DC Machines Lab

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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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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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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http://myelearning.sta.uwi.edu/

mailto:[email protected]

mailto:[email protected]

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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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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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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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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4.2.1. Using the diagram in Figure 2, connect the generator as a separately excited machine.


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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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.


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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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.


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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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


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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Table 2: Load Test Results

If/Amps Load Current, IL/Amps Terminal Voltage,VT/Volts

Armature resistance =

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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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DC Machines Lab

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