Transcript AC motor & drive - syncronous

EET421
Power Electronic Drives
– 2) synchronous motor
Abdul Rahim Abdul Razak
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2) SYNCHRONOUS MOTORS
Figure 1: Revolving-field
synchronous motor.
Synchronous motors have the characteristic of constant speed
between no load and full load. Their speed ns (synchronous speed)
only depends on AC line frequency f, and the # of motor poles per
phase, P.

120f
ns 
P
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Example 1:
1)
What pole number would be needed for a synchronous motor to
run at a speed of 300 rpm from a 60Hz supply?
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SYNCHRONOUS MOTORS
Advantages…
Synchronous motors have the following advantages over non-synchronous
motors:
• Speed is independent of the load, provided an adequate field current is applied.
• Accurate control in speed and position (stepper motors).
• They will hold their position when a DC current is applied to both the stator and
the rotor windings.
• Their power factor can be adjusted to unity by using a proper field current
relative to the load. Also, a "capacitive" power factor, (current phase leads
voltage phase), can be obtained by increasing this current slightly, which can
help achieve a better power factor correction for the whole installation.
• Their construction allows for increased electrical efficiency when a low speed is
required (as in ball mills and similar apparatus).
• More efficient compared to induction motor (large industrial scale)
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SYNCHRONOUS MOTORS
They are capable of correcting the low power factor of an inductive
loads when they are operated under certain conditions (overexcited).
Often used to drive DC generators or as replacement of a capacitor
banks to improve power factor .
Synchronous motors are designed in sizes up to thousands of
horsepower. They may be designed as either single-phase or
multiphase machines.
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SYNCHRONOUS MOTORS
Examples
1) Brushless permanent magnet DC motor – Toyota Prius,
Honda Civic Hybrid car
2) Stepper motor – printer, floppy drive
etc
3) Slow speed AC synchronous motor – cement factory
4) Switched reluctance motor. – washing
machine, electric car
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SYNCHRONOUS MOTORS
Steady states conditions…
Figure 2 :
Equivalent circuit
of a synchronous
motor
KVL Equivalent equation :
or
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SYNCHRONOUS MOTORS
Steady states conditions…
Figure 3 :
Equivalent phasor
diagram of a
synchronous motor
Torque equation :
or
Where δ is the load angle.
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SYNCHRONOUS MOTORS
Figure 4 :
Torque-speed
characteristic of a
synchronous motor
•
Loads are basically constant-Speed devices.
•
Terminal voltage and the system frequency will be constant
regardless of the amount of power drawn by the motor.
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•
The steady-states speed of the motor is constant from no load all the way
up to the maximum torque that the motor can supply (called the pullout
torque).
•
•
Thus the speed regulation SR, of this motor is 0 percent.
The torque equation can be stated as:
ind 
•
•
3V E A sin 
m X S
Thus, The maximum or pullout torque Tmax, occurs when δ =90º.
The maximum power can be produce is :
Pmax 
3V E A
XS
•
Normal full-load torques are much less than that, however. In fact, it
would be indicated at 1/3 rd of the pullout torque .
•
when the torque on the shaft of a synchronous motor exceeds the pullout
torque, the rotor can no longer remain locked to the stator and net
magnetic fields. Instead, the motor will slows down and at last will vibrate
severely.
•
The loss of synchronization after the pullout torque is exceeded is known
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as slipping poles.
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SYNCHRONOUS MOTORS
When the field current IF, changes … how does it
affect the synchronous motor?
1)
Consider a synchronous motor
operating at lagging power factor
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2)
Increasing IF will will result magnitude EA to increase. But it does not effect the
real power supplied from to the motor (power will only increase when torque
increased). Thus, P=constant.
3)
Input power to motor is given by:
or
4)
So the distance proportional to power on the phasor diagram (EA sin δ and IA
cos θ) must therefore
be constant. Thus, when
EA to increase, it will only
slide along the constant
power line (EA1 – EA4).
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6)
When value of EA increased, magnitude of IA will slightly decrease then
increased. But it can only do so within the constant power line (IA1 –
IA4).
Note:
At low EA, the armature current is lagging, and the motor is an
inductive load (consuming reactive power Q). As the field current is
increased, the armature current eventually lines up with VØ, and the
motor looks purely resistive. As the field current is increased further,
the armature current becomes leading, and the motor becomes a
capacitive load. It is now acting like a capacitor-resistor combination.
Consuming negative reactive power Q or, alternatively, supplying
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reactive power Q to the system.
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SYNCHRONOUS MOTORS
The V curves…
The several V curves drowns represent
a different motor power levels. For
each curve, the minimum armature
current occurs at unity power factor
(when only real power is being
supplied to the motor)
For field currents less than the value
giving minimum IA, the armature
current is lagging. Consuming Q.
For field currents greater than the
value giving the minimum IA., the
armature current is leading,
supplying Q to the power system
as a capacitor would.
Therefore, by controlling the field curent IF of a synchronous
motor, the reactive power consumed or supplied to by
the motor can be controlled.
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SYNCHRONOUS MOTORS
Underexcited
overexcited
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Example 2:
A 208V 45-kVA, 0.8-PF-leading, Δ- connected, 60Hz
synchronous motor has a synchronous reactance of 2.5Ω and
a negligible armature resistance. Its friction and windage
losses are 1.5 kW, and its core losses are 1.0 kW. Initially the
shaft is supplying a 15HP load with initial power factor of
0.85PF lagging. The field current IF at these conditions is 4.0
A.
a)
Sketch the initial phasor diagram of this motor, and find the
values lA and EA. 25.8L-31.8 deg 182L-17.5V
b)
lf the motor's flux is increased by 25 percent, sketch the new
phasor diagram of the motor. What are values lA and EA and
the power factor of the motor now? 227.5L-13.9 22.5L13.2
leading
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Example 3:
Develop a table showing the speed of magnetic field rotation in ac
machines of 2, 4, 6, 8, 10, 12, and 14 poles operating at
frequencies of 50, 60, and 400 Hz.
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Example 4:
At a location X, it is necessary to supply 300 kW of 60-Hz power.
But, the only power sources available at the site is 50 Hz. It is
decided to generate the power by means of a motor-generator set
consisting of a synchronous motor driving a synchronous
generator. How many poles should each of the two machines have
in order to convert 50-Hz power to 60-Hz power? 10 -12
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Synchronous motor & power factor correction
The following figure shows a large power system whose output is
connected through a transmission line to an industrial plant at a distant
point. The industrial plant shown consists of three loads. Two of the loads
are induction motors with lagging power factors, and the third load is a
synchronous motor with a variable power factor.
What does the ability to set the power factor of one of the loads do for the
power system?
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Synchronous motor & power factor correction
Example 5:
The power system in Figure 5-39 operates at
480V.
Load 1 is an induction motor consuming
0.78PF lagging, and load 2 is
consuming 200kW
100kW at
an induction motor
at 0.8PF lagging. Load 3 is a
synchronous motor whose real power consumption is
150kW.
a.
lf the synchronous motor is adjusted to operate at 0.85PF lagging, what is
the transmission line current in this system? 667A
b.
lf the synchronous motor is adjusted to operate at 0,85PF leading, what is
the transmission line current in this system? 566A
c.
Assume that transmission line losses PLL given as ; PLL = 3IL2RL
how do the transmission losses compare in the two cases? 28% less
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How does it works?..
Assume that the application of three-phase AC power to the stator
causes a rotating magnetic field to be set up around the rotor.
The rotor is energized with DC (it acts like a
bar magnet). The strong rotating magnetic
field attracts the strong rotor field activated
by the dc. This results in a strong turning
force on the rotor shaft. The rotor is therefore
able to turn a load as it rotates in step with
the rotating magnetic field. It works this way
once it’s started.
N
S
However, one of the disadvantages of a synchronous motor is that it
cannot be started from a standstill by applying three-phase ac power
to the stator… Why ??
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Because ..
…when ac is applied to the stator, a high-speed rotating magnetic field
appears immediately. This rotating field rushes past the rotor poles so
quickly that the rotor does not have a chance to get started.
Since the field is rotating at synchronous speed, the motor must be
accelerated before it can pull into synchronism.
Therefore, separate starting means must be employed.
A synchronous motor in its purest form has no starting torque. It has
torque only when it is running at synchronous speed.
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SYNCHRONOUS MOTORS – drive & control
Starting Methods :
1)
Variable AC frequency
2)
Mechanical drive - Turn the rotor into Synchronize speed
3)
DC motor drive – coupled on the common shaft
4)
Embedded Squirrel cage winding on rotor poles.
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SYNCHRONOUS MOTORS – drive & control
1) Variable AC frequency
Note : Electronic speed control – constrain:Modern solid state electronics has able to increase the options for
speed control. By changing the 50 or 60 Hz line frequency to higher
or lower values, the synchronous speed of the motor can be
changed.
However, decreasing the frequency of the AC current fed to the
motor also decreases reactance Xs which increases the stator
current.
This may cause the stator magnetic circuit to saturate with
disastrous results. Thus in practice, the voltage to the motor needs
to be decreased when frequency is decreased.
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SYNCHRONOUS MOTORS – drive & control
Example 6:
If a 60-Hz synchronous motor is to be operated at 50 Hz, will its
synchronous reactance be the same as at 60 Hz, or will it change?
(Hint: Think about the derivation of XS .)
SOLUTION:
The synchronous reactance Xs represents the effects of the armature reaction
voltage Estat and the armature self-inductance Ls. The Estat is caused by the
armature magnetic field Bs , and the amount of voltage is directly proportional to
the speed with which the magnetic field sweeps over the stator surface. The
higher the frequency, the faster Bs sweeps over the stator, and the higher the
armature reaction voltage Estat is.
Therefore, the armature reaction voltage is directly proportional to frequency (EA ~
f). Similarly, the reactance of the armature self-inductance is directly proportional
to frequency so the total synchronous reactance Xs is directly proportional to
frequency (XS ~ f),. If the frequency is changed from 60 Hz to 50 Hz, the
synchronous reactance will be decreased by a factor of 5/6 as well.
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Example 7:
What voltage should be used to allow a 420V, 60Hz, 4-pole
synchronous motor to be used on a 50Hz supply? 350v
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4) Embedded Squirrel cage winding on rotor poles.
Self starting method..
A squirrel-cage type of winding is added to the rotor of a synchronous
motor to cause it to start. The squirrel cage is shown as the outer
part of the rotor in figure 4-7.
It is so named because it is shaped and looks something like a
turnable squirrel cage. Simply, the windings are heavy copper
bars shorted together by copper rings.
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Self starting process..
Fleming′s Right Hand Rule
Also known as the Generator Rule this is a way
of determining the direction of the induced emf
of a conductor moving in a magnetic field.
Fleming′s Left Hand Rule
Also known as the Motor Rule this is a way
of determining the direction of a force on a
current carrying conductor in a magnetic
field.
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Starting process…
A low voltage is induced in these shorted windings by the rotating
three-phase stator field. Because of the short circuit, a relatively large
current flows (induced emf) in the squirrel cage.
This causes a magnetic field that interacts with the rotating field of the
stator. Because of the interaction (left hand rule), the rotor begins to
turn, following the stator field; the motor starts. We will run into squirrel
cages again in other applications, in more detail.
To start a practical synchronous motor, the stator is energized, but the
dc supply to the rotor field is not yet energized. The squirrel-cage
windings bring the rotor to near synchronous speed. At that point, the
dc field is energized. This locks the rotor in step with the rotating stator
field. Full torque is developed, and the load is driven.
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Starting process…
A mechanical switching device that operates on centrifugal force is
often used to apply dc to the rotor as synchronous speed is reached.
The practical synchronous motor has the disadvantage of requiring a
dc exciter voltage for the rotor. This voltage may be obtained either
externally or internally, depending on the design of the motor.
Constant speed
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Questions to ponder…
1. What requirement is the synchronous motor specifically designed to meet?
2. What is the speed regulation of a synchronous motor?
3. When would a synchronous motor be used eventhough its constant
speed characteristic is not needed?
4. Why can't a synchronous motor start by itself?
5. What techniques are available to start a synchronous motor?
6.What happens to a synchronous motor as its field current is varied?
7. A synchronous motor is operating at a fixed load. Once the field current is
increased, the armature current falls, was the motor initially operating at a
lagging or a leading Power factor?
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The end on synchronous motor
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