Download Lab 3

ECE 385
Lab 3 DC Synchronous Machines and Alternators
Due: Oct 18, 2007
Group 27Bravo
Authors:
David Gitz
Scot Shelton
Salman
B. Operation of the Alternator
Objective:
To become familiar with the operation of the alternator including secure shaft alignment, front
panel connections, and speed regulation; to measure the dc resistance of the stator.
Procedure:
1. Identify the DC synchronous machines of the alternator. Set the machines on the metal
plate and couple, the axes together through the double-head rubber coupler. Use the guard
around the coupling head. Spin the machine shaft manually to ensure the alignment. Turn
the plate screw tight to preserve the alignment.
2. Record the data off the name plates of the synchronous generator and the dc motor.
3. Configure the terminal panel of each machine as shown in fig. 7. Make sure the console
power is turned off. Do not connect to source or instruments yet. (a) Connect the dc motor
shut (self-excited); adjust the field resistance to the minimum value; set the Start wwitch to
off (down position). (b) Connect the synchronous generator stator terminals as Y—stator
terminals 1,2 and 3 are the line terminals; terminals 4,5 and 6 connect to the neutral—set
the machine rotor circuit to synchronous run mode (top flip-switch).
4. Stator Resistance Measurement: Use the handheld multi-meter, measure and record the
resistance between any two line terminals of the stator (say 1 and 2).
5. Connect the dc motor line terminals (L1-L2) to the 0-125 Vdc variable source. Make sure the
source knob is turned to zero. Connect the synchronous motor filed circuit to the 0-150 Vdc
variable source through a dc ammeter. Make sure the source knob is turned to zero. Do
not turn the console power on yet.
6. First turn on the console main power. Then turn on the 0-125 Vdc source. Set the start
switch on the dc motor panel in the up position (start). Gradually increase the supply
voltage, the motor will begin to run.
7. Speed Regulation: a) Set the digital stroboscope rpm dial to the synchronous rpm of the
generator (what you read on the name plate). B) Hold the stroboscope against the rotor
and press the trigger(held like a gun) it will help if the stroboscope is aimed in the front of
the rotor against the head. c) Adjust the dc motor supply until the rotor appears to be still
(or as close as you can possible get it to be still). When the rotor appears still to the eyes,
the rotation speed equals the stroboscope dial setting. Practice adjusting the spped for a
different value of the rpm. Do not power down, continue to the next experiment.
Results:
4. L1-L2: 8.9 Ohms
C. Open-Circuit Characteristic
Objective:
To obtain the excitation curve of the generator.
Procedure:
1. With the dc motor running at synchronous speed, turn the 0-150 Vdc supply of the
generator filed on. Adjust the field supply to adjust the generator field current as described
in the steps below.
2. Beginning from zero, increase the generator field current in steps of .25A until 1.25 . At
each step readjust the rotor speed to synchronous using the stroboscope, then with the
handheld voltage meter measure the line-to-line rms voltage across the stator terminals.
Record the values of the field current and the stator voltage.
3. Verify the line-to-neutral and line-to-line voltage relations: Adjust the generator field
current until the stator voltage between terminals 1 and 2 is 208V. Verify that the rpm rotor
speed is the synchronous. Record the field current. Without changing the generator field or
rotor speed, repeat the measurement across the terminals 2 and 3, 3 and 1, and also across
1 and the neutral, 2 and the neutral, and 3 and the neutral. Record all values. Return the
Questions/Problems:
1.
Synchronous Motor Nameplate
Alternating Current
Type:
SM-100
Volts:
PH:
No.:
N
208 Amps:
1.7 H.P.:
33
3 Cyc:
60 RPM:
1800
Code:
Duty:
Cont.
DC Machine Nameplate
Direct Current
type:
DM-100
Volts:
No:
125 Amps:
Winding:
RPM:
Compound
Duty:
1800 °C Rise:
2.
5243
2.4 H.P.:
.3kW
Continuous
40
The rated stator power of the Synchronous Motor is .33 Horse Power. Since it is a 3 Phase
Generator, there are 6 poles. The Stator Resistance per Phase is 8.9 Ohms, since it is Yconnected.
3. The Stroboscope works by setting the frequency of the light emitted to match the shaft
speed, and due to the nature of the human eye when the two frequencies match it will
appear that the shaft is standing still.
4. The synchronous machine needs a DC field to generate the excitation current that is
required in the stator.
I:
VacLL:
VacLN:
0.25
57.00
32.91
0.50
105.20
60.74
0.75
130.60
75.40
1.00
144.00
83.14
1.25
148.90
85.97
1.45
154.30
89.09
5.
The under-excited region on the graph is where the air-gap line is, when the current is from
.25 A to .5A. the over-excited region on the graph is when the current is from 1 A to 1.5 A.
6.
V1-V2
V2-V3:
V3-V1
V1-N
V2-N
V3-N
Voltage
Expected Line-Neutral
208.15
-208.30
-208.10
-124.00
120.18
123.00
120.26
124.80
120.15
Yes, the stator is symmetric and balanced because the Line-to-Line voltages are similar and
the Line-to-Neutral voltages are similar. However, the Expected Line-Neutral and the actual
values are off by ~4 V, but this would not indicate a un-balanced load.
Graphs:
D. Short-Circuit Characteristic
Object:
To obtain the SCC of the generator.
Theory:
SCR 
I f1
If2
Procedure:
1. With all the supplies off (knobs turned to zero) and the console off, short the stator
terminals 1,2, and 3. Short terminals 1 and 2 through an ac ammeter.
2. Power up the console and the dc motor supply. Gradually increase the motor speed to the
synchronous rpm.
3. Turn the generator field on and adjust the field current to zero.
4. A) Measure the stator current when the field is current is zero b) slowly increase the field
supply until the stator current is ¼ of its rated value. Record the field current. C) Slowly
increase the field supply until the stator current equals ½ of its rated value. Record the field
current. D) Slowly increase the field supply until the stator current equals its rated value.
Record the field current.
5. Reduce the field current to zero. Power the field supply off. Reduce the motor speed to
zero. Power the motor supply off. Power the console off. Break off the settings.
Results:
Questions/Problems:
1.
Field Current
Stator Current
0.000
0.024
0.425
0.758
0.850
1.500
1.350
2.000
2.
I f 1 was 1A which we found from part C, #6. I f 2 would be the current when the stator
current is at its rated value: 2.4 A. However, we were never able to measure that much
stator current; the highest we saw was 2.0 A. We extrapolated a value for I f 2 to be about
1.6A when the stator current is 2.4 A. SCR 
I f1
If2

1
 0.6 .
1.6
3.
4. The unsaturated field current is 0.25 A.
5. At the rated voltage (143.76 V), we have 1A of field current, and the synchronous reactance
is about 140 jΩ.
6. The excitation current will be 0.59 A as defined below.
Zeq = sqrt(140^2 + 8.9^2) + 100
Zeq =
240.2826
>> Iex = 143.76/Zeq
Iex =
0.5983
7.
jXs
1
Rs
2
140 jOhm
8.9 Ohm
Es
100 Ohm
RL
VL=143.76
Ix=.59A
Conclusion:
It is necessary for an AC motor to have an excitation current in place before it will start turning,
this will set up the rotating magnetic field. It is also necessary to vary the excitation current based on
the operating load to keep the operating voltage a constant.