Instructor: Assist. Prof. Dr. Sener AGALAR
TA: Res. Asst. H.Ersin EROL
Res. Asst. Mustafa DEMİRTAŞ
ANADOLU UNIVERSITY
DEPT. OF ELECTRICAL AND ELECTRONICS
ENGINEERING
EEM 471 ELECTRICAL MACHINERY
LABORATORY
EXPERIMENT V
3Φ Synchronous Machine
 Survey of the Magnetization Characteristic Curve
 Survey of the Short-Circuit Characteristic Curve
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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SURVEY OF THE MAGNETIZATION CHARACTERISTIC CURVE
The magnetization characteristic curve of an alternator is the graph showing the flux
generated by the poles as a function of the excitation magnetomotive force. In the common
technical language, anyway, the same name is given to the V0 = f( Iexc ) diagram, that shows the
same shape but is much more easily measurable. The perfect coincidence between the two
diagrams is evident considering that:
1) The V0 voltage generated by the machine in no-load conditions is perfectly coincident
with the induced e.m.f. In constant speed operation, therefore, it is directly
proportional to the inducing flux.
2) The excitation current and the magnetomotive force only differ for a coefficient which
is expressed by the turn number of the poles winding.
The importance of this characteristic curve in the alternator testing is very high; in fact, to
it all the indirect determination methods for the under load operation conditions (that follow
mainly graphic methods) make reference.
ELECTRIC DIAGRAM
NOTE : The test must be performed at the constant rated speed.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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NOTES:
a) The magnetization characteristic curve has to be measured, as said above, at the rated speed
of the machine, to which the rated output frequency corresponds. If that isn't possible, the
measurement can be still performed, but executing a reporting of all the testing results to
the rated speed. The reporting has to be executed with simple linear proportion.
In fact being
E1 E 2
=
n1 n 2
n
V0 = E it follows that; V2 = V1 2
n1
b) Unlike the DC machines, the effect of the residual magnetism is here normally neglectable.
This is shown by the percentually small value of the residual voltage (the magnetization
characteristic curve practically begins from the axis origin) and by the lack of difference
between ascending magnetization and descending magnetization. The reason of this
behavior is the air gap size, that in the alternators is kept very high to reduce the armature
reaction effects. (The above note, perfectly true for alternators with power greater than some
kVA, may find some exception when the machine is very small. In the latter case, especially
when the pole shoes are massive, a rather high residual magnetism may occur).
c) When the alternator is of salient poles kind, the detected magnetization characteristic curve
concerns the flux that flows through the air gap in front of the inducting pole shoes. In noload operation, the flux inside the machine, being essentially produced by the poles
excitation, follows this path. The flux generated by the armature reaction may anyway
follow different paths, according to the output cosΦ doesn't produce angular displacements
of the flux lines, and that is true only with cosΦ is very close to zero.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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SURVEY OF THE THREE-PHASE PERMANENT SHORT-CIRCUIT
CHARACTERISTIC CURVE
The short-circuit characteristic curve of an alternator is the graph showing the value of
the (permanent three-phase) short-circuit current as a function of the excitation current. Also this
diagram is essential for application of the main indirect testing methods for the alternator.
After that the speed of the group is regulated to the rated value at the alternator, read the
armature current ( Isc ) and the corresponding excitation current ( Iexc ). The results permit to draw
the short-circuit characteristic.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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NOTES :
a)
The right armature current that has been considered during the test is that corresponding to
the permanent and three-phase short-circuit, as said in the title; in other words:
- The measured values of the short-circuit current are those related to the steady
condition and haven't to be confused with those of the transient short-circuit current,
normally much higher, that occurs when the already excited machine is suddenly shortcircuited.
- The considered short-circuit is the three-phase one i.e. the symmetrical one. For this
purpose, it is necessary to keep in mind that in case of short-circuit of only one or two
phases, the asymmetrical armature reaction produces a high field in opposite, that may
induce high voltages in the open phases.
b)
The Isc = f ( Iexc ) graph has a linear shape and crosses the axis origin. This may be
explained when considering that the own impedance of the machine phases are highly
inductive. Therefore, the short-circuit current is very wattless (with reference to the
induced E0) and generates an armature reaction which is almost completely demagnetizing.
In this case :
- The whole flux inside the machine, although the excitation current is rather high, is
always very low (its value has to generate an induced e.m.f. able to balance only the Z.Isc
voltage drop due to the phases own impedance). The magnetic circuit works therefore
within the first section of the magnetization characteristic curve, whose shape is a straight
line; therefore, it can be assumed that:
where; K, K', K", K'" = constant proportionality coefficients.
-
The short-circuit current generated by the residual e.m.f., which is already very small in
the alternators is further reduced by the demagnetizing armature reaction, that tends to
zero the residual flux. Therefore, it can be assumed with great approximation that the
short-circuit characteristic curve crosses the axis origin.
Note : The small power value of the alternator can decrease the accuracy of the above results.
Therefore, a residual magnetism high enough to generate a small short-circuit current may
occur even in absence of excitation.
c) The short-circuit current only depends on the excitation current value and is largely
independent from the speed. This fact, which may seem strange, is easily explainable
when considering that the own impedance of the phases has the resistive component
practically neglectable with reference to the reactive component. Therefore, it may be
assumed that:
When the alternator's speed varies, therefore, both E and X vary of the same linear
amount and the short-circuit current (which is the ratio of them ) remains constant. Only at very
low speed the frequency may result so small to highly reduce the X value, making therefore the
resistive component of phases an unneglectable component. The test for survey of the shortcircuit characteristic curve is therefore validly performable even in case a driving motor is
available which isn't able to drive the alternator at its rated speed.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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Survey of the magnetization characteristic (Ignore V1 voltmeter)
SEQUENCE OF OPERATIONS
When the power supply is set, perform the following operations.
1
Set the controls of the modules:
VARIABLE DC OUTPUT :
(excitation)
Switch open.
Knobs fully turned CCW
VARIABLE DC OUTPUT :
STARTING RESISTANCE :
Switch open.Output on D.C.
voltage with about 220 V
Max. resistance
EXCITATION RESISTANCE :
Min. resistance
2
Start the driving motor until the rated speed at the alternator
3
Activate the variable D.C. output (excitation)
4
Through knob adjust the excitation current increasing it up to 20 - 30% over the
rated value.
5
For each value of the excitation current (ammeter A), read the correspondent
indications of the output voltage by the voltmeter V2.
6
De-excitate the alternator and stop the motor.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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TABLE OF MEASURED VALUES
Iexc
V2
V2
(forward) (backward)
n ( rpm )
0
0.05
0.10
0.15
0.20
0.25
0.30
3000
0.35
0.40
0.45
0.50
0.55
0.60
***
Plot Iexc versus Voutput in report
Survey of short - circuit characteristic (Ignore V1 voltmeter)
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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SEQUENCE OF OPERATIONS
When the power supply is set, perform the following operations.
1
Set the controls of the modules:
VARIABLE DC OUTPUT :
(excitation)
Switch open.
Knobs fully turned CCW
VARIABLE DC OUTPUT :
Switch open.Output on D.C.
voltage with about 220 V
STARTING RESISTANCE :
Max. resistance
EXCITATION RESISTANCE :
Min. resistance
2
Start the driving motor until the rated speed at the alternator
3
Close the switch of variable DC output module (excitation) and regulate the knob until the
rated current of the alternator is obtained
4
For each of different values of the excitation current, read the corresponding armature
current
5
When the armature current is quite near the rated value, change the rotation speed in a wide
range and verify that the short-circuit current is quite constant.
6
De-excitate the alternator and stop the motor.
TABLE OF MEASURED VALUES
Isc
Iexc
n ( rpm )
0
0.05
0.10
3000
0.15
0.20
0.25
0.30
0.35
0.40
0.45
***
Plot Iexc versus Isc in report
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT V
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