Homework answers / question archive / Experiment #1b: DC Generators Notice for this Semester: Due to the prescription of remote-learning this semester due to the COVID-19 pandemic, grading for this requirement depends entirely on the quality of the submitted report

Experiment #1b: DC Generators Notice for this Semester: Due to the prescription of remote-learning this semester due to the COVID-19 pandemic, grading for this requirement depends entirely on the quality of the submitted report

Electrical Engineering

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Experiment #1b: DC Generators

Notice for this Semester: Due to the prescription of remote-learning this semester due to the COVID-19 pandemic, grading for this requirement depends entirely on the quality of the submitted report. This document remains unaltered from its face-to-face version; only that data have already been provided.

Objectives of this Experiment

• Observe the effects of varying prime mover speed on the output voltage of a DC generator.
• Observe the effects of varying field excitation on the output voltage of a DC generator.
• Observe the effects of varying electrical load on the output voltage of a DC generator.
• Observe safety protocols when operating DC Generators and Induction Motors

Equipment List

(1) MV1028 DC Machine (2) MV1922 Analog Ammeter               Connecting wires

(2) MV1905 Shunt Resistor (1) MV1926 Analog Voltmeter         (1) MV1007 Slip-ring Induction Motor

(1) MV1105 1p Power Resistor (1) MV1054 Torque Meter

(1) MV1300 Power Supply (1) Digital Multimeter

Procedure: General

1. Take note of the ratings of the equipment that you will use in the experiment.¹

Equipment	Rating
MV1028 DC Machine (include serial number)
MV 1007 Induction Machine (include serial number)
MV 1905 Shunt Resistor

 

1 For this iteration, you are to read through the Terco catalog.

EEE 4: Industrial Electronics and Equipment

Experiment #1b: DC Generators

Equipment	Rating
MV1105 1p Power Resistor
MV1300 Power Supply
MV1922 Analog Ammeter
MV1926 Analog Voltmeter
MV1054 Torque Meter

 

NOTE: Observe proper use of resistance, voltage, and current meters, especially with regard to polarity. Resistance measurement is done when the circuit is not energized and the component is isolated from the system. Ammeter fuses easily break when improperly used - make sure it is working before connecting in a circuit. Use the largest setting of the ammeter when unsure of the circuit current then adjust accordingly. NEVER CHANGE

METER SETTINGS WHILE THE SYSTEM IS ENERGIZED!

Prime-mover and DC Generator Setup

DANGER: Perform (re)wiring and coupling procedures only on a fully de-energized system! Otherwise, you run the risk of electrocution and/or getting pinched by rotating elements.

2. Connect the rotor windings in wye.

3. Connect the stator windings in wye and connect it to the variable AC supply.

4. Couple the induction machine to one end of the sensing unit of the MV 1054 Torque meter. Couple the other end of the sensing unit to the DC machine.

5. Leave the armature winding of the DC machine open for now.

Procedure: Separately-Excited DC Generator

Shunt Regulator for Separate Excitation

Because the MV1300 power supply provides only one variable output, but 2 are needed; we need to use the MV1905 to provide the 2nd “variable source”.

6. Connect the fixed-resistance terminals of the shunt regulator to the fixed DC terminals of the power supply.

7. Connect the variable-resistance terminals of the shunt regulator to the field winding of the DC machine. The final setup should look similar to Figure x.

8. Turn on the power supply. Note that if you turn the knob of the shunt regulator, the voltage at the variable-resistance terminals vary from 0 to 200V.

9. Turn the knob of the shunt regulator such that the field voltage is 200V.

10. Couple the shaft of the DC machine to the sensing unit of the MV 1054 Torque meter.

Preliminary Measurements

11. Check that the field excitation is 200V and that the armature winding is open.

12. Start the induction motor using low voltage starting. Vary the input voltage until the speed is around 1680 rpm.

13. Measure the output voltage of the DC generator. Is this near the rated output voltage of the machine? Not really

Effect of Prime-mover Orientation

14. Change the direction of rotation of the prime mover. What happens to the output voltage of the DC generator? Make sure that the field excitation is 200 V and the speed is 1680 rpm. Becomes negative

1. NOTE: You may have to use a digital multimeter to measure the voltage.

Effect of Prime-mover Speed

15. With the field excitation constant at 150V, vary the prime mover speed, and measure the output voltage. Combine the readings of the class and plot the prime mover speed in the x-axis and the output voltage in the y-axis; a link will be made available via UVLE.

Prime Mover Speed (rpm)	Output Voltage (V)
1280	-143
1376	-159
1569	-170
1700	-188

 

16. Vary the field excitation then measure the output voltage at a constant prime mover speed of 1400 rpm. Plot the field excitation in the x-axis and the output voltage in the y-axis.

Field Excitation (V)	Output Voltage (V)
150	-153
160	-154
170	-155
190	-160

 

Effect of Generator Loading

17. Connect a resistive load (MV 1105) to the output terminals of the generator. Keep the field excitation constant at 150V and the prime mover voltage at 200VLL for all measurements. Combine the readings of the class. Plot the following: (1) load resistance vs. prime mover speed and (2) load resistance vs. output voltage.

Load Resistance (W)	Prime Mover Speed (rpm)	Output Voltage (V)
Infinite	1750	-193
223	1701	-185
72.6	1603	-174
42.3	1514	-156
30	1404	-143

 

Procedure: Self-Excited DC Generator

In the self-excited setup, we dispense for the need of a separate DC source just for the field, thus saving on equipment costs. However, there is always a trade-off to be made on some other aspect.

18. Turn off the supply and disconnect the MV 1905 shunt regulator and resistive load. Connect the field winding of the DC machine in parallel with the armature winding, making sure (+) is to (+) and (-) is to (-). Note that in this case, the field no longer has a separate supply.

19. Start the induction motor using low voltage starting. Vary the input voltage until the speed is around 1680 rpm, and then measure the output voltage of the DC generator. Is this near the rated output voltage of the machine? Not really

20. Change the direction of rotation of the prime mover. What happens to the output voltage of the DC generator? Make sure that the speed is 1680 rpm. Almost no output produced

1. NOTE: You may have to use a digital multimeter to measure the voltage.

21. Revert to the correct orientation giving the higher magnitude of voltage output. Vary the prime mover speed (same speeds as in Step 15) and measure the output voltage. Plot the prime mover speed (x) vs. the output voltage (y).

Prime Mover Speed (rpm)	Output Voltage (V)
1280	133
1376	149
1569	175
1700	192

 

22. Connect a resistive load (MV 1105) to the output terminals of the generator. Keep the field excitation at 150V and the prime mover voltage at 200V_LL for all measurements. Use the same loading as in Step 17. Plot the following: (1) load resistance vs. prime mover speed and (2) load resistance vs. output voltage.

Load Resistance (W)	Prime Mover Speed (rpm)	Output Voltage (V)
Infinite	1752	195
223	1658	184
72.6	1564	167
42.3	1460	150
30	1371	135

 

Guide Questions for the Report

The laboratory report must be submitted as a PDF file. Answers to the questions within the

NOTE and WARNING boxes must also be included as part of the report.

1. How does the magnetization curve look like? Is it linear or not? Explain.

2. How does the output voltage vary with the following?

1. Field voltage

2. Mechanical speed

3. Loading

3. How does the self-excited DC generator build up its output voltage?

4. What are the strengths and drawbacks of each configuration (self-excited, separately- excited?)

5. Draw the power flow from the 3-phase mains input, to the load. Indicate where possible losses occur, if any.

Experiment #1a: DC Motors

Notice for this Semester: Due to the prescription of remote-learning this semester due to the COVID-19 pandemic, grading for this requirement depends entirely on the quality of the submitted report. This document is almost-identical to its face-to-face version; only that some data have already been provided.

Objectives of this Experiment

• Properly start and stop a self-excited shunt DC motor.
• Control the speed and direction of a shunt DC motor using different methods.
• Appreciate the effect of mechanical loads on a shunt DC motor.
• Observe safety protocols when working with DC machines and power equipment.

Equipment List

(1) MV1028 DC Machine (2) MV1922 Analog Ammeter       Connecting wires

(2) MV1905 Shunt Resistor (1) MV1926 Analog Voltmeter    (1) MV1007 Slip-ring Induction Motor

(1) MV1105 1p Power Resistor (1)MV1054 Torque Meter

(1) MV1300 Power Supply (1) Digital Multimeter

Procedure

1. Take note of the ratings of the equipment that you will use in the experiment.¹

Equipment	Rating
MV1028 DC Machine (include serial number)
MV 1007 Induction Machine (include serial number)
MV1905 Shunt Resistor
MV1105 1p Power Resistor

 

1 For this iteration, you are to read through the Terco catalog.

Equipment	Rating
MV1300 Power Supply
MV1922 Analog Ammeter
MV1926 Analog Voltmeter
MV1054 Torque Meter

 

NOTE: Observe proper use of resistance, voltage, and current meters, especially with regard to polarity. Resistance measurement is done when the circuit is not energized and the component is isolated from the system. Ammeter fuses easily break when improperly used - make sure it is working before connecting in a circuit. Use the largest setting of the ammeter when unsure of the circuit current then adjust accordingly. NEVER CHANGE

METER SETTINGS WHILE THE SYSTEM IS ENERGIZED!

Wiring the DC Shunt Motor

DANGER: Perform (re)wiring and coupling procedures only on a fully de-energized system! Otherwise, you run the risk of electrocution and/or getting pinched by rotating elements.

2. Connect the field winding of the DC machine in series with an ammeter and a 440 ohm resistor (MV 1905). Connect these to the variable DC terminals of the power supply.

3. Connect the armature winding of the DC machine in series with an ammeter and a power resistor (MV 1105). Connect the armature circuit in parallel with the field circuit.

4. Determine the MV1105 setting such that the starting armature current is equal to two amperes when rated supply voltage is used. Don’t forget to consider the effect of the armature and field resistance of the machine. NOTE: You don’t want the starting resistance too small or too large.

5. When changing the armature resistance, make sure that the current does not exceed 8 amperes under worst-case conditions.

6. Couple the shaft of the DC machine to the sensing unit of the MV 1054 Torque meter. Your setup should look like the setup shown in Figure 1.

NW (4)

+ RA

MV1105

SHUNT

FIELD ARMATURE anes. TOrQue

VT WINDING WINDING Meter

4

RF S

MV1905

FIGURE 1

Starting a DC Motor

7. Connect the machine to the power supply. Turn on the MV 1300 power supply as follows:

1. Make sure that the fixed and variable AC supply switches are ON.

2. Make sure that the main switch is ON.

3. Make sure that the breakers are ON.

4. Turn on the main toggle switch. You should hear a magnetic contactor latch inside the power supply.

5. Turn the adjustable knob to vary the variable DC output up to the rated voltage of the machine.

8. NOTE: Turn off the variable DC output using its selector switch. If you turn off the supply using the main switch, you need to restart the power supply using the described procedure.

9. Make sure that the MV 1105 resistor has the setting derived from Step 4, and R; is zero.

Connect the DC machine to the power supply.

1. What is the instantaneous starting current? 1.15A

2. What is the steady-state current? 0.53A

10. Gradually decrease the armature resistance. Take note of the instantaneous and steady- state armature current as the armature resistance is decreased.

1. What is the overshoot in current after setting MV 1105 to #5? 1.9A

2. What is the steady-state current after setting MV 1105 to #5? 0.53A

3. What is the overshoot in current after setting MV 1105 to #7? 2.4A

4. What is the steady-state current after setting MV 1105 to #7? 0.53A

Speed Control

WARNING: Whenever you start the DC machine, make sure that you UNDERGO THE PROPER STARTING PROCEDURE! MV1105 must have the setting derived from Step 4, and R_F must be zero. Why are these steps necessary?

11. Set the input voltage to 150V and set MV1105 to #7. Control the speed of the motor by changing the field resistance. Plot the field current in the x-axis and the speed in the y- axis. NOTE: Make sure the field current does not go below 0.25 A.

Field Current	Armature Current	Speed	Input Power
0.38 A	0.4 A	1111 rpm
0.31 A	0.4 A	1129 rpm
0.29 A	0.38 A	1146 rpm
0.27 A	0.39 A	1153 rpm
0.26 A	0.38 A	1173 rpm

 

12. Set the input voltage to 200V and set R_F to zero. Control the speed of the motor by changing the armature resistance. Plot the armature resistance in the x-axis and the speed in the y-axis. NOTE: Make sure that the armature current does not exceed 7 amperes even when the field is open-circuited!

Armature Resistance	Field Current	Speed	Input Power
147 W	0.48 A	800 rpm
89 W	0.48 A	1132 rpm
65 W	0.48 A	1275 rpm
49.7 W	0.48 A	1340 rpm
43 W	0.48 A	1383 rpm

 

13. With R_F = 0 ohms, and MV1105 at Setting #7, control the speed of the motor by changing the terminal voltage. Plot the input voltage in the x-axis and the speed in the y-axis.

Input Voltage	Field Current	Armature Current	Speed	Input Power
50 V	0.1 A	0.6 A	496 rpm
75 V	0.15 A	0.43 A	712 rpm
100 V	0.2 A	0.4 A	858 rpm
120 V	0.25 A	0.39 A	995 rpm
150 V	0.43 A	0.36 A	1150 rpm

 

WARNING: When varying the supply voltage, make sure that you do not exceed 110% of the rated voltage. Why?

Direction Control

14. Reverse the direction of the motor by doing each of the specified steps below one after the other. Record your observations on the direction of rotation of the machine with respect to a base case. Mind the meter polarities.

WARNING: Always turn OFF the machine before disconnecting the field winding, and then start the motor appropriately. Why are these steps necessary?

Step Taken	Observation
Reverse the field winding	Counterclockwise
Reverse the armature winding	Counterclockwise
Reverse the polarity of the supply	Clockwise

 

Effect of Mechanical Loading

15. Couple the DC motor to an adjacent machine to simulate mechanical loading. What happens to the armature current, field current, and the speed of the motor when the mechanical load of the machine is increased?

	No-Load	With Mechanical Load
Armature Current	0.4 A	0.9 A
Field Current	0.32 A	0.35 A
Speed	1150 rpm	985 rpm
Input Power	106.5 W	180 W
Output Power	0 W	73.0839 W

 

Stopping a DC Machine

16. Stop the motor by doing each of the specified steps below. Measure the duration from the start of braking until the rotor comes to a full stop. Recall that you need to do the following steps in order to perform dynamic braking: (1) disconnect the armature winding from the supply and (2) connect the armature winding to a suitable braking resistance. Show your instructor a mock-up demo before actual implementation.²

NOTE: When implementing dynamic braking, make sure that you do not connect the braking resistor to the supply voltage. Why is this important?

Step Taken	Observation
Disconnect the supply voltage	Stopped at 4 seconds
Implement dynamic braking	Stopped at 1.5 seconds

 

2 In lieu of an actual demo, include the proposed schematic as part of the laboratory report.

Guide Questions for the Report

The laboratory report must be submitted as a PDF file. Answers to the questions within the

NOTE and WARNING boxes must also be included as part of the report.

1. Explain the starting procedure for a DC motor. What is the importance of an external resistance in series with the armature?

2. Is there too high a starting resistance? Why or why not?

3. What factors affect the ability of a DC motor to drive mechanical loads?

4. Make a comparison among the different methods of speed control, explaining the strengths and drawbacks of each.

5. What determines the direction of rotation for a DC motor? Explain.

6. Does dynamic braking stop a motor quicker? Why or why not? If yes, how does the value of resistance affect the time taken to stop the motor?