anadolu university dept. of electrical and...
TRANSCRIPT
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 1
Instructor: Assist. Prof. Dr. Sener AGALAR
TA: Res. Asst. H. Ersin EROL
ANADOLU UNIVERSITY
DEPT. OF ELECTRICAL AND ELECTRONICS
ENGINEERING
EEM 471 ELECTRICAL MACHINERY
LABORATORY
EXPERIMENT II
• No Load Test of a 3Φ Asynchronous Motor
• Short Circuit Test of a 3Φ Asynchronous Motor
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 2
NO LOAD TEST AND Pm - Piron SEPARATION
General
The no-load test is useful not only to point out the working conditions of the motor's
magnetic circuit, but also to obtain significant elements both for drawing the circular diagram (Io
and cos Φ) and for calculation of the conventional efficiency(Pm and Piron).
It consists in supplying the induction motor with its rated voltage, leaving the rotor free to rotate
without any braking torque. In these conditions, the input current is represented by the vectorial
sum of the magnetization current and of the small active component due to both the (stator) iron
and (friction and ventilation) mechanical losses.
The input power, on the other hand, corresponds to the sum of all the no-load losses, i.e. :
- losses in the stator's copper
- losses in the stator's iron
- mechanical losses (friction and ventilation)
The iron losses are fully located in the stator magnetic circuit, which is crossed by a
sinusoidal flux with mains frequency. On the other hand, as the rotor is almost syncronously
rotating with the field, it is crossed by a practically constant flux and, therefore, there is no
reason for losses to be generated in it due to eddy currents or magnetic hysteresis.
Unlike the transformer, here the primary (stator) copper losses aren't neglectable, as the input
current in no-load condition is in percent much higher. Anyway, as the stator windings resistance
has been already measured (experiment 1), it's rather easy to determine these losses to subtract
them from the total losses.
Normally, the no-load test isn't executed with only one measurement at rated supply, but
it is repeated by gradually reducing the voltage, so to obtain a series of values that allow to draw
in a diagram the interesting quantities. In this way a dual benefit is obtained: the random
measuring errors are reduced, by adequately interpreting the testing points, and the shape of the
various measured quantities can be observed.
The stator windings may be indifferently connected in the way that shows the best
adjustment and measuring convenience. In fact, the interesting values are the phase values,
which may anyway determined when the line values are known.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 3
EXTRAPOLATION OF THE Pm + Piron CURVE
The Pm + Piron = f(Vo) curve is practically a parabola, with an offset from the V axis equal
to Pm. In fact, when Vo varies, the mechanical losses don't change, they are related to the speed,
which remains significantly constant. On the other hand, the iron losses change (as the voltage
varies, therefore practically varying the generated flux of the same amount) and, as a quadratic
proportion exists between the iron losses and the induction, the graph representing them will
have a parabolic shape.
The Pm and Piron separation is therefore graphically possible when the intersection point between
the curve and the Y axis is located. This point isn't experimentally measurable, as with too small
supply voltages the induction motor tends to stop. Therefore, the intersection point has to be
obtained by graphic extrapolation of the measured curve section. To reduce the difficulty of this
operation the consideration may be helpful that in that point the curve is tangent to a parallel to
the V axis. Moreover, through a graphic contrivance it is possible to practically eliminate the
uncertainty of the extrapolation: by drawing the Pm+Piron graph as a function of Vo
SHORT - CIRCUIT TEST
The short-circuit test main purpose is the determination of the absorbed current and of
cosfi when the motor is supplied with locked rotor. Moreover, the test allows to calculate the
series equivalent parameters of the motor (Re – Xe - Ze) and, when a dynamometer is available,
to perform the starting torque measurement. When the motor is operating in short-circuit
condition, the input current is only limited by the equivalent impedance of its windings
(resistance and leakage reactance), as the main flux and the corresponding self-induced counter
electromotive force are practically zero.
Therefore, in this condition the motor should be supplied with full rated voltage, very
high absorptions would occur (3 to 7 times the rated current), that could thermally damage the
windings. Therefore, the short-circuit test is normally executed by supplying with adequately
reduced voltages the motor, so that currents not exceeding the rated values may circulate.
The values of the absorptions at full rated voltage (which are the interesting values) are then
calculated assuming the direct proportionality between current and voltage and the quadratic
proportionality between power and voltage.
In other words it is assumed that when the supply voltage varies both the leakage
reactance and the resistance of windings remains constant. In operating with locked rotor, the
induction motor is assumed to be in short-circuit condition, as its stator and rotor windings are in
Pm + Piron
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 4
perfect electric similarity with the primary and secondary of a static transformer operating in
short-circuit condition.
It is necessary not to misunderstand the rotor winding connection, that in normal
operation is always short-circuited on itself; during the rotation, in fact, the power which the
motor converts into mechanical is electrically equivalent to the generation of an adequate
resistance within the rotor phases. Therefore, the electric circuit of the motor becomes very
similar to the circuit of a transformer with ohmic load. Only when the rotor is locked the
mechanical generated power becomes zero and therefore the equivalent resistance. In this case,
the rotor is in perfect electric short-circuit on itself.
As for the no-load test, also in this case a single measurement (for instance at rated input
current) is normally not accepted, but a sequence of measurements is performed for
different input currents, so that the interesting quantities may be drawn in a diagram. Also
in this test the stator connection may be indifferently selected for convenience of
adjustment and measurement of the concerned quantities.
PRACTICAL TEST N0: 2a - NO-LOAD TEST
OPERATION SEQUENCE
Build the circuit in heavy drawn that is shown in the diagram:
1 - Before power on the circuit, preset the control in the module:
VARIABLE THREE-PHASE OUTPUT: Turn the Variac completely
counter clock-wise
2 - Power on the circuit
3 - Slowly adjust the Variac to supply the motor with its rated voltage (whose value is 220V
being the stator delta connected). Switch the Ra rheostat off turning it to position 5 and
leave the motor free to rotate for some minutes, to stabilize the friction in the bearings.
4 - Adjust the variac to obtain, in sequence, the supply voltage values shown in the table; for
each of them read the instrument indications.
5 - Power off the system.
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 5
PRACTICAL DIAGRAM
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 6
VO IO WA WB PO cos Φ Pm+Piron
230
220
210
200
190
180
170
160
150
140
130
120
110
100
75
50
25
0
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 7
PRACTICAL TEST N0: 2b - SHORT CIRCUIT TEST
PRACTICAL DIAGRAM
OPERATION SEQUENCE
Build the circuit in heavy drawn that is shown in the diagram:
1 - Before power on the circuit, preset the control in the module:
VARIABLE THREE-PHASE OUTPUT: Turn the Variac completely counter clock-
wise
2 - Hold the rotor of the motor, being the required effort minimum, stucking the rotor can be
EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT II 8
done with simple manual action on the joint.
3 - Power on the circuit
4 - Slowly adjust the Variac to supply the motor with its current values reported in the table;
for each value read and note the instrument indications.
5 - Power off the system.
ISC VSC WA WB PSC cos Φ
3
2.5
2
1.5
1
0.5
Please Study on Two-Wattmeter method before you came to laboratory !!!
In Report
For open circuit test plot VO versus IO, PO , cos Φ, Pm+Piron
For short circuit test plot ISC versus VSC, PSC, cos Φ
Prove the cos Θ formula which was given in text.