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Step 1: |
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Set the DMM to mA.
Remove the probes from the DMM and the plug ground pin of the
BNC-banana adapter (the one with the bump) into the
Common
terminal of the DMM.
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Step 2: |
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Plug a BNC clip lead into the adapter and connect the clips to the
terminals of the motor.
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Step 3: |
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Plug a black banana patch cord into the
COMMON
terminal of the power supply (blue binding post).
Plug the other end into the hole in the BNC-banana adapter
which is not plugged into the DMM.
Plug a red banana patch cord into the
0 TO +20V
terminal of the power supply.
Plug the other end into the right terminal of the DMM
(the
300 mA
terminal on the Fluke meters).
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Step 4: |
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Verify that you have the following circuit:
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Step 5: |
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Turn on the power supply and adjust the voltage to 4 V.
The motor should be turning slowly.
Note and record the current reading on the DMM.
This is the
no load
current.
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Step 6: |
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Increase the voltage to 18 V.
Note the no load current at this voltage.
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Question 1: |
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Is our assumption that the load torque is approximately constant
with speed correct?
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Step 7: |
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Turn off the power supply and
use the
C-clamp
to clamp the motor to the bench,
with the shaft hanging over the edge.
Be careful not to overtighten the clamp and damage the motor.
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Step 8: |
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Get a piece of string about 2 ft. long and tie a loop in each
end.
Wrap the string 1 1/2 times around the spool on the end of the
motor shaft, with the loops pointing up.
Hook a spring balance into each loop.
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Step 9: |
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Turn the motor back on.
Hold both scales in the same hand and pull upward
until the motor begins to slow down.
While maintaining a steady force (as indicated by a steady
current reading on the DMM), record the current and the force reading
on each scale.
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Note: |
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Some of the scales are calibrated in grams.
They aren't actually measuring mass, but rather the
weight
of the indicated mass due to Earth's gravity.
To convert to force you must multiply by the gravitational
acceleration, 
(i.e. F=mg
).
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Step 10: |
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The tangential force on the spool is the
difference
in the two scale readings.
Since the radius of the spool is 4 mm,
the torque in newton-meters is given by
.
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Step 11: |
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Make measurements of torque for several different values of current
between 50 mA and 200 mA (or whatever the maximum value is
before the motor stalls.
Try to get evenly spaced steps.
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Note: |
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Be sure the motor shaft is turning when you make these measurements.
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Step 12: |
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Plot the torque vs. current.
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Question 2: |
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Using the data from this and the previous experiment,
compute the torque and back-emf constants, 
and 
.
If our theory (and measurements) are correct, the two constants
should be equal if we use consistent units.
One set of consistent units are volts/radian/sec and newton-meters/ampere.
Convert your constants to these units and compare them.
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