ECE 4501Power Systems Laboratory Manualrev 1.0

ECE 4501Power Systems Laboratory ManualRev 1.0

7.0SYNCHRONOUS MOTORS

7.1SYNCHRONOUS MOTOR STARTING CHARACTERISTICS

7.1.1OBJECTIVE

To examine Synchronous Motor construction and study its starting characteristics.

7.1.2DISCUSSION

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.

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.

7.1.3INSTRUMENTS AND COMPONENTS

Power Supply ModuleEMS 8821

Synchronous Motor/Gen ModuleEMS 8241

Electrodynamometer ModuleEMS 8911

Synchronizing Switch ModuleEMS 8621

Three-Phase Watt-VarmeterEMS 8446

AC Ammeter ModuleEMS 8425

AC Voltmeter ModuleEMS 8426

Hand TachometerEMS 8920

7.1.4PROCEDURE

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)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.

2)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)What is the rated speed and horsepower of the machine? ______RPM ______HP

3)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.

Rotation: CW or CCW /

Line Current = Amps

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.

Rotation: CW or CCW /

Line Current = 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).

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.

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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.

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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.

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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.

24)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

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:

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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?

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34)Turn OFF the power supply and return all wiring to its proper place.

7.1.5CONCLUSIONS

1)What two precautions should always be taken when starting a synchronous motor?

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2)If the squirrel cage winding were removed from the rotor, would the synchronous motor start?

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3)Give two reasons why the rotor winding of a synchronous motor is usually shunted by an external resistor during start-up:

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4)If a synchronous motor is over-excited, what can be concluded about its power factor?

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