EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT IX 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 IX

Polyexcitation Direct Current (DC) Machine

• Effective Efficiency test of the DC Motor with Braking DC

Generator

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT IX 2

CONVENTIONAL EFFICIENCY

The conventional efficiency of DC motor can be calculated using the data measured in the

previous tests.

3.1 Friction and ventilation losses, at rated speed, measured through the method of no-load

motor operation (test 2) Pm = ....

3.2 No-load iron losses, at rated voltage and speed, measured through the method of no-load

motor operation (point 2) Pir = ...

3.3 Joule effect losses in inductor windings and in rheostats, at rated voltage and power.From

the no - load test the excitation current is calculated corresponding to the rated voltage,

then the excitation losses are calculated.Pe = Vr . lexc

3.4 Joule effect losses in armature windings. The ohmic resistance of the armature, measured

at room temperature (test 1), has to be referred to the conventional temperature of 75°C.

Ra75 = Rart[l + 0.004 (75 - rt)] When the rated absorbed current of the dc motor is indicated with la-, the total

armature current is given by la = I2r - lexc The armature losses can be therefore calculated

Pa = Ra75 . la2

3.5 Electric losses due to brush contacts. As the contact resistance between brush and

commutator follows an anomalous behaviour, the brush losses are conventionally

evaluated.

Pb = 2 la (carbon or graphite brushes)

Pb = 0.6 la (metal brushes) . Where la is the total armature current.

3.6 Additional losses. As the switching from no-load to load condition implies an alteration of

the pole flux distribution due to the armature reaction, this is kept into account through

additional losses that are conventionally evaluated

Padd = 0.5% Vr la

3.7 Conventional efficiency. The total losses of the DC motor operating at rated voltage and

load are given by P1 = Pm + Pir + Pe + Pa + Pb + Padd the conventional efficiency can be

therefore calculated:

where Pabs = Vr • lar

3.8 Efficiency curve. To draw the conventional efficiency curve, it is necessary to repeat the

evaluations of points 3.1 to 3.7 for different load conditions, normally of 0, 1/4 Ia, 2/4 Ia , 3/4 Ia,

4/4 Ia, 5/4 Ia

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT IX 3

DIRECT TEST WITH BRAKING DC GENERATOR

The purpose of this test is effective efficiency of the motor with direct test.

The motor is loaded with a braking DC generator equipped with arms and weights.

ELECTRIC DIAGRAM

NOTE : For the test is required a tachometer for measuring the rotation speed.

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT IX 4

PRACTICAL DIAGRAM

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT IX 5

TEST PROCEDURE

After the set up of the motor under test and of the DC braking generator make all the connections in accordance with the attached diagrams and set the commands as follows:

1 Preset the module:

VARIABLE DC OUTPUT :

VARIABLE DC OUTPUT : (excitation)

EXCITATION RESISTANCE:

STARTING RESISTANCE :

LOAD RESISTANCE :

Switch open

Output with about 230V

Switch open

Variac fully turned in CCW direction

Min. resistance

Max. resistance

Indifferent position

2 Set the main switch on ON and, acting on variac, adjust the voltage supply to the exact value

shown on the motor plate. Gradually switch Ra off. Verify that the rotation sense of the motor

is correct with reference to the position weight and then take out the shunt current. Adjust with

the variac the supply voltage to exactly the motor's rated value. Acting on Re adjust the speed

to exactly the rated value.

3 Take up the instruments readouts and the motor rpm. through a tachometer.

4 Switch off the main switch which causes the stop the motor.

NOTE: Instead of DC braking generators, we will use magnetic brakes

for this test !!!

EEM471 ELECTRICAL MACHINERY LABORATORY / EXPERIMENT IX 6

Table of Measuring Results

Magnetic

Brake

Voltage

Levels

C (Nm) n (rpm) Pr (W) U (V) Iind Iexc Pabs η

0

25

50

75

100

125

150

175

200

Pabs = V.(Iind + Iexc) Iabs = Iind + Iexc η = Pr / Pabs

*** In report Plot Pr versus Iabs, n, η