ECE 321 Experiment No:3
Energy Systems Lab 1 FALL 2010
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Experiment No. 3
DC Machines
OBJECTIVE:
The objective of this experiment is to record and compare performance data for a shunt and cumulatively compounded (long shunt) DC motor.
APPARATUS:
- 2 Test Tables
- DC Motor – Generator Set (lime green or cow)
- Motor Starter Box
- 1 – 100 field rheostat
- 1 – DC Power Supply, either a 5A Kenwood or a 18A Sorensen
- 1 – Single phase resistive load bank
- 3 – Multimeters (Used to measure motor armature voltage on the starter box, motor field current, and generator armature voltage)
- 1 – 50A DC Ammeter for generator armature current
- 1 – 75A DC Ammeter for motor armature current
- 2 – Ammeter insertion plugs
- 1 – Ammeter insertion plug with multimeter connectors
- 1 – Tachometer (Transducer and Display Box)
PROCEDURE:
1)Setup
a)Record the nameplate data of the machines used. Indicate which side is used as the motor and the generator. Note, for this experiment, it is best to use the machine with the higher rated current as the motor side.
b)Plug in the test tables.
c)Separately Excited DC Generator Connection: On the generator side, use the following instructions (1.c.i)-(1.c.vi) to set up a separately excited generator. Use Figure 1 as a visual reference.
i)Connect the 120V single phase resistive load bank to Terminals 1 and 2 on the output of the generator test table (right hand side). The generator armature current Iagen will flow through the generator test table to the resistive load bank.
ii)Connect Terminal 1 on the left hand side of the generator test table to A1 output of the generator panel. Similarly, connect Terminal 2 to A2.
iii)Connect the field of the generator to the output of the DC Power Supply. This will be used to supply the generator field current, Ifgen. (Note, this lab was written using the 5A Kenwood, but your team may be using a different brand of power supply.) Be sure to plug in the DC Power Supply. Be sure the knobs are turned all the way down.
iv)Use the 0-50A DC ammeter and an insertion plug to measure Iagen in Line 1 on the generator test table. Remember the proper method from DC Measurements lab (i.e. wait for the power to be on, close in switches to provide a path for current to flow, then ‘tap’ the insertion plug into the connector to make sure the meter up-scale).
v)Use a multimeter on the output of the voltage reversing switch to measure the armature voltage of the DC generator, Vagen. Be sure to protect the multimeter by opening the voltage circuit breaker switch before power is applied.
vi)Connect the tachometer to the generator shaft. Adjust the height so the tachometer cable is level with the shaft. Note the direction of the arrow on the top of the generator. The way the arrow is pointing will indicate the positive lead on the tachometer.
Figure 1: Separately Excited DC Generator
d)Shunt Excited Motor Connection: Use the diagram in Figure 2 to connect the other side of the DC motor-generator set as a shunt excited motor. A good method to follow is to color code the lines on Figure 2 and coordinate your diagram with the cables used in the lab. Always start with the machine side and work back towards the supply.
i)Connect a multimeter to the starter box to read the DC motor armature voltage, Vamotor.
ii)The motor field current, Ifmotor, and armature current, Iamotor, are displayed on the starter box. To obtain a more accurate reading, follow steps 1 and 2 below:
(1)Ifmotor: When facing the back side of the motor test table, there are four sets of terminals on the bottom. For Ifmotor, use the two sets of terminals on the right hand side (remember, facing the BACK of the test table). Connect F1 from the motor input/output panel to the second set of terminals. Then, run a cable from the second set of terminals, to F1 on the motor starter box.
(a)On the front bottom left of the motor test table, there is a line with a shorting switch and insertion plug connection below Line 4. This indicates the motor test table back connection.
(b)Use a multimeter and an ammeter insertion plug with banana plugs to measure Ifmotor at the insertion plug connection described in (d.ii.1.a.).
(2)Iamotor: Use the 0-75A DC ammeter and an insertion plug to measure Iamotor in Line 1 on the motor test table. Remember the proper method from DC Measurements lab (i.e. wait for the power to be on, close in switches to provide a path for current to flow, then ‘tap’ the insertion plug into the connector to make sure the meter up-scale).
iii)Turn on the field circuit breaker located on the 100 field rheostat.
iv)Check that the line circuit breakers are open.
v)Have the instructor check your connections.
Figure 2: Shunt Connected DC motor
2)Shunt Motor Performance
a)Read all Section 2 instructions before proceeding.
b)Close the circuit breaker on the 120V DC distribution panel.
c)On the motor test table, close the circuit breakers to complete the motor circuit.
d)Using the rheostat, set the field current to a maximum. The rheostat will be at a minimum.
e)Double check that the resistive load bank is set for an open circuit so Iagen = 0A.
f)Check for the rotation of motor by lightly raising the handle on the starter box.
i)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 until it clicks into a vertical position.
ii)If the motor rotates backwards;
(1)Stop by the motor by releasing the handle on the starter box
(2)Open the circuit break on the 120V DC distribution panel
(3)Reverse the field leads, (F1 and F2)
(4)Close the circuit break on the 120V DC distribution panel
(5)Try restarting.
g)Adjust the 100 field rheostat until the motor is rotating at nameplate speed. (Note: the generator and motor are connected by the same shaft, and therefore rotating at the same speed.)
h)Set the generator field current, Ifgen, equal to the motor field current, Ifmotor.
i)Measure and record Iagen, Ifgen, Vagen Ifmotor,Iamotor, Vamotor, and speed. This will be your data for the no load condition.
j)Maintain constant field currents during this experiment.
k)Use the resistive load bank to vary the generator load current, Iagen, to approximately no load, ¼, ½, ¾, and full rated generator load current.
(1)At each step, measure and record Iagen, Ifgen, Vagen Ifmotor,Iamotor, Vamotor, and speed. Motor speed will tend to decrease as the load increases.
l)After recording all of the data, follow the proper shut-down procedure:
i)reduce the generator field current Ifgen to zero and disable the output,
ii)use the load bank to systematically reduce Iagen to zero,
iii)decrease speed to a minimum,
iv)press the ‘stop’ button on the starter box and make sure the motor stops.
v)Open the circuit breakers on the motor and generator test tables and isolate all equipment.
vi)Finally, open the circuit breaker on the 120V DC Distribution Panel and disconnect the input cables from the distribution circuit box so you have a visual disconnect to your experiment setup.
3)Long Shunt Motor Performance
a)Connect the motor as a cumulatively compounded (long shunt) machine by taking the motor armature current into the series (S1) terminal on the motor input/output panel. Then connect S2 in to A1. Refer to Figure 3.
b)Using the Long Shunt Motor configuration, follow all of the instructions in Section 2 only through part l.ii where you reduce the generator armature current to zero. Wait for the instructor at this point.
Figure 3: Long Shunt DC Motor Configuration
4)Measuring rAtotal, rField, rbrush, rip and VBD
a)With Ifgen and Iagen at zero, the instructor is going to deflux the generator using the following method:
i)Double check that the generator armature is at no load and breakers are open so no current can flow
ii)With the DC supply at zero, reverse the generator field current leads one at a time.
iii)While monitoring Vagen, increase Vfgen using the DC supply until Vagen = 0V.
b)Open breakers on the table and open the circuit breaker on the 120V Distribution Panel.
c)Disconnect all leads to the starting box, motor-generator set and the resistive load bank.
d)Read all the instructions for Section 4 before proceeding. Talk about how you’re going to take the measurements before you actually do them.
e)Refer to Figure 4. Connect the armature of the generator through a test table and the single phase resistive load bank to 120V DC. Make sure the resistive load bank is at no load (open circuit).
Figure 4:Circuit set up for measuring rAtotal, rField, rbrush, rip and VBD
f)Turn on the circuit breaker on the 120V DC distribution panel.
g)To protect the machine, do not do the following for a long time.
h)Take 6-8 readings with current ranging from 0-70% of rated generatorcurrent. For each current level measure:
i)the current, Iagen,
ii)the voltage across the armature, Vagen, (can use a multimeter in parallel to measure Vagen between A1 and A2 on the generator input/output panel),
iii)the voltage across a brush, Vb1 or Vb2 which is measured between the brush pigtails and a commutator bar under the brush. The total brush drop voltage VBD=2Vb1=2Vb2
iv)the voltage between commutator bars under the two consecutive brushes, VC. Use the handheld multimeter to measure the voltages.
i)Systematically reduce the load bank to no load and then open the circuit breakers on the motor test table.
j)Open the circuit breaker on the 120V DC distribution panel and disconnect the input cables from the distribution circuit box so you have a visual disconnect to your experiment setup.
5)Measurement of rfield
a)Use a multimeter to measure the resistance of the shunt field, rfield, of both the DC machines. Simply measure across F1 and F2 on both of the motor and generator input/output panels. Record rfield_gen and rfield_motor.
Figure 5: Diagram of armature circuit.
Helpful Equations:
Vagen=Vt
2Ra=(Vagen-2Vb1)/Iagen (use the data from the experiment to calculate Ra)
Vt = Va = Ea + IaRa
Ea=kϕωτ = Ia*kϕsubstitute to find: τ = Ia*Ea/ω
REPORT: Please reference pages 539-543 in your text.
Show your raw and calculated (if applicable) data from each section in the discussion portion of your report.This will help you reference your data and support your results as you use the data throughout your report. Make observations and explain the cause and effects. Graph relationships if applicable. (As an example: The rheostat was used to adjust the motor field current. Decreasing the motor field current increased the speed of the motor-generator set as shown in Figure 1.)
Also, as this is a long report, you may need data, calculations and conclusions from DC Machines 1 experiment for a complete and thorough discussion of what was learned and observed about DC Machines in the last two experiments.
1)Shunt Motor Performance
In this section of the experiment,graph the torque versus speed curve for the shunt motor. Calculate the torque from the recorded data, and use the average value of armature resistance obtained from this experiment (Note: If you have one data point that is irregular, you may omit that data when taking your average).
2)Long Shunt Performance
In this section of the experiment, you will graph the torque versus speed curve for the long shunt motor.Calculate the torque from the recorded data, and use the value of armature resistance obtained from this experiment.
3)Graph both the torque vs. speed curves on the same graph. Compare the curves and explain your results. You may use your book to help explain the differences/similarities.
4)Lessons Learned
Following the conclusion section of the report, please answer the following questions:
a)What was done to change the speed of the generator? Please be specific.
b)Describe what differentiates a motor and a generator in the lab experiment setup. You may use a diagram along with your description if necessary.
c)When increasing the load current, what happened to the speed? Why?
Helpful Equations/Information:
- You may assume the armature resistance of the motor is the same as the armature resistance of the generator.
- A motor produces torque
- VT = The terminal voltage as described in your text. To make the lab clear, we specified Vagen and Vamotor. These are voltages across the stator armature terminals, hence the name terminal voltage.
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