EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 1

Instructor: Assist. Prof. Dr. Şener AGALAR

TA: H. Ersin EROL

Abdülkadir ZİREK

DEPT. OF ELECTRICAL AND ELECTRONICS

ENGINEERING

2020-2021 FALL

EEM 471 ELECTRICAL MACHINERY

LABORATORY

EXPERIMENT I

• Measuring the Stator Resistance of a 3Φ Asynchronous Motor

• Measuring the Rotor Resistance of a 3Φ Asynchronous Motor

• Transformation ratio of Stator / Rotor

• Transformation ratio of Rotor / Stator

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 2

RATED CHARACTERISTICS OF THE MACHINE UNDER TEST

Before starting to discuss the execution of the individual tests, it is useful to fully examine

the rated characteristics of the machine under test. These data, reported on a plate, represent the

"Identity Card" that every manufacturer fixes on its machines, so that who will use them will be

able to recognize their most important operating features.

These rated values, of course, are the result of tests that the manufacturer has performed on several

prototypes at the beginning of the series production; each manufactured machine may show per-

formances which are slightly different from the rated values, due to the unavoidable manufacturing

tolerances.

To perform the checking test on a series machine, therefore, is to verify when the standard

performances reported on the plate are really respected and to detect those special operating char-

acteristics that, though not reported on the plate, are nevertheless important for the practical use of

the machine.

TEST NO: 1

MEASURING THE STATOR AND ROTOR WINDING RESISTANCES

The value of the stator and rotor winding resistances in the induction motor are useful to

calculate:

• the conventional efficiency

• torques and slips under load (through the drawing of the circular diagram).

Being the three-phase induction motor, its stator winding is constituted by three separate circuits

(U, V, W), while the rotor windings are star connected (phases K, L, M), whose terminals are

reported on the terminal board according to the following diagram:

The three phases may be either star or delta interconnected in a very simple way, as the follow-

ing diagrams show:

rotor winding

stator winding

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 3

STAR CONNECTION DELTA CONNECTION

Being the three stator phases designed for the rated current, they have a rather small re-

sistance (the value is proportionally smaller for greater machines). Therefore, it is necessary to

follow the required contrivances to avoid the influence of the contact resistances both selecting a

suitable measuring method and adequately wiring the voltmetric connections in the circuit itself.

Obviously, the measurement must be performed in DC current and with steady machine; in any

case, it is necessary to act on "cold" machine, i.e. not operating since several hours, to be sure that

every part of it has a temperature equal to the room temperature. In this case, when the measuring

method will not produce an appreciable heating, it will be possible to say that the measured re-

sistance values are referred to a winding temperature equal to the room temperature (which may

be easily measured with a common thermometer).

ELECTRICAL DIAGRAM NO: 1 (Stator Resistance)

Among the several available methods, we will choose, for greater convenience, the volt-

ampere method.

Nameplate of the Asynchronous Motor.

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 4

To avoid warming the windings, the test current is limited to 10% of the rated phase current

thus reading the rated current value from data plate

𝐼𝑓𝑛 = ________𝐴

The max test current shall be 𝐼𝑝𝑚𝑎𝑥= 0.1 × 𝐼𝑓𝑛 = ________𝐴

ELECTRICAL DIAGRAM NO: 2 (Rotor resistance)

NOTES

• To measure the rotoric phase resistances it has been suggested to directly connect the volt-

meter to the rings. This contrivance is essential not to measure the contact resistance be-

tween brushes and ring too.

• As the measurement concerns two phases in series connected, to obtain the rotor R phase

it is necessary to divide the result by 2.

TEST NO: 2

MEASURING THE STATOR-ROTOR TRANSFORMING RATIO

All the theoretic description of the induction motor's operation is based on the perfect sim-

ilarity existing between this machine and the static transformer. In fact, the induction motor, too

has a primary winding (the stator), and a secondary winding (the rotor), struck by a sinusoidal

alternate linked flux; the composition of the three fluxes in the individual phases generates a ro-

tating flux in the space. Therefore, for the induction motor, too it is possible to speak about a

- o

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 5

transforming ratio, relating the primary (stator) and the secondary (rotor) quantities. Its measure-

ment may be performed following the same methods used for the transformer.

ELECTRICAL DIAGRAM NO: 3 (Stator - Rotor Ratio)

Supply the stator with approx. rated voltage value and with the rotor open-circuited measure rotor

voltage.

𝐾1 =𝑉𝑆

𝑉𝑅 (Phase value)

ELECTRIC DIAGRAM NO: 4 (Rotor - Stator Ratio)

𝐾2 =𝑉𝑆

𝑉𝑅 (Phase value)

Supply the rotor with voltage value obtained in the stator - rotor ratio test and measure the

stator voltage V’s.

The real transforming ratio value can be calculated: 1 2K K .K

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 6

PRACTICAL TEST NO: la (Stator Resistance)

Practical Diagram

OPERATION SEQUENCE

Build the circuit in heavy drawn that is shown in the diagram:

1. Preset the control in the module;

VARIABLE DC OUTPUT

EXCITATION RHEOSTAT

Variac completely counter

clock-wise rotated

"R” completely counter clock-

wise rotated

2. Activate the power supply and close the switch.

3. Adjust the variac and "R" to obtain a current of approximately 0.2 A.

4. Directly insert the voltmeter to the asynchronous motor terminals.

5. Read the values on the voltmeter and on the ampere-meter.

6. Disconnect the voltmeter and open the switch of variable DC output.

7. Repeat the steps 2&3 after having inserted in the measuring circuit successively into the V

phase and the W phase.

Phase Ampermeter (A) Voltmeter (V) Rphase (Ω) Mean Rphase (Ω)

U

V

W

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 7

PRACTICAL TEST NO: 1b (Rotor Resistance)

Practical Diagram

OPERATION SEQUENCE

Perform the operations already described in steps 1 to 6 for the measurement of the stator

phases, taking care to directly connect the voltmeter on the rotor rings, downstream the brushes.

Phases Ampermeter (A) Voltmeter (V) Rphases (Ω) Mean Rphases (Ω)

K-L

L-M

M-K

Note that Mean Rphases gives the resistance of two phases. To find the mean resistance of

a rotor winding you need to take the half of this value.

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 8

PRACTICAL TEST NO: 2a (Transformation Ratio: Stator/Rotor)

Practical Diagram

OPERATION SEQUENCE

Build the circuit in heavy drawn that is show in the diagram.

1. Preset the control in the module:

VARIABLE THREE-PHASE OUTPUT: turn the variac completely counter clock wise

before power on the system

2. Adjust the supply to the stator rated value (380V) and measure the three stator and rotor line

voltages, deducting the associated phase values.

3. Open the switch.

Stator Voltage (V) Rotor Voltage (V) K1 K1m

U-V

K-L

V-W

L-M

W-U

M-K

Formulas:

stator1 1m

rotor

VK K MeanValue

V

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT I 9

PRACTICAL TEST NO: 2b (Transformation Ratio: Rotor/Stator)

Practical Diagram

OPERATION SEQUENCE

Build the circuit in heavy drawn that is show in the diagram.

1. Preset the control in the module:

VARIABLE THREE-PHASE OUTPUT: turn the variac completely counter clock wise

before power on the system

2. Adjust the supply to the value previously measured test 2a on the rotor and measure the three

stator and rotor voltages, deducting the associated phase values.

3. Open the switch.

Rotor Voltage (V) Stator Voltage (V) K2 K2m

K-L

U1-V1

L-M

V1-W1

M-K

W1-U1

Formulas:

rotor2 2m

stator

VK K MeanValue

V

Measuring results: Real Transformation Ratio: 1m 2mK K .K