EE2683 Lab Manual

7/31/2019 EE2683 Lab Manual

1/13

EE2683

Electrical Circuits and Machines

Laboratory Manual

University of New Brunswick

Department of Electrical and Computer Engineering

March 2008


2/13

page 2 of 11

EE2683 Electric Circuits & Machines

Experiment #1: Phase relationships in A-C circuits

Purpose:

The purpose of this experiment is to review and reinforce your understanding of the phase

relationships in series and parallel a-c circuits. It should also enable you to get familiar with the use

of a dual beam oscilloscope and to conduct basic measurements and calculations.

Equipment:

The equipment required includes a Dual beam oscilloscope, an oscillator, individual R, L, C

components, and digital meters.

Experiment:

1. Equipment Familiarization

Familiarize yourself with the dual beam oscilloscope, including identification of the two inputs and

all adjustable settings. Start it up and get two horizontal traces.

2. Understanding the time base on the scope

a) Calculate the time period for the sweep circuit to display one cycle of a 60 Hz wave.

b) Display a 60 Hz voltage waveform on the screen. Check the calculated period, and that obtained

from oscilloscope measurement.

3. Resistance Measurement

The test circuit you will be using in this experiment is shown in Figure 1-1below. Use the digital

multi-meter as a digital ohmmeter.

a) Measure the resistance of the resistor.

b) Measure the resistance of the inductor.

c) Assume that the resistance of the capacitor is infinite.

Figure 1-1. A Simple RLC Test Circuit.

4. Reactance Calculation

Values of L and C will be provided in the lab. Calculate XL and XC at 1000 Hz.


3/13


4/13

page 4 of 11


Experiment #2: Transformers

Purpose:

The purpose of this experiment is to examine the operating characteristics of a transformer.

Equipment:

The equipment required includes a single phase transformer, a voltmeter, ammeters, a variable ac

supply, and loading resistors.

Experiment:

Observe how a transformer behaves relative to the predicted performance. A comment should be

added about the transformers being used. Most transformers have two windings, but the one being

used has three windings (even though only two windings are used for this experiment). Besides

that, all the windings have tap positions which means that we can use part or all of the turns in each

winding.

1. Nominal Voltage Ratios

Record the nominal voltage ratio (or turn ratio) of winding 1-2 (primary) and winding 5-9

(secondary).

2. Measured Voltage Ratios

Apply rated voltage of 120 volts to winding 1-2, and measure the open circuit voltage at winding 5-

9.Compare the measured and the nominal values.

3. Predicted Currents

If winding 5-9 carries rated current, calculate the current in winding 1-2.

4. Measured Currents

Arrange a circuit with winding 5-9 connected to a resistor bank to measure the currents calculated

in (3), and have it checked by the instructor. Energize the circuit and measure the primary and

secondary currents. Compare the measured and predicted currents.

5. Voltage Regulation

Using the same circuit as for part (4), making sure that the applied voltage is 120 volts as in Part

(2), measure the voltage at 5-9.

a) How does this value of loaded secondary voltage compare with the open circuit value of voltage

measured in Step (2)?

b) Calculate the value of voltage regulation (VR).


5/13

page 5 of 11

6. Polarity Checks

Assume that winding 1-2 has a polarity mark at position 1.Apply 120 voltages to this winding, with

the other winding open-circuited. Take a single lead and connect terminal 2 to terminal 9.Place a

voltmeter between terminal 1 and terminal 5.

a) What two values could the voltmeter read?

b) Based on the value that the voltmeter reads, decide whether a polarity mark should be placed on

terminal 5 or on terminal 9.

Take the circuit apart.

7. Saturation Presence

Use a variable ac voltage (line-to-line voltage of the supply module) as the source to winding 1-2,

with winding 5-9 open circuited. Place an ammeter in winding 1-2 to measure the excitation

current. Measure and plot the applied voltage versus excitation current in 15 volt steps from 0 to

135 volts.

Comment on any saturation effect which may be present.

Report:

Complete the writing up of the report and hand it in within 1 week.


6/13

page 6 of 11


Experiment #3: Direct Current Machines

Purpose:

To investigate the voltage generation characteristics of a dc generator, and the speed-load control

characteristics of a dc motor. A dc machine may operate either as a dc generator or as a dc motor.

Equipment:

A dc shunt motor/generator, ammeter, voltmeter, tachometer, load resistor, a prime mover (i.e.

synchronous motor) for the generator, a load for the motor, a variable voltage supply.

When experimenting with electric machines, there is always some equipment required. Every

generator needs a prime mover, and in our case we are going to use a synchronous motor as a drive

because it operates at constant speed. Every motor needs a load, and here we are going to use a

synchronous generator supplying a resistor bank as a load.

Experiment:

DC Generator Tests

1. Prime Mover

Arrange a synchronous motor as a prime mover using the circuit shown in Figure 3-1. Your

instructor will show you how to start the synchronous motor. Keep the same connection for all the

DC generator tests.

Figure 3-1. Synchronous Motor Connections.

2. Generator Open Circuit Generation Curve

Connect the generator as a separately-excited machine as in Figure 3-2, but do not connect the load

resistor. Place an ammeter and an adjustable resistor in the field circuit as shown. Connect a

voltmeter across the armature of the d-c generator in order to read the terminal voltage.


7/13

page 7 of 11

Figure 3-2. Separately Excited DC Generator Connections.

Measure the values of field current and terminal voltage, by varying the field current from zero to

the maximum terminal voltage of about 120 volts. Plot the resultant no load (or open circuit) voltage

as function of field current.

3. Voltage Regulation With Separate Excitation

Again, connect the generator as a separately-excited machine (Figure 3-2).Make the necessary

adjustments in the field to give rated voltage.

Load the generator in steps up to the rated current (1.0 A) by use of a load resistor bank. And take

readings of terminal voltage and load current. Plot terminal voltage versus load current. By what

percentage does the voltage drop as the load goes from zero to rated?

4. Voltage Regulation With Self Excitation

This is a repeat of Part (a.3) except that the dc machine should be self-excited (shunt-excited

generator as in Figure 3-3).

Figure 3-3. Shunt Excited DC Generator Connections.

After taking the measurements, plot terminal voltage versus load current on the same graph as for(a.3).By what percentage does the voltage drop as the load goes from zero to rated?

Note: This ends the part of the experiment on dc generators. Disconnect the synchronous motor. We

now use the machine as a dc motor.


8/13

page 8 of 11

DC Motor Tests

1. DC Motor Starting

Arrange the synchronous machine as a generator in order to provide a load for the dc motor under

test as shown in Figure 3-4. Your instructor will check your circuit.

Figure 3-4. Synchronous Generator Connections.

A schematic motor hook up diagram for the dc motor is shown in Figure 3-5. Note that there are a

variable resistor in the field circuit, and a variable voltage source for the armature. Also, note that

there are ammeters in both the armature circuit and the field circuit.

Figure 3-5. Separately Excited DC Motor Connections.

Set the field resistance to its minimum. Start the motor up by means of the variable armature

voltage source, and finally set the armature voltage at rated. Then adjust the variable field resistor

to give rated speed at the no load condition.

2. Loading the Motor

Load the motor so that the armature current is approximately 75% of its rated value of 2.8 A by

varying the switch settings on the 3 load. Record the speed of the motor (one value).

3. Speed Control by Variable Armature Voltage

Vary the armature supply voltage from 100% to 75% of the rated value. Record the range of speed

control that results and the corresponding armature voltages used (two sets of data).

Return the source voltage to the rated value.


9/13

page 9 of 11

4. Speed Control by Variable Field Resistor

Adjust the variable field resistor over its entire range (unless the speed gets excessively high, do not

exceed 2500 rpm). Record the range of speed control that results and the corresponding field

currents used (two sets of data).

Report:

Complete the writing of the report and hand it in within 1 week.


10/13

University of New Brunswick Department of Electrical and Computer Engineering

w11_ee2683lm.docx 14 March 2011 @ 9:51 AM page 11 of 14


Experiment #4: Alternating Current Motors

Purpose:

The purpose of this experiment is to examine the the loading characteristics of both induction

and synchronous motors.

Equipment:

This experiment requires an induction machine, a synchronous machine, a dc machine, a load

resistor bank, a tachometer, a sorobe light, and four Digital Voltmeters.

Motor Torque Measurement:

Since you will be testing the load characteristics of motors you will have to provide a variable

load for them. A seperately excited dc generator will be used for this purpose. A connection

diagram for the dc generator is shown in Figure 4-1.

Figure 4-1. The DC Generaqtor Load Connections.The loading on the motors is determined by the power dissipated by the 3 Phase load resistor

module connected to the dc generator. The load power can be adjusted by varying the field

current of the generator, Ifg, or by changing the load resistance using the switches on the front

pannel of the resistor module.

To calculate the loading on the motors under test, measure the motor speed, N, the armature

voltage,Va and armature current, Ia , of the DC generator. The motor torque can then be

estimated using the relationship

Tm VaIa

m

=

60VaIa

0.752N=

40

VaIa

N=12.73

VaIa

N. (4-1)

Note that the speed is in radians/second, not RPM, and the efficiency, , is approximatly 75%.

Experiment:

1. Connect the induction motor as shown in Figure 4-2. Connect meters to monitor both the line to

line ac voltage to the motor, and the current for one of the stator windings. You should test the

operation of the motor before you connect up the dc machine to load the motor.


11/13



Figure 4-2. Induction Motor Connection Diagram.

To start the induction motor:

a) Turn the voltage control knob on the power supply all the way counterclockwise before you

turn on the power supply.

b) Turn on the power supply and slowly turn voltage control knob clockwise. The motor should

start turning when the control is at about 30 on the dial.

c) Continue slowly turning the Voltage control clockwise until it is turned fully clockwise. At

no load, the induction motor should be running at about 1750 RPM.

To turn the Induction motor off:

a) Turn the stator voltage down by turning the voltage control knob counterclockwise until the

applied voltage is zero.

b) Wait for the motor to stop turning and then turn off the power supply.

2. The speed, N, of an induction motor will change as the loading on the motor changes. This

behavior is characterized using a torque-speed curve. To experimentally determine the

torque-speed curve for your induction motor, start up the induction motor with no load, and

increase the the loading until the motor current, Im

, reaches the rated value. Record the no load

value ofVm. Record the speed of the inducton motor, N, and both the armature voltage, Va , and

current, Ia , of the dc generator in Table 4-1 for various values of Im.

Table 4-1. Induction Motor Loading Data.


12/13



When you have finished your measurements, shut down the motor and disconnect it. You should

leave the dc generator connected if another group is waiting for your motor.

Calculate the the output torque of the inducton motor using the expression of Equation 4-1, and

plot the resulting torque-speed characteristic.

3. Connect up your synchronous machine as a motor using the diagram shown in Figure 4-3.

Figure 4-3. Synchronous Motor Connections.

` To start the synchronous motor:

a) Make sure the field winding is turned off and the voltage control knob on the power supply is

turned all the way counterclockwise before you turn on the power supply.

b) Turn on the power supply and slowly turn voltage control knob clockwise. The motor should

start turning when the control is at about 30 on the dial.

c) Continue slowly turning the voltage control clockwise until it is turned fully clockwise.

d) When the motor has had a chance to reach a constant speend, switch on the field current.

The motor will synchronize with the frequency of the stator voltage and the stator currentshould drop. It the stator current increases, the motor did not synchronize correctly. In this

case, turn the field current off, temporarially reduce the stator voltage to roughly 50% and

then bring it back to the rated value and repeat the synchronization process. Repeat until

the motor properly synchronizes with the line voltage.

e) Adjust the field current, Ifm, for minimum stator current.

To turn the synchronous motor off:

a) Turn off the field current.

b) Turn the stator voltage down by turning the voltage control knob counterclockwise until the

applied voltage is zero.

c) Wait until the motor stops turning and then turn off the power supply.

5. The speed of a synchronous motor does not change as the motor is loaded. Therefore it is not

possibleto measure a torque-speed characteristic for a synchronous motor. However, loading

the motor causes the spatial relationship between the rotating magnetic field generated by the

stator currents and the rotor, the space angle or torque angle of the motor, to change.


13/13



When the load on a synchronous motor changes, the machine speed momentarily changes as it

goes to a new torque angle. However, it always quickly locks onto synchronous speed, unless

the new load is sufficient to cause the motor to fall out of synchronization.

Observe that there is a black mark on the shaft of your motor. A strobe light which is

synchronized with the AC line frequency will make this mark appear to be stationary when themotor is running at synchronous speed. However, the position of the mark will chnge when the

loading on the motor changes. This enables a crude measurement of the space angle of the

motor.

Start up the synchronous motor with no load on it. Once you have it synchronized, adjust the

field current for the minimum motor current, Im

, and record the measured values of the field

current,Ifm, and mtor line-to-line voltage,Vm, in Table 4-2.

Vary the load on the synchronous motor from no load to full load and record the motor current,

Im

, the space angle of the synchronous motor, and both the armature voltage, Va , and current,

Ia , of the dc generator the dc generator in Table 4-2 for various values of

Im.

Table 4-2. Synchronous Motor Loading Data.

When you have finished your measurements, shut down the motor and disconnect it. You should

leave the dc generator connected if another group is waiting for your motor.

Calculate the load torque on the motor using the expression of Equation 4-1 and then plot the

load torque versus the space angle characteristic. (You should note that this is a fairly crude

curve in that measurement of the angle is obviously not precise, but rather an indication.)

Report:

Complete the writing of the report and hand it in within a week.

Documents

EE2683 Lab Manual