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.