Download CIRCUIT #1, To measure generator open circuit voltage:

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 = _________________
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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.
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