The No Load Circuit And Short Circuit Characteristics Biology Essay

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The Ward-Leonard system is a conventional speed control method. It consists of a 3 phase induction machine controlling a separately excited DC generator. The DC generator in turn supplies a variable DC voltage to a DC motor. It is basically a DC variable speed drive [2]. The Ward-Leonard system is shown below in Figure 1.

Figure : Ward-Leonard system setup

The principle behind the Ward-Leonard system is that the DC generator can actually influence the motor to develop a torque and speed required by the load [3]. Thus the speed of the generator is directly proportional to the armature voltage applied to the DC motor [2]. The output voltage of the DC generator is controlled by adjusting the exciting voltage (field voltage), this then controls the speed of the DC motor [2].

Applications

Travelling cranes

Lifts

Mine hoists

Boring machines

Table : Ward-Leonard system advantages and disadvantages

Advantages

Disadvantages

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Very wide range of speeds

High cost

Provides step less speed control

Low over-all efficiency

Experiment

Apparatus

2- coupled induction machine and dc motor (as shown below in Figure 2)

4- digital multimeters (DMM)

2- Variac (Excitation field)

Tachometer

Figure : Coupled induction machine and dc machine

Objectives of the experiment

Characterise the DC machines and determine the equivalent circuits.

Derive the power flow equations between the DC machines in terms of the equivalent circuits.

Control the power flow between the DC machines by adjusting the field currents. Then compare the measured results with the expected theoretical power flow.

Experiment procedure and setup

No-load Test

This test was used to determine the armature voltage.

Before the experiment began the armature and field resistance were both measured.

The Variac (exciter) was then connected to the field port on the DC machine.

The Digital multimeter was connected to the armature port on the DC machine in order to measure the armature voltage.

The DC machine was coupled to a three phase induction machine which was first turned on to run the DC machine. The setup is shown below in Figure 3.

Using the knob on the Variac, increase the field voltage with an increment of 10V ( also increases) and for each case determine the armature voltage. This was done from 0V to the rated field voltage 110V.

Now decrease the field voltage to demagnetise the DC machine from 110V to 0V also with an increment of 10V. Note the residual magnetism.

Figure : No-Load test setup

Short-Circuit Test

This test was used to determine the armature current.

The same procedure for the No-Load test was followed but in this case the digital multimeter was connected in series in the armature port in order to measure the current.

Using the knob on the Variac, increase the field voltage with an increment of 10V and for each case determine the armature current. This was done from 0V to the rated field voltage 110V.

The DC machine was demagnetised from 110V to 0V also with an increment of 10V recording the armature current.

Ward Leonard experiment

This setup was used to determine the power flow between the machines.

The two coupled machines were connected together as shown below in Figure 4. A coupled machine is shown in Figure 1. The coupled machines were connected together through the armature.

The positive terminals of the armature were connected together and the negative terminals were connected together.

A digital multimeter was connected in between the positive terminals of the armature in order to measure the current.

Each DC machine was connected to Variac through the field port. Both the Variac machines were turned down to 0V.

The two induction machines were switched on both at the same time from the 3 power supply.

The Variac knobs were both turned at the same time with an increment of 10V from 0V. This is done up until the multimeter reads 0A.

The 0A was obtained at a field voltage of 110V.

At this stage the second machine was left constant and the field voltage of the first machine was turned down at an increment of 10V, whilst recording the current and the speed of the machine without exceed the speed of 1502 rpm. A tachometer was used to measure the speed.

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After that the first machine was calibrated back to 110V, were the multimeter reads 0A.

Now the first machine was left constant and the field voltage of the second machine was turned down at an increment of 10V, whilst recording the current and the speed of the machine without exceed the speed of 1502 rpm. A tachometer was used to measure the speed.

After this then the practical is complete, the next step is to deduce an equation for the power as a function of excitation (field current) based on the machine characteristics. Then plot the graphs.

Figure : Power flow setupC:\Users\Mash\Desktop\f.bmp

Safety

Do not exceed the ratings of the machines and all the other equipment.

Switch off the equipment after completing the practical.

Results

Characterisation of DC machine

Table : Armature and field resistance

Resistance

Before

After

7.3 Ω

9.8 Ω

573 Ω

542 Ω

No-load characteristics

G:\Machine Prac\opennn.bmp

Figure : No-load circuit

Table : No-load test results

Magnetizing

Demagnetizing

Field volts (V)

Armature volts (V)

Field volts (V)

Armature volts (V)

0

0

1.6

9

10

27.8

10

35

20.8

61.1

20.5

68

30

88.9

30.1

97

40

116.8

38.2

119

50.1

141.1

50.1

148

60.6

165.1

60.1

170

70

181.9

69.5

185

80

196.2

80.4

200

90

209

90.5

211

100.2

218

100.9

220

110.3

227

110.3

227

Figure : No load test results plot

The following table shows the calculated field current using the measured field resistance of 573 Ω.

Table : Amperes in the field coils

Magnetizing

Demagnetizing

Field volts (V)

Field Amperes (A)

Field volts (V)

Field Amperes (A)

0

0.0000

1.6

0.0028

10

0.0175

10

0.0175

20.8

0.0363

20.5

0.0358

30

0.0524

30.1

0.0525

40

0.0698

38.2

0.0667

50.1

0.0874

50.1

0.0874

60.6

0.1058

60.1

0.1049

70

0.1222

69.5

0.1213

80

0.1396

80.4

0.1403

90

0.1571

90.5

0.1579

100.2

0.1749

100.9

0.1761

110.3

0.1925

110.3

0.1925

Figure : DC generator no-load characteristics

Comments

The graph shows the relationship between the no-load armature voltage and the field current at a constant speed of 1496 rpm. The magnetization curve is a straight line up to a field current of 0.1A, after this point the graph approaches a condition known as saturation, thus any increase in the field current does not result in an increase in the armature voltage.

Consequently the demagnetizing plot is above the magnetizing plot, this is due to the residual magnetism and hence the curve begins just above the 0 mark (a little way up).

Closed circuit test

G:\Machine Prac\shortt.bmp

Figure : Closed circuit diagram

Table : Closed circuit results

Magnetizing

Demagnetizing

Field volts (V)

Armature current (A)

Field volts (V)

Armature current (A)

1.7

2.54

1.6

0.21

10.8

3.63

10.7

1.1

20.5

4.52

18.8

1.81

30.8

4.72

29.5

2.64

40

5.54

39.9

3.37

50.9

6.4

50.2

3.93

60.7

5.59

60.8

4.44

70

6.01

70.7

5.05

80

6.1

80.8

5.5

90

6.52

90

5.9

100

6.92

100

6.54

111

7.4

111

7.4

Figure : Short-circuit characteristics

Nameplate Information

No-load circuit calculations

Figure : No-load Fitted-curve

This can be written as

Using Figure 10 we can use the fitted plot of the no-load saturation curve above to determine the constant. The measured speed is used.

From

Thus we can calculate:

But in practice we can approximate the value of the torque constant

Short-circuit calculations

Figure : Short circuit fitted plot

This can be written as

As calculated above

Thus by substitution

Thus now we can determine the armature resistance

Coupled machines (Ward-Leonard system)

)

110.1

110

0.1

1492

1491

0.205224

0.15597

110.1

100

0.15

1492

1492

0.186567

0.212687

110.1

90

0.49

1490

1494

0.16791

0.625299

110

79.9

0.86

1486

1499

0.149067

0.974303

110.1

70

1.22

1490

1498

0.130597

1.210896

110

60

1.59

1480

1499

0.11194

1.352687

110.1

50

2.03

1484

1501

0.093284

1.439179

110.1

40.3

2.45

1484

1503

0.075187

1.399974

Power flow

)

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110

110

0.1

1492

1492

0.205224

0.15597

100

110

-0.83

1492

1496

0.186567

-1.17687

90

110

-1.17

1480

1500

0.16791

-1.49306

80

110

-1.56

1470

1500

0.149254

-1.76955

70

110

-2.05

1474

1500

0.130597

-2.0347

60.1

110

-2.25

1483

1502

0.112127

-1.91737

Derivation of power equation

Figure : Ward-Leonard system setup

Figure : Ward-Leonard system equivalent circuit

Now from Figure above we expect that

Where

(For the generator)

(For the motor)

(For the generator)

(For the motor)

Equate equation (1) and (2)

Rewrite the equation

Now we observe that

Let

This equation remains the same, it just depends which machine is a generator and which machine is a motor. As mentioned above to determine which machine acts as a generator or motor, we look at the following sign conversion.

Conclusion

DC machine Characterisation

The DC machine characterisation of the generator was successfully done, both the no-load test and the short circuit test were done and all the parameters were calculated. The parameter calculated include the armature resistance which was found to be 6.25 Ω as compared to the measured and the rated armature resistance it is within range ( difference).

The characterisation also helps us understand how the dc works; by using the saturation curves we can determine the point where the machine starts saturation and determine the critical resistance. We can also determine information about the machine which would normally be given in the nameplate.

Ward-Leonard system and the power flow

The power flow equation was successfully derived and found as the equation below

The equation was derived from the Ward-Leonard system that was setup in the practical. The practical showed that the power can be controlled between two DC machines using this setup. In the practical the power flowed from the generator to the motor, this was seen through the current having a negative current flowing in one direction and a positive current flowing to the other direction. The practical was successful and it clearly corresponds to the theory.

What I leaned

The practical was useful in terms of helping us understand the concept of residual magnetism which is the same as in the theory.

The practical was also a good representation in terms of how an elevator/lift works.