increasing field
current
• What is the meaning of the power factor of the motor
changes from lagging to leading
– can support Q to the electrical system
– The motor is now acting like a capacitor-resistor combination
load, and the magnitude of the capacitor can be changed by
the field current
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Synchronous motor V curve
• A plot of IA versus IF for a synchronous motor is the V
curve.
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Underexcited and overexcited
• The internal voltage can be smaller or larger than the
terminal voltage
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Example 6-2
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Homework - 2
• Use the
MATLAB to
complete the
Figure 6.12
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EX 6-3 Power factor correction
• Why the power factor is so important in power system
?
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EX 6-3 Power factor correction
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The reason for power factor correction
4. Meanwhile, the harmonic content is also important reason
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The synchronous capacitor or
synchronous condenser
• A synchronous motor can be operated overexcited
to supply reactive power Q for a power system.
• No real power have been drawn from the load and
the shaft of the motor is removed
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The V curve of the synchronous capacitor
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Starting the synchronous motors
• How to start the synchronous motor ?
• What is the problem of the synchronous motor in
starting?
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The problem in starting
• The induced torque between rotor flux and stator flux
• The directions of the induced torque depends on the
angle relation between the BS and BR
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The problem in starting
• At starting, the motor speed is increased from zero
speed
• The speed of the rotor is slower than the speed of the
stator flux
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The problem and how to solve
The motor will vibrate but not rotate !!
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Reducing the stator electrical frequency
• At starting, the motor operates at a low enough
speed.
• The speed of the stator magnetic field can then be
increased gradually up to 60Hz
• Notably
– The stator voltage must also be reduced to prevent the overcurrent in stator winding. (EA = Kfw)
• The power electronics can build the inverter to
achieve the variable frequency and voltage drive
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Motor starting by using damper windings
• The damper or
amortisseur windings
are special bars laid
into notches carved in
the face of a
synchronous motor’s
rotor, and then
shorted out on each
end by a large
shorting ring.
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The simplified diagram of the damper
winding
• The rotor shows an
damper winding with
the shorting bars on
the ends of the two
rotor pole faces
connected by wires
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The reason of adding the damper
windings
• At starting, the bars of damper winding are shorted,
and the field winding is disconnected from the field
voltage
• The motor will act as an induction motor
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Starting the synchronous motor by
damper winding
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The effect of damper windings on motor
stability
• The damper windings speeds up slow machines and
slows down fast machines
• Thus, the stability can be increased by adding the
damper windings
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The differences between the synchronous
generators and the synchronous motors
The differences between them are the direction of the induced
torque and the angle relation between the internal and terminal
voltage
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Phasor
diagram
Synchronous motor rating
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Recap
• The synchronous speed
• The field current and the power factor of the
synchronous motor
• Synchronous condenser
• The starting problem of the synchronous motor
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Appendix C
Salient-Pole Theory of Synchronous
Machines
Edit by Chi-Shan Yu
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Why we discuss the salient-pole effect ?
• Salient pole rotor and cylindrical rotor
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Until now the result is
• The following result is only valid for cylindrical rotor
• The salient pole rotor has another reluctance torque
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The effect of armature reaction in a
salient-pole synchronous generator
• The rotor magnetic field
and the induced voltage
on the stator conductor
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The effect of armature reaction in a
salient-pole synchronous generator
• If a lagging load is
connected to the terminals
of this generator
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The effect of armature reaction in a
salient-pole synchronous generator
• The stator magnetic
field is no more 90
degrees behind the
armature current
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The stator voltage
• Each component of the stator
magnetic field produces a
voltage in the stator winding by
armature reaction. The total
voltage in the stator is thus
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Now we may include the armature selfresistance and reactance
• The armature self-reactance is independent of the
rotor angle
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How to plot the phasor diagram ?
• The armature current IA is broken into Id and Iq by
angle d+q
• Usually, the torque angle d is unknown, and the
power factor angle is q known.
• Without the knowledge of d, how to plot the phasor
diagram ?
• Once the angle d is known, the armature current IA
can be broken into Id and Iq.
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How to plot the phasor diagram ?
• The EA’’ has the same angle as the EA
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Example C -1
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Torque and Power equation of salientpole machine
• The power produced by d and q axis current
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Power equation
• The power is
• The d-axis
current is
• The q-axis
current is
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Power equation
• The power is
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Torque equation
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