THE UNIVERSITY OF THE WEST INDIES ST. AUGUSTINE, TRINIDAD & TOBAGO, WEST INDIES
FACULTY OF ENGINEERING Department of Electrical & Computer Engineering
B. Sc. in Electrical & Computer Engineering
ECNG 3030
Electromechanical Energy Conversion
DIRECT CURRENT MACHINES
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THE UNIVERSITY OF THE WEST INDIES
ST. AUGUSTINE, TRINIDAD & TOBAGO, WEST INDIES
FACULTY OF ENGINEERING
Department of Electrical & Computer Engineering
Contents
1. General Information................................................................................................................4
2. Lab Learning Outcomes..........................................................................................................5
3. Pre-Lab....................................................................................................................................5
3.1. Required Reading Resources...........................................................................................5
3.2. Pre-Lab Exercise.............................................................................................................6
4. In-Lab.......................................................................................................................................7
4.1. D.C. MOTOR SPEED CONTROL.................................................................................7
4.2. OPEN CIRCUIT CHARACTERISTICS........................................................................8
4.3. SELF-EXCITATION.....................................................................................................10
4.4. EXTERNAL CHARACTERISTIC OF SHUNT GENERATOR..................................11
5. Post-Lab.................................................................................................................................13
5.1. In-Lab Results................................................................................................................13
5.2. Calculations and Results................................................................................................15
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THE UNIVERSITY OF THE WEST INDIES
ST. AUGUSTINE, TRINIDAD & TOBAGO, WEST INDIES
FACULTY OF ENGINEERING
Department of Electrical & Computer Engineering
Listof figures
Figure1: Diagram of DC Motor shunt connections........................................................................7 Figure2: Diagram showing generator connections for the open circuit test, separately excited.....8 Figure3: Diagram showing the DC Generator self-excited shunt connections..............................10
Figure4: Diagram showing the connections for load test..............................................................11
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THE UNIVERSITY OF THE WEST INDIES
ST. AUGUSTINE, TRINIDAD & TOBAGO, WEST INDIES
FACULTY OF ENGINEERING
Department of Electrical & Computer Engineering
ECNG 3030
Electromechanical Energy Conversion http://myelearning.sta.uwi.edu/
Semester I
1. GENERAL INFORMATION
Lab #: 3
Name of the lab:
Direct Current Machines
Lab weighting: 2.5% Estimated total
study hours1:
3
Delivery mode: √Lecture √Online
√Lab
ˆ Other
Venue for the lab: Energy Systems Lab
Lab dependencies2 The theoretical background to this lab is provided in ECNG 3030
Theoretical content link: http://myelearning.sta.uwi.edu/
Recommended
prior knowledge
and skills3:
To undertake this lab, students should be able to: Understand the basic concepts of DC Machines
Course Staff Position/Role E-mail
Phone
Office Office
Hours
Mr. Tariq Wadi Lecturer [email protected] Ext
82677
Pwr Elec Lab
-
Mr. Andrew
Balgobin
Instructor [email protected] Ext
82676
Pwr Sim Lab
-
1 Estimate includes teaching time, study time, and student preparation time for classes and labs. 2 Include any Co-requisites, Post-requisites, or Forbidden course /lab combinations with respective code (C/P/F). 3 Lecturers can state lab input requirements in terms of student behaviours.
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LAB #3: D.C. Machines
2. LAB LEARNING OUTCOMES
Upon successful completion of the lab assignment, students will be able to: Cognitive Level
1. To understand the D.C. Motor Starter. 3 2. To obtain the Open-Circuit characteristics of the D.C. Generator 3
3. To investigate the phenomenon of self-excitation and to obtain the external
characteristics of a D.C. Shunt Generator
3
3. PRE-LAB
Due Date: The day of the In-Lab Exercise
Submission Procedure: Must be typewritten and submitted before In-lab is performed
Estimated time of
completion:
1 hr.
3.1. Required Reading Resources
• ECNG 3030 Class notes
3.2. Recommended reading resources
• Basic Electric Machines -Del Toro
• Electric Machines & Power Systems -Del Toro
• Electric Machinery -Fitzgerald & Kingsley
• Electric Machines and their Applications -Hindmarsh
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LAB #3: D.C. Machines
3.3. Pre-lab Questions
3.3.1 Any D.C. Motor whose rating exceeds 2 Hp requires starters or controllers. Why?
3.3.2 Sketch a diagram of a D.C. motor and labels its parts. What is the significance of each element
of the D.C. motor? 3.3.4 With the aid of suitable diagrams of the equivalent circuit and equations, discuss the
implications of the different field/armature circuit arrangements of the D.C. generator.
3.3.5 Nameplate Specifications of the D.C. Generator:
125 Volts
1 Amp.
0.3 Field Amps.
0.125 kW
1750 rpm
Using the specifications above, calculate the ratings of the meters that will be used in the In-lab
experiments.
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LAB #3: D.C. Machines
4. IN-LAB
Allotted completion
time:
3hrs
Required lab
equipment:
All provided
4.1. D.C. MOTOR SPEED CONTROL
Figure 1: Diagram of D.C. Motor shunt connections
4.1.1. Using Figure 1, connect the circuit of the D.C. motor.
DO NOT energize the circuit until the demonstrator checks it.
NB. The motor field regulator should always be kept at the minimum (i.e.
fully anticlockwise) resistance position at the time of starting.
4.1.2. Turn on the D.C. voltage switch
4.1.3. Slowly increase the input voltage and observe the speed of the motor shaft.
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LAB #3: D.C. Machines
4.1.4. Adjust the motor field regulator and again observe how the speed is affected.
4.1.5. Turn the motor field regulator fully anticlockwise and turn off the D.C. voltage switch.
4.2. OPEN CIRCUIT CHARACTERISTICS
In this section, the Open Circuit Characteristics of the D.C. Motor is investigated.
The Open Circuit characteristic is a graph relating the Open-Circuit armature voltage of a D.C.
generator versus its field current when the machine is driven at its rated speed. In this exercise
the D.C. generator field is excited by a separate D.C. source and the current is varied using a
generator field regulator (rheostat).
Figure 2: Diagram showing generator connections for the open circuit test, separately excited
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LAB #3: D.C. Machines
4.2.1. Using the diagram in Figure 2, connect the generator as a separately excited machine.
DO NOT energize the circuit until the demonstrator checks it.
NB. The generator field regulator should always be kept at the maximum
(i.e. fully clockwise) resistance position at the time of starting.
4.2.2. Start the D.C. motor and increase the speed until rated speed is obtained.
4.2.3. With the D.C. Generator being driven at rated speed and open circuited, measure the
armature voltage from zero field current. This is due to residual magnetism.
4.2.4. Increase the excitation current in steps and record the excitation current and armature
voltage for every 10V step in armature voltage (until armature voltage ≈ 130V). Record
your results in Table 1 in the Post Lab section.
4.2.5. Repeat steps while decreasing excitation current. Record your results in Table 1 in the
Post Lab section.
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LAB #3: D.C. Machines
4.3. SELF-EXCITATION
Figure 3: Diagram showing the D.C. Generator self-excited shunt connections
In this section, Self-excitation and the External Characteristics of the D.C. Shunt Generator
are investigated.
{The maximum value of Rext should be higher than the critical resistance}
4.3.1. Using the diagram in Figure 3, connect the circuit for self-excitation.
DO NOT energize the circuit until the demonstrator checks it.
4.3.2. Set the field rheostat to its maximum value. Drive the DC Generator at its rated
speed. Decrease Rfield to a lower value and observe whether the machine self excites
(Develops e.m.f). If the machine does not self excites, the field terminals of the
generator have to be reversed. This should be done only after the D.C. Machine
is switched off.
4.3.3. Record observations made.
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LAB #3: D.C. Machines
4.4. EXTERNAL CHARACTERISTIC OF SHUNT GENERATOR
Figure 4: Diagram showing the connections for load test
The external characteristics of a shunt generator is a graph relating terminal voltage and the
load current of a D.C. Generator when driven at its rated speed with the field current
maintained at its normal no-load value.
4.4.1. Load the D.C. Generator by connecting a Resistive Load Bank across its terminals.
DO NOT energize the circuit until the demonstrator checks it.
4.4.2. Drive the D.C. Generator at its rated speed. Adjust its field current such that the
machine develops rated voltage on NO-LOAD.
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LAB #3: D.C. Machines
4.4.3. Vary the load connected across the Armature of the D.C. Generator in steps until
rated armature current flows. Record all measurements taken in Table 2 in the Post-
Lab section.
Ensure that the motor speed is always set at its rated value and the generator field
current is maintained (the motor and generator field regulators may have to be
adjusted).
End of In-Lab
Direct Current Machines 12 | P age
LAB #3: D.C. Machines
5. POST-LAB
Due Date: Two (2) weeks from the In-Lab Exercise
Submission
Procedure:
Submit to the technicians in the Energy Systems Lab by 4:00PM
Deliverables: Cover page
Signed Plagiarism Form
Typewritten responses to the postlab questions ONLY
Signed results from lab session (in pen)
All bound in a folder.
ID #:
5.1. In-Lab Results
Table 1: Open Circuit Test Results
Reading No. If increasing/ Amps VOC/Volts If decreasing/ Amps VOC/Volts
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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LAB #3: D.C. Machines
Table 2: Load Test Results
If/Amps Load Current, IL/Amps Terminal Voltage,VT/Volts
Armature resistance =
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LAB #3: D.C. Machines
5.2 Questions and Calculations
Estimated time of completion: 3 hrs.
5.2.1 Obtain the critical field resistance of the D.C. Generator at rated speed using the graph
plotted in the exercise above for the open circuit (magnetization) characteristic of the
DC Generator.
[6marks]
5.2.2 Plot the open circuit (magnetization) characteristic of the D.C. Generator at half the
rated speed and determine the corresponding critical resistance of the D.C. Generator.
[6marks]
5.2.3 Identify and explain the two different methods of speed control of the D.C. motor observed
in experiment 4.1. Describe another method by which the speed can be varied.
[6marks]
5.2.4 Discuss the observations made while studying the phenomenon of self-excitation
of the D.C. Generator.
[6marks]
5.2.5 Plot the external characteristics of the D.C. generator (VT vs IL).
Using the value of the armature resistance, Ra of the D.C. Generator, determine the internal
characteristic of the DC Generator (EA vs IA).
[6marks]
Total: 30 marks Updated by A. Balgobin 11/09/2015
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