Thursday, June 15, 2006

Cummulative Short- Shunt DC Motor

Experiment no. 9 Cummulative Short- Shunt DC Motor

I. OBJECTIVE

To be familiar with the actual characteristics of a cummulative short- shunt dc motor.

II. CONCEPTUAL FRAMEWORK

The cumulative compound motor. It is so called because the shunt field is connected so that its coils are aiding the magnetic fields of the series field and armature. The shunt winding can be wired as a long shunt or as a short shunt. The shunt field is connected in parallel with only the armature. The motor connected as a long shunt where the shunt field is connected in parallel with both the series field, interpoles, and the armature.

The short shunt motor as a cumulative compound motor, which means the polarity of the shunt field matches the polarity of the armature. The top of the shunt field is positive polarity and that it is connected to the positive terminal of the armature. The shunt field has been reversed so that the negative terminal of the shunt field is now connected to the positive terminal of the armature. This type of motor is called a differential compound because the polarities of the shunt field and the armature are opposite.

The cumulative compound motor is one of the most common DC motors because it provides high starting torque and good speed regulation at high speeds. Since the shunt field is wired with similar polarity in parallel with the magnetic field aiding the series field and armature field, it is called cumulative. When the motor is connected this way, it can start even with a large load and then operate smoothly when the load varies slightly.

You should recall that the shunt motor can provide smooth operation at full speed, but it cannot start with a large load attached, and the series motor can start with a heavy load, but its speed cannot be controlled. The cumulative compound motor takes the best characteristics of both the series motor and shunt motor, which makes it acceptable for most applications.

III. EQUIPMENT & MATERIALS

1- DC GENERATOR/ MOTOR

1- DIGITAL MULTIMETER (DMM)

1- DIGITAL WATTMETER (DWM)

1- TACHOMETER

1- RUBBER COUPLING

BANANAPLUG WIRES

CONNECTING WIRES

IV. CIRCUIT DIAGRAM



V. DATA AND RESULTS

Vs

If

IL

Ia

EbIa

PIN

IaRa

N

Ta

%Eff.

20.0

.04

4.69

4.65

18.29

61

30.27

660

.26

.30

25.5

.06

5.09

5.03

23.65

76

35.42

1250

.18

.31

30.3

.07

5.41

5.34

28.33

89

39.92

1852

.15

.32

35.4

.08

5.68

5.60

33.34

101

43.90

2410

.13

.33

40.8

.10

6.02

5.92

38.62

113

49.06

2918

.13

.34

45.7

.11

6.14

6.03

43.87

126

50.91

3290

.13

.35

50.3

.12

6.44

6.32

47.87

137

55.92

6584

.13

.35

55.8

.14

6.72

6.58

53.38

149

60.61

4006

.13

.36

Ra= 1.4 ohm; Rf= 205 ohm; Rs= 0.6 ohm

VI. OBSERVATION

In performing this experiment, we observed that the variac has a great effect in controlling the terminal voltage. During its operation, as it varies increasingly, all its parameters also increases as you refer to our given data and results.

VII. CONCLUSION

Therefore, in a separately-excited dc generator, the terminal voltage can be controlled by changing the internal generated voltage of the machine. By Kirchhoff’s voltage law VT = EA –IA RA, so if EA increases, VT will increase, and if EA decreases, VT will decrease. When the load supplied by the generator is increased, IL (and therefore IA) increases. As the armature increases, the IA RA drop increases, so the terminal voltage of the generator falls.

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