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Download CIRCUIT #1, To measure generator open circuit voltage:
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Transcript
Using the Wind Tunnel Electric Test Box to Measure Voc, Isc, Req and PLmax Measure the generator open circuit voltage, Voc Insert the generator into the wind tunnel holder, attach both wires from the test box to the generator, and turn the power switch on the test box to the ‘on’ position.. If the wires are connected backward you will just get negative readings. You can then change the connections. No harm will be done. The electric generator can be represented as being an ideal voltage source, Vg, in series with an equivalent resistor, Req. Req + + Vout Vg - Adjust the setting on the test box to the ‘open’ position. This attaches the voltmeter across the generator being tested as shown below. The voltmeter has a high impedance of about 100K ohms so it acts like an open circuit and can be used to measure the open circuit voltage, V oc. Voltage Current (volts) (milliamps) Req + Short Open Motor load Vg Load test Load adjust V voltmeter Off on Voc can now be determined experimentally by rotating the generator at any angular velocity and measuring the resulting open circuit voltage. Voc is proportional to the angular velocity of the rotor, thus changes in the speed of the rotor will cause proportional changes in V oc. Turn on the wind tunnel fan and measure the resulting angular velocity. Note: since there is an open circuit the current is zero and there is no voltage drop across Req. Therefore, Vg = Voc. Voc = _________________ 1 Measure the generator short circuit current, Isc With the generator still running at the angular velocity used to measure the open circuit voltage, adjust the setting on the test box to ‘short’. This will remove the voltmeter from the circuit and place the ammeter across the terminals of the generator as shown below. The ammeter has a low impedance of about 1 ohm so it acts like a short circuit. Measure the short circuit current, Isc. Voltage Current (volts) (milliamps) Req + Vg Short Open Motor load ammeter - Load test Load adjust I Off on Isc = _________________ Determine the value of Req, the equivalent resistance of the generator If a measurement of the short circuit current, Isc, is made at the same angular velocity at which Voc was measured, and Ohm's law is used to determine the voltage drop across Req, it must be the case that Vg - Isc Req = 0 Noting that Vg = Voc, and solving for Req, Req = Voc / Isc The previously measured values of Voc and Isc are thus used to obtain Req! Req = _________________ Note: One can expect about a 5-10% error in calculating Req because there will be a slightly different electric loading on the generator between the open circuit voltage measurement and the short circuit current measurement. This will cause the angular velocity to change slightly between the measurements. This can be heard as a slight slowing of the generator when the test box switch is moved from the ‘open’ to the ‘short’ position. It is estimated that the difference in the two angular velocities is about 5-10%. Measure the maximum power output to the load, P Lmax The generator can deliver power to a load resistor RL placed across its output terminals as indicated below. Req + Vg I The power consumed by this load resistor is P L, where PL = I2RL = VI 2 + V _ RL The resistor RL can be chosen so as to consume maximum power from the generator. As indicated in the text, maximum power is transferred to RL when RL = Req. To accomplish this the test box contains a variable load resistor. The load resistor, RL, must be adjusted until it equals Req. This is done by first setting the test box switch to ‘load test’ which places ~5 volts across the adjustable load resistor in the test box as indicated below. Then determine what current will be present when RL = Req. To do this, read the exact voltage (~5 volts) on the voltmeter and then vary R L by rotating the ‘load adjust’ knob on the test box until a current of I = V/Req is measured. At this setting RL = Req. Voltage Current (volts) (milliamps) Short Open Motor load Load test Load adjust + I ~5V RL _ Off on Now switch the test box switch to ‘motor load’, which will place the adjusted load resistor across the generator and set the ammeter and voltmeter so they read load voltage and current as indicated below. Voltage Current (volts) (milliamps) Req I Short Open Motor load + Vg Load test V RL Load adjust Off on Measure V and I for this load setting. The product of V and I is the power transferred to the load, P L = V I. Since RL = Req, this is PLmax. V = _________________, I = _________________, P Lmax = _________________ While it is not necessary, to verify that this value of R L gives maximum power to the load resistor it is only necessary to set the test box switch to ‘motor load’ and adjust the ‘load adjust’ knob for a series of current values. Choose these about 5ma apart. For each current setting measure the corresponding voltages. Then plot PL versus I, where PL = V I. PL I It should be the case that the maximum power is approximately the same as you found earlier. Turn the power switch on the test box to the ‘off’ position. 3