ELECTRICAL MACHINES LAB
Experiment Nine
Synchronous Motor
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Electrical Engineering Department
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Theory
Synchronous motor has a special property in maintaining a constant running speed
under all conditions of no- load to full load. This constant running speed can be
maintained even under variable line voltage conditions. It is, therefore, a useful motor in
applications where the running speed must be accurately known and unvarying. It
should be noted that, if a synchronous motor is severely overloaded, its operation will
be pulled out of synchronism and suddenly lose its synchronous properties and the
motor will come to a halt.
Synchronous motors in their simplest form are not self-starting motor. The rotor is
heavy and it’s not possible to bring it in magnetic lock with the rotating magnetic field.
For this reason, all synchronous motors are equipped with some kind of starting
device that brings the rotor speed to 90% of its synchronous speed. The most
commonly starting method is to have the rotor core fitted with squirrel cage rotor. This
achieves a strong magnetic field that provides a good interaction with the stator’s
rotating field. Starting the motor from the standstill is now possible by firstly energizing
the armature windings and waits till the rotor picks up a good speed (that is equal to
synchronous speed). Thin switch the motor from asynchronous mode to synchronous
by exciting the field windings.
Synchronous motor has an additional interesting property which is the ability of acting
like a variable 3-phase inductor/capacitor during no-load. The value of reactance is
determined according to the amount of field excitation.
When running the motor at zero excitation current a considerable amount of positive
reactive power will be drawn from the supply to create the necessary magnetism in the
stator windings. The reactive power has the disadvantage of low power factor. The
motor in this case act in much the same way as any other type of motors (a
combination of inductive-resistive load).
Once the rotor is excited, some of the magnetism in the motor will be produced by
which the stator draws less reactive power, when field excitation is brought further up,
a point can be reached where all the magnetism produced by rotor is used to
compensate for the stator magnetism. As a result the power supply will only produce a
real-power and the power factor becomes unity.
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Electrical Engineering Department
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Tending to create more magnetism is likely achievable by over exciting the field
winding. The power supply in this case produces a negative reactive power in an
attempt to keep the total flux constant. The synchronous motor acts like a capacitive
load.
The armature current drawn by a synchronous motor for a definite power output is a
function of its field current. For a given load on the motor, as the field current is varied,
both the input current and the input power factor change. the plot of armature current
as the function of field current for a constant power output is called a V curve because
of its characteristics shape.
At given load, the armature current is minimum at a particular value of field current. If
field current is gradually decreased below this value, the armature current will gradually
increase till a point is reached where the motor starts hunting.
A similar phenomenon is observed if the field current is increased above this value. The
points on the V - curve where the armature current is minimum corresponding to unity
power factor of the input current. The curve joining the minimum current points of a set
of V-curves is often called a unity power factor-compounding curve.
‘V’ CURVES OF A SYNCHRONOUS MACHINE
Theory
V-curve of a synchronous machine shows its performance in terms of variation of
armature current with field current when the load and input voltage to the machine is
constant. When a synchronous machine is connected to an infinite bus, the current
input to the stator depends upon the shaft-load and excitation (field current). At a
constant load, if excitation is changed the power factor of the machine changes, i.e.
when the field current is small (machine is under-excited) the P.F. is low and as the
excitation is increased the P.F. improves so that for a certain field current the P.F. will be
unity and machine draws minimum armature current. This is known as normal
excitation. If the excitation is further increased the machine will become overexcited
and it will draw more line current and P.F. becomes leading and decreases. Therefore,
if the field current is changed keeping load and input voltage constant, the armature
current changes to make V.I.cosφ constant. Because of their shape, graphs of
variation of armature current with excitation are called ‘V’ curves. If the ‘V’ curves at
different load conditions are plotted and points on different curves having same P.F. are
connected the resulting curve is known as “compounding curves”.
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
Starting of synchronous machines
A synchronous machine does not possess starting torque but once brought near to
synchronous speed, it is capable of developing torque. Synchronous machine can be
started either by means of some auxiliary machine coupled to its shaft or by running it
as an induction machine till it attains a speed near to the synchronous speed.
-Synchronous motor is not self-starting if it is coupled with a d.c. Machine, the latter
may be used as an auxiliary motor for starting. The synchronous machine is run up to
its synchronous speed using d.c. motor drive and then synchronized with three phase
main supply. If the D.C. motor is now switched off the synchronous machine will start
running as a synchronous motor off the a.c. supply. Now the field current is increased
in steps and corresponding armature current are noted.
-Usually starting synchronous machines as an induction motor is quite common. In
this case, the field circuit of the synchronous machine is closed through high
resistance and reduced voltage is applied across the stator windings. Due to threephase balanced supply into the windings of the stator a rotating magnetic field is set
up which rotates at synchronous speed. This rotating field interlinks with the damper
winding (short circuited copper grids placed on the pole shoes), which behaves just
like the squirrel cage rotor of an induction motor. Thus the motor starts as an induction
motor in the direction of rotating field and comes to full speed which is slightly less
than synchronous speed and field circuit of the synchronous machine can be
connected to the D.C. supply. During starting, the field winding is closed through high
resistance to reduce the current in the field winding.
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
Phasor diagrams for different excitations and constant power of a synchronous motor
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
Practice : Three Phase Synchronous Motor
Objectives:
This experiment is designed to:
-Starting Procedure.
-Load characteristics.
-V-Curves.
Equipment:
-Three-phase synchronous machine (Salient pole): 380 V, 1 KVA, 1500RPM, 4-Pole,
Excitation current 0.6 A .
-Exciter for synchronous machines.
-3-Pole circuit breaker.
-3-Phase supply: 380 V, 50 HZ.
-Control Unit : 1 KW.
-Load : Magnetic power break: 1 KW.
-2 Ammeters.
-1 Voltmeters.
-1 Wattmeters.
-1 Power Factor meter.
-Panel mounting frame.
-Connection cables and plugs.
-Rubber coupling sleeves and coupling guards.
-Prime Mover set ( DC shunt motor).
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
Setting of the control unit:
-Set the control unit as follows: Speed N = 1500 RPM, Torque M = 10 N.m
Operating mode: m= constant
Note (1) : In any of the practical procedure, thermal and earth protection of each of the
machines involved should be assured.
Note (2) : Synchronous motor is not a self motor. if the rotor field poles are excited by the
field current and the stator terminals are, connected to the AC supply, the motor will not
start; instead, it vibrates. three methods are normally used to start Synchronous motor :
1-By another machine as a prime mover.
2-By using a variable-frequency supply.
3-Start the machine as an induction motor.
Notice that the last method, an additional winding, which resembles the cage of an
induction motor, is mounted on the rotor. this cage-type winding is known as its damper
winding, which without exciter, operates in the same way as short-circuit cage rotor and
permits asynchronous starting of the machine. the exciter voltage is interrupted during
start-up by a normally closed contact and is connected only when the motor has
reached its highest asynchronous speed, to provide the easiest possible transition from
asynchronous to synchronous running, the motor should be started without a load being
applied.
Prime mover connection and control:
-A prime mover is required to conduct test procedure (2). The prime mover to be used in
this experiment is a DC shunt motor. This wring diagram is shown in figure (1). The
sequence of steps that should be followed t start and control the speed of this motor is
as follows:
-While the voltage knob is fully counterclockwise (i.e, zero output voltage) switch S1 ON
(Position I).
-Press the START switch (green indicator ON).
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
-Set the resistance of the field regulator Rfm to almost 200 ohm.
-To start the drive motor, increase Vs gradually up to 120 V.
-Use the field regulator of the motor field to set the motor speed to the required value
(1500RPM).
Figure (1)
Practical procedure (1): measurement of armature resistance
-Connect the circuit of figure (2).
-Switch S ON. Carefully and gradually increase the applied voltage in steps to match the
requirements of table (1), inch step record the value of VDC.
-Reduce VDC gradually to minimum. Switch S OFF.
S
U1
U2
V1
V2
A
DC
V
Stator Circuit
DC Test
W1
W2
Figure (2)
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
Table (1)
I DC (A)
0.5
1.0
V DC (V)
R DC (ohm)
DC armature resistance
Practical procedure (2): No-load and short circuit tests
-Connect the circuit of figure (3).
-Switch S1 ON and set the exciter current to zero Ampere.
-Start the prime mover as previously mentioned, set its speed at 1500 RPM and keep
this speed fixed throughout the test.
-Measure the voltage at the terminals of the synchronous machine.
-Increase the exciter current in steps to match the requirements of table (2), for each
step, record the terminal voltage.
-Reduce the exciter current to zero once more.
-Short circuit the terminals of the synchronous machine by closing S2.
-Measure the stator current and recored it in table (3), for each step, record the stator
current (short circuit current).
-Reduce the exciter current to zero. switch S2 OFF.
Table (2)
If (mA)
0.0
50.0
100
150
200
300
400
500
600
300
400
500
600
Ea (V)
No-load test
Table (3)
If (mA)
0.0
50.0
100
150
200
Ea (V)
Short circuit test
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Electrical Engineering Department
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S2
E2
Exciter
E1
A
Field circuit
Figure (3) :No load / short circuit tests
Practical procedure (3): Load characteristics
-Connect the circuit of figure (4).
-Switch ON the control unit. adjust its set-start value such that the motor can be started
without a load being applied (i.e, position 10).
-Set the measurement instruments as follows:
-100 V AC for the Voltmeter at the stator terminals.
-10 A AC for the Ammeter in the field circuit.
-(0.1-30) A RMS, (0.3-1000) V RMS, real power mode for the wattmeter.
Remember to choose the appropriate scale for each instrument without overloading it,
this overloading is indicated, when occurs, by red light.
-Adjust the exciter current to 0.6 A.
-Press the push button switch on the motor terminal board, and while doing so, switch
ON the circuit breaker S. Keep the push-button switch pressed until the motor has
reached its highest asynchronous speed, then release this switch so that the motor
continuous to run in a synchronous behavior.
-Set the torque on the control unit in steps in table (4), in each step record line voltage,
speed, stator current, input real power (single phase), input reactive power (single phase)
and input power factor.
-Note that the measured power should be multiplied by 3 to evaluate the total input
power.
-Reduce the load torque gradually to minimum.
-Switch OFF S.
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Electrical Engineering Department
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Table (4)
T (N.m)
2.0
4.0
6.0
Ia (A)
Vt (V)
N (RPM)
PI (W)
Qi (Var)
P.F
Load test
Practical procedure (4): V- characteristics
-Connect the circuit of figure (4).
-Switch ON the control unit. adjust its set-start value such that the motor can be started
without a load being applied (i.e, position 10).
-Set the measurement instruments as follows.
-Set the excitation current to 0.6 A.
-Set the torque by control unit to zero N.m (no-load).
-Record the stator current and the input active power and reactive power, input power
factor in table (5).
Note that when the machine is under excited if the exception current falls below a certain
value, the operation becomes un-stable, this will limit the minim value, the operation
becomes un-stable, this will limit the minima value of the excitation current, if this
happens while conduction the test switch S OFF immediately.
-Repeat steps (3)-(5) but for T= 2 N.m
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Electrical Engineering Department
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Figure (4)
T (N.m)
0.0 N.m
0.0 N.m
0.0 N.m
0.0 N.m
0.0 N.m
0.0 N.m
If (mA)
600
500
400
300
200
100
I st (A)
N (RPM)
Pi (W)
Qi (Var)
PF
Table (5-A)
T (N.m)
2.0 N.m
2.0 N.m
2.0 N.m
2.0 N.m
2.0 N.m
2.0 N.m
If (mA)
600
500
400
300
200
100
I st (A)
N (RPM)
Pi (W)
Qi (Var)
PF
* Please indicate if the p.f is capacitive or inductive.
Table (5-B)
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
Results and comments
-Calculate the average value of the armature resistance Ra, what can you suggest to
correct for AC operation.
-Plot, on one graph paper, the open circuit and short circuit characteristics of the tested
machine, show scales.
-Calculate the synchronous reactance of the armature, plot the synchronous reactance
versus the field current and comment on the variation.
-If the speed of the machine is not fixed during the short circuit test, do you expect the
short circuit current to vary?, please justify your answer.
-From the load test, calculate the developed torque, recored the results obtained in table
(4).
-Plot, for the load test, the output horsepower, developed horsepower, developed torque,
stator current, input power factor, input reactive power and torque angle versus the load
torque, comment on the curves.
-Plot, in one graph paper the V-Curves from the results of test procedure (5), also plot in
one graph paper, the input power factor versus the field current.
-What can you deduce from plots of (7) above? , give full explanation of the shapes and
meanings of these plots.
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Electrical Engineering Department
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ELECTRICAL MACHINES LAB
MATLAB Journey
-Download the file in clearing website (exp9), Follow the link below which will
lead you to a Matlab m-file that have the procedure to Plot the values of
synchronous motor, but modify the values for the machine ratings by the lab
instructor:
-Plotting Stator abc Currents.
-Plotting Rotor abc Currents.
-Plotting Rotor Speed in Radian/sec and Revs/min.
-Plotting Mechanical and Electrical Torque.
-
-Plotting Mechanical and Electrical Torque vs Rotor Speed (rpm).
-Plotting Stator abc Voltages.
-Plotting Rotor abc Voltages.
- https://bit.ly/2sczwQ8
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Electrical Engineering Department
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