The No Load Circuit And Short Circuit Characteristics Biology Essay

The No Load Circuit And Short Circuit Characteristics Biology Essay 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 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. 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:UsersMashDesktopf.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 à ¢Ã¢â‚¬Å¾Ã‚ ¦ 9.8 à ¢Ã¢â‚¬Å¾Ã‚ ¦ 573 à ¢Ã¢â‚¬Å¾Ã‚ ¦ 542 à ¢Ã¢â‚¬Å¾Ã‚ ¦ No-load characteristics G:Machine Pracopennn.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 à ¢Ã¢â‚¬Å¾Ã‚ ¦. 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 Pracshortt.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 ) 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 à ¢Ã¢â‚¬Å¾Ã‚ ¦ 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.