ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
Experiment No. 2-1
DC Machines
Objectives:
The objectives of this experiment are to investigate the operation of dc generator under load,
determine the characteristic Kaφ, the resistances rA (total resistance rA= ra + rb, where ra is the
armature resistance and rb is the total brush resistance), rF (field winding resistance), rb (brush
resistance) and the brushdrop voltage VBD for a particular DC machine.
Apparatus:
1. 2 test tables
2. DC motor-generator set (lime green or cow)
3. 1 - 100 Ω field rheostat
4. 1 - 5 A Kenwood DC power supply
5. 1 - 1 phase resistive load bank
6. 4 Multimeters (To measure armature voltage on the Starter box, Field current for the motor
using insertion plug, motor terminal voltage, and generator armature voltage.)
7. 1 - 0-50 A DC Ammeters for generator armature
1 – 0-75A DC Ammeter for motor armature
2 - Ammeter insertion plugs
1 - insertion plug with multimeter connectors
8. 1 Tachometer
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
Procedure:
1.Setup
(a) Record the nameplate data of the machines used.
(b) Plug in the test table.
(c) Separately Excited Generator: Connect one of the machines of the DC motor-generator set
as a separately excited generator. Please reference Figure 1.
(i)
Connect the shunt field of the generator to the output of the 5A Kenwood DC power
supply. Be sure to plug in the power cord for the Kenwood.
(ii)
Connect the resistive load bank in parallel to the 120V connection.
(iii)
Connect the resistive load bank through the test table to the armature of the DC
generator. Keep the switches on the left side of the table open.
(iv)
Connect one of the 0-50 A DC ammeters such that the armature current of the DC
generator can be measured.
(v)
Connect a multimeter such that the armature voltage of the DC generator can be
measured. (Connect into the reversal switch on the test table. Be sure the selection knob
is selecting the correct lines on the voltage test table.)
Figure 1: Separately Excited DC Generator
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
(d) Shunt Excited Motor: Connect up the other machine of the DC motor-generator set as a
shunt excited motor. Please refer Figure 3 ON THE NEXT PAGE. A good method to follow
your connections in an orderly fashion is to color code the lines on the diagram and
coordinate with the color of connectors you use in the lab. Always start from the machine
side when making connections!
(i)
Connect a multimeter to the starter box such that the armature voltage of the DC motor
can be measured.
(ii)
The field and armature current of the motor should be measured from the starter box.
Alternatively, when connecting through the test table, the field current can be measured
with a multimeter from the table ‘back connection’ line. (Instructor will demonstrate.
This is where you will need the insertion plug with the multimeter connectors) The
armature current can be measured on line through the test table with the 0-75A DC
Ammeter.
(iii)
Turn on the field circuit breaker located on the 100 Ω field rheostat.
(iv)
Connect the tachometer to the shaft of the MG set. Adjust the height of the tach so that
the connecting line is level with the shaft.
Note the direction of rotation arrow on the MG set, and connect the tachometer to the end
that will not cause the connector to unscrew while the machine is operating.
(v)
Make sure the switches on the left side of the table are off and insure that the ammeter
shorting switches are closed.
(vi)
Have the instructor check your connections.
Figure 2: Shunt Connected DC motor.
(Do not use this diagram for making connections!!)
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
Figure 3: Connection of starter box to motor
2. Measure the magnetization curve of the DC generator
(a) Close the circuit breaker on the 120 VDC supply panel.
(b) Turn on the switches on the left side of the table that are connected to the motor.
(c) Set the field current to a maximum i.e. set rheostat to minimum.
(d) Check for the rotation of motor by lightly raising the handle on the starter box. If the rotation is
in the correction direction and the generator field current set at zero, start the motor by slowly
rotating the handle of the starter box. Pay attention to the armature current on the starting box
to make sure that it doesn’t spike, and bring the handle all the way up.
If the motor rotates backwards;
i. stop the motor,
ii. open the circuit breaker on the 120 VDC panel,
iii. reverse the shunt field leads (F1 and F2),
iv. close the circuit breaker on the 120 VDC panel,
v. try restarting.
(e) Adjust the 100 Ω field rheostat until the motor is rotating at the nameplate speed of the motor.
Continuously adjust the field rheostat such that the machine stays at the nameplate speed during
the load and KAφ tests.
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
(f) Record the generator armature voltage due to the residual flux.
(g) Note: To connect the Kenwood safely, be sure the voltage knob is turned all the way down
before you enable the output.
(h) With no load on the generator and the motor rotating at rated speed, use the Kenwood DC
supply to increase the field current of the generator from zero to a value so that Vagen is 110% of
rated voltage.
Incrementally increase the field current in 6 to 8 steps. Keep the speed at rated. Measure the
armature voltage, Vagen, and the field current, Ifgen. The generated emf equals the armature
voltage with no generator armature current; i.e., Eg = Vagen
WAIT FOR OTHER GROUP TO REACH THIS POINT – THE NEXT SECTION NEEDS TO BE DONE SIMULTANEOUSLY.
3. Measure the magnetization curve at 3/4 of the rated speed.
(a) Adjust the 100 Ω field rheostat until the motor is rotating at the lowest speed possible.
(b) The laboratory instructor will adjust the supply voltage such that the speed is 3/4 of rated
speed.
(c) With no load on the generator and the motor rotating at 3/4 of rated speed increase the field
current of the generator from zero to a value so that Vagen is 80% of rated voltage. Increase
the field current in 6 to 8 steps and approximately match your field current readings from
part 2(h). Keep the speed at ¾ rated. Measure the armature voltage, Vagen, and the field
current, Ifgen. The generated emf equals the armature voltage with no generator armature
current; i.e., Eg = Vagen
(d) Leave the motor rotating.
4. DC Generator Load Test (Rated Speed)
(a) The laboratory instructor will return the supply voltage to 120 V DC.
(b) Adjust the 100 Ω field rheostat until the motor is rotating at the nameplate speed
(c) Adjust the generator field current until the generator terminal voltage equals 120 V DC. Keep
the generator field current constant at this value during the load test.
(d) Measure Ia (armature current), If (field current), V (terminal voltage) of the motor.
(e) Keeping the field current constant, for no load, 1/4, 1/2, 3/4, full load on the generator, measure
and record: Ia,gen, Ia,motor, VT,motor, Va,gen Ifmotor.
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
Be sure to have a person monitoring each of the ammeters to not exceed any limits (either the
50A or 75A of the meters or the machines nameplate values, whichever is lower). Also, be sure
to systematically change the load to avoid any large changes in current which may damage the
equipment.
(f) Decrease the load current to a zero.
(g) Turn off the armature circuit switches on the left side of the generator test table.
(h) Bring the speed to a minimum value with the rheostat that controls the motor field current.
(i) Turn off the motor by pressing the stop button on the starting box.
(h) Turn off the switches on the left side of the motor test table.
5. Measurement of rA, rF, rb, rip and VBD
(a) Make sure that the circuit breaker on the 120 VDC panel is open.
(b) Disconnect all leads to the starting box, motor-generator set, and the resistive load bank.
(c) Connect the armature of the motor through a test table and the 1-φ resistive load bank to the
distribution box. Make sure that originally the resistance bank is open circuited.
(d) Turn on the circuit breaker on the 120 VDC panel.
(e) To protect the machine, do not do the following for a long time.
(f) Take six readings with current ranging from zero to 110% of rated current. For each current
level measure the current, Ia the voltage across the armature, Vt (between A1 and A2), the
voltage across a brush, VBD/2=Vb1 or Vb2 (between the brush pigtails and a commutator bar
under the brush), and the voltage between commutator bars under the two consecutive brushes
VC. Use the handheld multimeter to measure the voltages.
(g) Open the circuit breaker on the 120 VDC panel.
Table
A1
Supply
120 V
+
_
A2
Line 1
Load
Line 2
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
Figure 4 :Circuit set up for Part 5
Vt
Vb1
Vc
ra
Vb2
ra
Figure 5: Diagram of armature circuit.
6. Measurement of rf
(a) Use a multimeter to measure the DC resistance of the shunt field resistance, rf, of both the DC
machines.
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ECE 321
Experiment No:2
Energy Systems Lab
FALL 2009
_______________________________________________________________________________________
Report:
1. From the readings of Part 2
(a) Plot Eg versus Ifgen.
(b) Kaφ versus Ifgen, where Kaφ = Eg/ω
(c) Use MathCAD’s cspline and interp functions to plot 40 points from a curve fit of Kaφ on the
same graph as Kaφ.
Note: Plot both Eg and Kaφ on the same graph, but with different scales.
(d) Predict and comment on the magnetization curve for higher field currents.
2. From the readings of Part 3
(a) Plot Eg versus Ifgen.
(b) Kaφ versus Ifgen, where Kaφ = Eg/ω
(c) Use MathCAD’s cspline and interp functions to plot 40 points from a curve fit of Kaφ on the
same graph as Kaφ.
Note: Plot both Eg and Kaφ on the same graph, but with different scales.
(d) Predict and comment on the magnetization curve for higher field currents.
3. From the readings of Part 4
Plot Vagen versus Iagen,
4. From the readings of Part 5
(a) Plot VBD versus Ia
(b) Select reasonable values for rb and VBD
(c) Plot ra versus Ia
where: ra ≡ rA – rb = (Va-VBD)/Ia
5. From the armature resistance value obtained from the experiment, calculate the starter
resistance that would be required to be inserted in series with armature to limit the starting
current to twice the rated value.
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