ECE 4501
Power Systems Laboratory Manual
Rev 1.0
7.0 SYNCHRONOUS MOTORS
7.1
SYNCHRONOUS MOTOR STARTING CHARACTERISTICS
7.1.1
OBJECTIVE
To examine Synchronous Motor construction and study its starting characteristics.
7.1.2
DISCUSSION
Synchronous motors are designed to run at synchronous speed, hence the name. Synchronous
speed defines the natural rate of rotation of the magnetic field generated in the stator of an AC
machine. This rate of rotation is governed by the frequency of the voltage applied to the machine
and the number of pole pairs built into the stator, and is defined by the following equation:
Synchronous Speed, Ns = (60 seconds/minute) * (Cycles per Second) RPM
Number of Pole Pairs
OR,
Ns = 120 f/p RPM
, where f is the frequency of the voltage waveform in Hz
and p is the total number of poles
The stator of a synchronous motor is wound in the same manner as a three-phase induction motor,
with distributed poles. When a three-phase source is applied to the stator, a rotating magnetic field
develops at synchronous frequency. The rotor of a synchronous motor, however, differs greatly
from that of an induction motor. In an induction motor, voltages are induced in the rotor as its
windings cut lines of flux created by the rotating stator field. The induced voltages cause a flow
of current in the rotor winding, creating a magnetic field in the rotor and developing torque.
In a synchronous motor, applying a DC current to the rotor winding via a separate source creates
the magnetic field in the rotor. This constant magnetic field could be considered a “sail” which is
caught in the rotating stator field, forcing the rotor to turn at synchronous frequency. However, it
must be noted that synchronous motors are not self-starting. The DC field in the rotor will only
synchronize with the rotating field in the stator if the rotor is already turning at some high
percentage (say 90%) of synchronous speed. Therefore, synchronous motors are never started
under load and must use some sort of starter that will accelerate the rotor to near-synchronous
speed while the DC excitation circuit is de-energized. A starter may be an external motor,
connected to the rotor shaft during startup and disconnected when the rotor is able to synchronize
with the stator field. More commonly, a squirrel cage winding is incorporated into the rotor
assembly, allowing the synchronous motor to self-start as an induction motor. NEVER START A
SYNCHRONOUS MACHINE WITH THE EXCITING WINDING ENERGIZED.
-1-
ECE 4501
Power Systems Laboratory Manual
Rev 1.0
Figure 7.1
When a synchronous motor has achieved synchronous speed and a load is applied, the rotor will
momentarily lag behind the rotating stator field (but still turn at synchronous speed). As the rotor
briefly slows, the DC field in the rotor makes a torque angle with respect to the rotating stator field
and the motor develops more torque at the shaft and accelerating the rotor. This angle between the
DC field and the rotating stator field is called the Power Angle. If the load is variable, the power
angle will be constantly changing (hunting), overshooting and undershooting the desired level of
torque. To minimize hunting, synchronous machines have a damper winding on the rotor,
sometimes called the Amortisseur winding. In synchronous motors that contain a squirrel cage
winding for self-starting, the squirrel cage will also provide damping. When a synchronous motor
is overloaded, the rotor is forced out of synchronism with respect to the rotating stator field and
the motor slows down. The load level at which a motor gets pulled out of synchronism is called
the pullout torque.
The primary reason for applying a synchronous motor lies in its ability to alter its own power
factor. By varying the DC voltage applied to the rotor winding, it is possible to control the
reactive power consumed by the motor. It is even possible to “over-excite” the motor in this
fashion, making it create reactive power (leading power factor). This is a distinct advantage in
applications involving a very large load. By applying a synchronous motor of sufficient size to the
largest load in an industrial facility, it is often possible to greatly improve the power factor of the
entire plant.
7.1.3
INSTRUMENTS AND COMPONENTS
Power Supply Module
Synchronous Motor/Gen Module
Electrodynamometer Module
Synchronizing Switch Module
Three-Phase Watt-Varmeter
AC Ammeter Module
AC Voltmeter Module
Hand Tachometer
EMS 8821
EMS 8241
EMS 8911
EMS 8621
EMS 8446
EMS 8425
EMS 8426
EMS 8920
-2-
ECE 4501
7.1.4
Power Systems Laboratory Manual
Rev 1.0
PROCEDURE
CAUTION! – High voltages are present in this Experiment. DO NOT make any
connections with the power supply ON. Get in the habit of turning OFF the power supply
after every measurement.
1)
2)
Examine the Synchronous Motor/Generator Module EMS 8241, paying particular
attention to the slip rings, brushes, DC field rheostat, stator winding and rotor windings.
Note that the rotor has two field coils (one for each pole-pair) and also contains a squirrel
cage for starting and damping.
View the face plate of the motor and answer the following:
a)
What is the rated voltage of each stator winding? ___________ Volts
b) What is the rated current of each stator winding? ___________ Amps
c)
Locate the DC exciter and notice that the rotor windings are in series with a toggle
switch, S, and a 150-Ohm rheostat.
d) What is the rated voltage of the rotor winding? ____________ Volts, DC
e)
3)
What is the rated speed and horsepower of the machine? _______ RPM ______HP
Now connect the circuit shown in Figure 7.2, noting that the stator windings are
connected in WYE:
FIGURE 7.2 Induction Start of Synchronous Motor
4)
Turn ON the main power supply and observe the motor start and run as a squirrel cage
induction motor. Measure and record the direction of rotation and the line current drawn
by the motor.
Line Current =
Rotation: CW or CCW
-3-
Amps
ECE 4501
Power Systems Laboratory Manual
Rev 1.0
5)
Turn OFF the power supply. Interchange any two of the three connections from the
power supply (i.e. swap phases).
6)
Turn ON the power supply and record the new rotation direction and line current.
Line Current =
Rotation: CW or CCW
Amps
7)
Turn OFF the power supply.
8)
Now connect the circuit shown in Figure 7.3 below, making sure that the motor is
coupled to the electrodynamometer via a timing belt.
FIGURE 7.3
9)
The Synchronizing Module will be used as an on-off switch for applying 3-phase power
to the motor. Set its switch to the zero (Open) position.
10)
Set the load control knob on the Dynamometer for approximately 40% loading. Set the
Exciter Rheostat on the Synchronous Motor at maximum excitation (fully CW).
-4-
ECE 4501
Power Systems Laboratory Manual
Rev 1.0
11)
Close the Exciter switch (position 1). Turn ON the power supply.
12)
Now apply 3-phase power to the stator by closing the switch on the Synchronizing
module. CAREFUL: Do not leave this switch closed for more than 10 seconds.
13)
Quickly assess what happens to the motor, then open the switch on the Synchronizing
Module and turn OFF the power supply.
14)
Describe the motor’s operation under the above conditions, including what was heard and
what the ammeter displayed.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
15)
Should a synchronous motor be started under load when its excitation circuit is
energized? __________
16)
Leaving all other connections intact, disconnect the DC exciter circuit from the fixed, 120
V DC supply and connect it to the variable, 0 – 120 V DC supply (7 – N).
17)
Make sure the synchronizing module switch is open and the power supply voltage control
is at zero percent and turn ON the power supply.
18)
Apply power to the stator windings by closing the synchronizing module switch and
record what happens to the motor, including RPM, Watts and VARs.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
19)
Is the motor operating as an induction motor? ________
20)
Now turn the voltage control knob until the variable DC supply voltage is 120 V DC;
record what happens to the motor, including RPM, Watts and VARs.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
21)
Is the motor operating as an induction motor now? _________
22)
Now adjust the Exciter Rheostat until the VARs supplied to the motor are Zero. Did the
Watt reading change? ________
23)
Return the voltage control to zero percent and turn OFF the power supply.
-5-
ECE 4501
24)
Power Systems Laboratory Manual
Rev 1.0
Connect the circuit shown in Figure 7.4 below:
FIGURE 7.4 – STARTING TORQUE of SYNCH. MOTOR
25)
Set the load control on the Dynamometer to 100% load (fully CW), turn the Exciter
Rheostat to maximum excitation (fully CW) and close the Exciter Switch, S, on the
Synchronous Motor.
26)
Turn ON the power supply and measure V1, V2, I1 and the starting torque supplied by the
motor.
Line Voltage, V1
Induced Voltage, V2
Line Current, I1
Starting Torque
27)
Turn OFF the power supply.
28)
Calculate the three-phase Apparent Power, S, for the motor using V1 and I1.
S = 3 Vline Iline = _____________ VA
-6-
ECE 4501
Power Systems Laboratory Manual
Rev 1.0
29)
Calculate the Full Load Torque (Rated Torque) using the rated watts and rated RPM for
the motor:
T = P/ = ____________ Newton-Meters
30)
Calculated the ratio of starting torque (measured above) to rated torque:
Ratio = Tstart/Trated = _______________
31)
Try to explain why V2 is present, and why it is a large value:
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
32)
Leave the circuit unchanged and turn ON the power supply. Slowly turn the
dynamometer load control counter-clockwise until the motor is able to run freely
(approximately 1700 RPM).
33)
What is the effect on the induced voltage, V2?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
34)
7.1.5
Turn OFF the power supply and return all wiring to its proper place.
CONCLUSIONS
1) What two precautions should always be taken when starting a synchronous motor?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
2) If the squirrel cage winding were removed from the rotor, would the synchronous motor start?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
-7-
ECE 4501
Power Systems Laboratory Manual
Rev 1.0
3) Give two reasons why the rotor winding of a synchronous motor is usually shunted by an
external resistor during start-up:
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
4) If a synchronous motor is over-excited, what can be concluded about its power factor?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
-8-