ECE 4501Power Systems Laboratory Manualrev 1.1

ECE 4501Power Systems Laboratory ManualRev 1.1

6.0 INDUCTION MOTORS

6.1WOUND ROTOR INDUCTION MOTOR

6.1.1OBJECTIVE

To examine the construction of a three-phase wound rotor induction motor and understand the concepts of exciting current, synchronous speed, slip and induced voltages.

6.1.2DISCUSSION

When the three windings in the stator of an induction motor are connected to a three-phase voltage source, currents flow in the windings and a rotating magnetic field is established in the stator. If there is no load connected to the motor shaft, the three-phase current drawn by the stator windings is called the exciting current. This current, at line voltage, provides the reactive power necessary to establish the rotating magnetic field in the stator and the real power dissipated in the copper windings and core.

The speed of the rotating stator field is determined by the frequency of the three-phase waveforms supplied by the source, 60 Hz in North America, and by the number of magnetic poles with which the stator is built. This speed is known as the synchronous speed and is measured in revolutions per minute, RPM. Poles always come in pairs ( a north pole and a south pole) and for a two-pole motor, the field will complete 60 revolutions every second and thus synchronous speed is 3600 RPM. This is as fast as any induction motor can ever turn when excited by 60 cycle waveforms. Electric utilities maintain system frequency with great precision (in order to make electric clocks run accurately, among other things). Therefore, synchronous speed may be considered a constant value for a given motor.

The rotor of an induction motor consists of a laminated steel core with slots and some type of winding. The two most common types of winding are the squirrel cage rotor and the wound rotor (using copper windings). The squirrel cage rotor will be discussed in a later section of the experiment. In the wound rotor, three sets of windings are set in the slots of the core material. Each winding is brought out to a slip ring on the shaft of the rotor. Terminating the windings on slip rings allows flexibility in the manner in which the windings are configured by allowing resistors to be placed in WYE or DELTA across them. The resistors are sized to accurately control the magnitude of currents in the rotor windings.

The rotating three-phase magnetic field produced by the stator induces an alternating voltage on each of the rotor windings. If the rotor is not turning, the rate at which each rotor winding cuts the lines of flux produced by the magnetic field will be equal to the synchronous speed and the induced voltages in the rotor will be at the same frequency as the source voltage. This condition is called 100% slip. As the rotor is turned in the same direction as the rotating magnetic stator field, the rate at which the rotor windings cut lines of flux will decrease and the induced voltages in the rotor windings will decrease in frequency and magnitude. If the rotor is turned at a rate equal to the synchronous speed, its windings will not cut any lines of flux and the induced voltages will be zero in magnitude and frequency. This condition is called 0% slip. The torque produced by the motor drops to zero at 0% slip and thus, for all practical purposes, an induction motor cannot actually achieve synchronous speed. Conversely, if the rotor is turned in the opposite direction with respect to the stator field, but at synchronous frequency, the induced voltages will have twice the magnitude and frequency as compared to the 100% slip condition.

6.1.3INSTRUMENTS AND COMPONENTS

Power Supply ModuleEMS 8821

AC Metering Module (2.5 A)EMS 8425

AC Metering Module (250V)EMS 8426

DC Motor/Generator ModuleEMS 8211

Wound Rotor IM ModuleEMS 8231

Three-Phase Wattmeter ModuleEMS 8441

Hand TachometerEMS 8920

6.1.4PROCEDURE

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 construction of the Wound Rotor Induction Motor, EMS 8231, paying close attention to the slip rings, the rotor windings, the stator windings, and the connection schematic.

2)What is the rated current of the stator windings? ______Rated voltage? ______

3)What is the rated current of the rotor windings? ______Rated voltage? ______

4)Are the rotor windings configured WYE or DELTA? ______

5)What is the rated speed of the induction motor? ______Rated Horsepower? ______

6)Connect the following circuit, coupling the two motors with a timing belt:

FIGURE 6-1-1: INDUCTION MOTOR TURNED BY A DC MOTOR

7)Note that the DC Shunt Motor* will be used to turn the rotor of the Induction Motor. Also note that the DC motor is connected with fixed (120 Vdc) shunt field excitation and variable (0 – 120 Vdc) excitation for the armature.

* Alternatively the Prime Mover/ Dynamometer can substitute for the DC shunt motor, in which case supply power to it using 1-N terminals at power supply to 1-2 terminals of Dynamometer.

8)Turn the field rheostat on the DC motor to its full clockwise position (for minimum field resistance). If Dynamometer is used instead of DC motor adjust the load control to minimum

9)Note that the stator of the Induction motor is WYE connected and that voltmeter, V1 will measure input voltage and V2 will measure the induced voltage on the open circuited rotor windings.

10)Make sure that both motors are coupled by a timing belt.

11)Turn on the 24 Vac power supply and the Main Power Supply, but DO NOT turn the voltage control (the DC motor should not turn).

12)Measure and record the following (it is OK if W1 and W2 have different signs):

V1 = Volts / W1 = Watts / I1 = Amps
V2 = Volts / W2 = Watts / I2 = Amps
I3 = Amps

13)Turn OFF the main power supply.

14)Calculate the following (by the 2-wattmeter method, three phase real power will be W1 + W2):

Apparent Power, S3 / Reactive Power, Q3
VA / VAR
Real Power, P3 / Power Factor, pf
W

15)Turn on the power supply and adjust the voltage control until the DC motor turns at 900 rpm.

16)Measure and record the following: (Note: If the value of V2 is less than the previous test, turn OFF the power supply and interchange any two of the stator connections, then repeat step 15)

V1 = Volts / W1 = Watts / I1 = Amps
V2 = Volts / W2 = Watts / I2 = Amps
I3 = Amps

17)Is the three phase real power, P3, the same as before? ______

18)Increase the voltage control to 100 percent and adjust the field rheostat until the DC motor turns at 1800 rpm.

19)Measure and record the following:

V1 = Volts / W1 = Watts / I1 = Amps
V2 = Volts / W2 = Watts / I2 = Amps
I3 = Amps

20)Turn the voltage control to zero percent and turn OFF the power supply.

21)In the procedures at 900 and 1800 rpm, is the rotor of the induction motor being turned with or against the direction of the rotating stator field? Explain:

______

______

______

22)Interchange the connections 1 and 2 on the armature of the DC Motor in order to reverse its direction. Double check that the rheostat is in its full clockwise position.

23)Turn on the power supply and adjust the voltage control for a DC motor speed of 900 rpm.

24)Measure and record the following:

V1 = Volts / W1 = Watts / I1 = Amps
V2 = Volts / W2 = Watts / I2 = Amps
I3 = Amps

25)Increase the voltage control to 100 percent and adjust the field rheostat until the DC motor turns at 1800 rpm.

26)Measure and record the following:

V1 = Volts / W1 = Watts / I1 = Amps
V2 = Volts / W2 = Watts / I2 = Amps
I3 = Amps

27)Return the voltage control to zero percent and turn OFF the power supply.

28)In the last two procedures, was the rotor of the induction motor being turned with or against the direction of the rotating stator field? Explain:

______

______

______

6.1.5CONCLUSIONS

6.1.5.1 Since the voltage induced on the rotor winding is zero volts when the rotor is turned at synchronous speed, what is the synchronous speed of the motor, in rpm?

______

6.1.5.2The following equation defines the relationship between frequency, number of poles in a motor and its synchronous speed:

Synchronous Speed, Ns = 120 f / p , where f is frequency in Hz, and p is the number of poles

Determine the number of poles in the motor tested:

______

______

6.1.5.3Calculate the slip rpm and percent slip for each of the 5 motor speeds tested, -1800, -900, 0, 900, 1800 RPM:

Slip rpm, Nslip = Ns – NrotorPercent Slip = ( Nslip / Ns ) x 100%

ROTOR RPM / Slip RPM / Percent Slip
-1800
-900
0
900
1800

6.1.5.4How much reactive power is required to produce the rotating magnetic field in the stator of the induction motor (i.e. the amount consumed when the rotor is open circuited and not turning)?

______

6.1.5.5How much real power is dissipated under the same conditions? ______

6.1.5.6Plot the rotor speed versus the induced rotor voltage on the graph below. Should the resulting

curve be a straight line? ( i.e. is Vrotor Nrotor ) ______

6.2INDUCTION MOTOR STARTING CHARACTERISTICS

6.2.1OBJECTIVE

To determine starting current and starting torque of a three-phase, wound rotor induction motor

6.2.2DISCUSSION

In the previous section, it could be observed that the open circuit voltage across the rotor windings varied linearly with respect to the slip RPM of the rotor. When the rotor windings are short-circuited, this induced voltage causes large circulating currents in the rotor, creating a magnetic field in the rotor that opposes the rotating stator field. The strength of the rotor field is proportional to the rotor currents, which are proportional to the induced rotor voltage which, as stated, varies linearly with slip RPM.

Therefore, it should be expected that a three-phase induction motor with shaft at standstill (100%slip) would produce high torque. An induction motor with shaft turning at synchronous speed will produce zero torque.

6.2.3INSTRUMENTS AND COMPONENTS

Power Supply ModuleEMS 8821

AC Metering Module (2.5 A)EMS 8425

AC Metering Module (250V)EMS 8426

DC Motor/Generator ModuleEMS 8211

Wound Rotor IM ModuleEMS 8231

Electrodynamometer ModuleEMS 8911

6.2.4PROCEDURE

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)Connect the circuit shown below:

FIGURE 6-2-1: INDUCTION MOTOR STARTING TORQUE

2)Set the Dynamometer control fully clockwise for maximum load torque.

3)Make sure the voltage control is at zero percent and turn ON the power supply.

4)Turn the voltage control clockwise until the AC voltmeter, V1 reads 100 Vac. The motor should turn slowly. Measure and record the three ammeter currents and the torque developed by the rotor.

I1 = / I2 = / I3 = / T =

5)Gradually reduce the load on the motor by turning the Dynamometer control knob slowly in the counterclockwise direction.

Do the three rotor currents increase or decrease as the load is reduced and the motor speeds

up? ______

6)Reduce the load on the motor to 1 Lbf-in (one inch-pound = 0.12 Newton-meters) and record the three rotor currents:

I1 = / I2 = / I3 =

7)Return the voltage to zero percent and turn OFF the power supply.

8)Connect the circuit shown below. NOTE that the fixed power supply is now being used.

FIGURE 6-2-2: INDUCTION MOTOR FULL VOLTAGE START

9)Set the Dynamometer control fully clockwise for maximum load torque.

10)To avoid damage to the motor, complete the next three steps QUICKLY.

11)Turn on the power supply.

12)Measure and record the motor voltage, V1, and currents, I1 and I2, and the developed starting torque.

V1 = / I1 = / I2 = / T =

13)Turn OFF the power supply.

14)Calculate the apparent power delivered to the motor by the source using the equation:

S3 = 3 VLine ILine

6.2.5CONCLUSIONS

1. Assume that this motor is rated ¼ HP and its full load motor speed is 1500 RPM. Calculate the full load torque using the following equation:

Tlbf-in = (100,000) HP/ (1.59) RPM OR TN-m = (100,000) HP/ (14.07)RPM

______

______

1. Calculate the ratio of starting torque (the torque measurement taken in Step 12 above) to the full load torque (just calculated above):

______

______

1. Assume that the full load stator current is 1.2 amperes per phase and calculate the ratio of starting current to full load current:

______

______

1. If the stator voltage of a wound rotor motor is reduced by 50% of rated value, what will be the percent change in:

a)Starting current:

______

______

b)Apparent power:

______

______

c) Starting Torque:

______

______

6.3INDUCTION MOTOR STARTING CHARACTERISTICS

6.3.1OBJECTIVE

To examine the construction and starting characteristics of the three-phase squirrel induction motor.

6.3.2DISCUSSION

The squirrel cage induction motor is the workhorse of industry. It is similar to the wound rotor induction motor in construction, except that the rotor winding is replaced by copper or aluminum bars. The bars are terminated at each end of the rotor with short-circuit rings, forming a ‘cage’. The advantages to the cage include durability, ease and cost of manufacturing, and minimal maintenance. Disadvantages, including reduced starting torque, are numerous but rarely outweigh the price and maintenance advantage held by the squirrel cage motor over a comparable wound rotor motor.

6.3.3INTRUMENTS AND COMPONENTS

Power Supply ModuleEMS 8821

AC Metering Module (2.5/8 A)EMS 8425

AC Metering Module (250V)EMS 8426

Dynamometer ModuleEMS 8911

Squirrel Cage IM ModuleEMS 8221

Three-Phase Wattmeter ModuleEMS 8441

Hand TachometerEMS 8920

6.3.4PROCEDURE

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 construction of the Squirrel Cage Induction Motor, paying close attention to the manner in which the stator windings are arranged, the presence of fan blades for forced-air cooling, and the short-circuit rings on each end of the rotor cage. Note also that unlike the wound rotor motor, no slip rings are necessary since the bars of the cage are short-circuited.

2)Enter the following nameplate data from the front of the motor module:

Current Rating / Voltage Rating / Rated Speed / Rated Horsepower

3)Connect the following circuit:

FIGURE 6-3-1: Squirrel Cage Power and Torque Characteristics

4)Turn on the 24-Volt switch (if using the Prime Mover/Dynamometer), adjust the Dynamometer for minimum load (full ccw), then turn ON the main power supply, and adjust the voltage control for 208 Vac operation.

5)Measure the three line currents, the two-wattmeter readings and the motor speed and record them in the first line of the table below. (It is OK if the two wattmeter readings have different sign)

6)Turn the Dynamometer control clockwise until the load on motor is 3 Lbf-in (0.35 N-m) and record the same measurements in the second line of the table. Repeat until all lines of the table are filled.

TORQUE

Lbf-In / N-m

/

I1

Amps

/ I2
Amps / I3
Amps / P1

Watts

/ P2
Watts / SPEED

RPM

0 / 0
3 / 0.35
6 / 0.7
9 / 1.05
12 / 1.4

7)Return the voltage control to zero percent and turn OFF the power supply.

8)Connect the circuit shown below, noting that the fixed voltage source is now used:

FIGURE 6-3-2: SQUIRREL CAGE FULL-VOLTAGE START

9)Perform the next three steps quickly:

10)Set the Dynamometer control for maximum load torque (fully cw) and turn ON the power supply.

11)Measure and record the motor voltage, V1, and currents, I1 and I2, and the developed starting torque.

V1 = / I1 = / T =

12)Turn OFF the power supply.

13)Calculate the apparent power delivered to the motor by the source using the equation:

S3 = 3 VLine ILine

6.3.5CONCLUSIONS

1. Using the 9 Lbf-in Torque data from the table in Step 6) of 6.3.4 Procedure, calculate the following full-load characteristics:

Apparent Power: ______

______

Real Power: ______

______

Reactive Power: ______

______

Power Factor: ______

______

1. Using the data from Step 11) above and the 9 Lbf-in Torque data from the table in Step 6), compute the following ratios:

Starting Current / Full Load Current: ______

______

Starting Torque / Full-Load Torque: ______

______

1. Compare the Wound Rotor Induction Motor to the Squirrel Cage Induction Motor with respect to a) Starting Torque, b) Starting Current, c) Speed Regulation, and d) Full-Load Speed.

______

______

______

______

1. Assume that the Full-Load Speed of a 4-Pole Induction Motor is 1650 RPM. If power frequency were decreased to 50 Hz, what would the new Full-Load speed be?

______

______

______

______

Would the exciting current (no-load current) increase or decrease?

______

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