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Lab #3 – Synchronous Motor
ECE 325 - Electric Energy System Components
Instructor: Dr. Kai Sun
Lab TA’s: Denis Osipov and Wenyun Ju
Objective:
The objective of this lab is to examine the terminal characteristics of a synchronous motor under load and
no-load conditions.
Pre-lab:
1. What is an important feature of synchronous motors?
2. Draw an equivalent circuit model for a synchronous motor. Show Ef, Xs, Ra, Ia, Vt, If, and the field
winding.
3. Describe the open-circuit test. What is measured? What is varied? Sketch the circuit.
4. Describe the short-circuit test. What is measured? What is varied? Sketch the circuit.
5. Sketch an open-circuit characteristic and a short-circuit characteristic. Show Ia (rated), Vt (rated), and
a modified are gap line.
Lab Exercise:
Part A
The open-circuit and short-circuit tests will be performed to determine values for Xs and K, using the
synchronous machine as a generator.
Procedure:
1. Connect the DC machine as a self excited shunt generator as shown in Lab #3 Part A wiring
diagram.
2. Couple the DC machine to the synchronous machine.
3. Set the 0-125 VDC knob to zero.
4. Set the 0-150 VDC knob to zero.
5. Set the synchronous machine to ‘IND START’.
6. Bring the DC machine terminal voltage up to 125 V, and adjust the field rheostat to obtain a speed of
1800 RPM.
7. Set the synchronous machine to ‘SYNCH RUN’.
8. Perform the open-circuit test: With the synchronous machine’s terminals open, measure the voltage
across the winding (Voc) as field current (If) in increased. Increase the field current by increasing the
DC voltage across the field winding. Vary If from 0 to 1A in increments of 0.1A.
9. Set If to zero.
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10. Perform the short-circuit test: Short the phase windings of the synchronous machine and put an
ammeter in series with the shorted windings. Measure the average winding current (Isc) as If in
increased. Vary If from 0 to 1A in increments of 0.1A.
Calculation:
1. At Vt=VOC=120V, calculate Xs and K. (Xs= VOC / Isc and K= VOC / If)
2. Plot |Ia| vs. If. (|Ia| =|( Vt -K* If)/Xs|)
Part B
The armature current will be measured as field current is varied under load and no load conditions.
No Load Procedure:
1. Connect the system as shown in Lab #3 Part B wiring diagram, but do not couple the synchronous
machine to the DC machine.
2. Set the synchronous machine to ‘IND START’.
3. Turn the 0-150 VDC knob to position ‘0.5’.
4. Turn on main power.
5. Set the synchronous machine to ‘SYNCH RUN’. The motor speed should be at 1800 RPM.
6. Measure Ia and If as If is varied up to 1A in increments of 0.1A.
Load Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Turn off main power.
Mechanically couple the synchronous machine to the DC machine.
Set the first eight switches of the resistance load to ‘ON’.
Set the synchronous machine to ‘IND START’.
Turn on main power.
Turn the 0-150 VDC knob to position ‘2.0’.
Turn the 0-125 VDC knob to position ‘8’.
Set the synchronous machine to ‘SYNCH RUN’. The motor speed should be 1800 RPM.
Measure Ia and If as If is varied up to 1A in increments of 0.1A.
Calculation:
1. Plot Ia vs. If for load and no load conditions and compare with calculated graph.
2. Is there a best field current at which the synchronous machine should be operated?
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Lab #3 Part A Wiring Diagram
POWER PANEL
120V AC
0-240/140V AC-30-4W
A
B
C
120V AC
N
0
N
PRI 0-150V DC SEC
MAIN AC
A
B
C
N
+
PRI 0-125V DC SEC
-
+
-
VM
(DC)
+
+
SYNCHRONOUS MACHINE
THREE PHASE AC
IND
START
+
AM
(DC)
120V AC
-
-
1
FIELD RHEOSTAT
2
1
SHUNT FIELD
2
WINDING
DC FIELD
ARMATURE
1
2
1
3
2
SERIES FIELD
1
2
T1
4
T1
5
T2
T2
6
T3
L1
L1
L2
L2
L3
Short-circuit test
Open-circuit test
1
2
3
4
5
6
1
2
3
-
VM
(AC)
+
+
3
6
5
4
AM
(AC)
-
Lab #3 Part B Wiring Diagram
POWER PANEL
120V AC
0-240/140V AC-30-4W
A
B
C
120V AC
N
0
AM
(DC)
-
PRI 0-150V DC SEC
MAIN AC
N
A
B
C
N
+
PRI 0-125V DC SEC
-
+
120V AC
-
+
SYNCHRONOUS MACHINE
THREE PHASE AC
IND
START
+
-
1
FIELD RHEOSTAT
1
SHUNT FIELD
2
+
AM
(AC)
WINDING
2
DC FIELD
ARMATURE
1
2
1
3
2
SERIES FIELD
1
4
T1
L1
5
6
T2
T3
L2
2
T1
T2
L1
L2
L3
RESISTANCE LOAD
1
2
3
4
5
6
8
7
ON
OFF
1000Ω
1000Ω
1000Ω
1000Ω
750Ω
750Ω
750Ω
750Ω
100Ω
100Ω
150Ω
150Ω
250Ω
250Ω
375Ω
375Ω
LOAD
ON
OFF
16
15
14
13
12
4
11
10
9