Maybe there was a bit of an anomaly here?
This is in regards to and an addendum to the Marinoff Ball Bearing
Motor experiments documented here:
http://www.mtaonline.net/~hheffner/HullMotor.pdf
At the request of vort Harry Veeder, I did a test to show how much
time it takes to heat up the resistor when the motor is running vs
when stopped. I added a little green LED just below the filament so
you can see exactly when the current comes on, and also provided a
clock to see the time. Here it is:
http://www.youtube.com/watch?v=PWlVn-uqxig
Looks to me like about 8 seconds when the motor is running, and about
5 when stopped. This at first glance appears to be yet another
indication this is an ordinary magnetic effect. The reduction in
final current can be attributed to a back-emf. To some degree it
might also be attributed to non-conduction time when the motor is
running, but the scope traces have indicated pretty much full time
current conduction in all runs since the bearings were cleaned.
However, the current sense resistor voltage drop doesn't look like
what I'd expect.
http://www.mtaonline.net/~hheffner/HullRunningTrace.jpg
http://www.mtaonline.net/~hheffner/HullStoppedTrace.jpg
The traces show: (1) motor running, takes about 5 seconds to go from
7 V to 9.6 V, but about 9 seconds to heat orange, (2) motor stopped,
takes about 5 seconds to go from about 6 V to 10.8 V, and to heat
orange.
The difference in peak voltage makes sense in that the running motor
peaks at 1.2 V less, so the back emf must be about 1.2 V.
However, the traces don't make sense with regards to how the filament
heats.
P = I^2 R = V^2/R
The resistance R = 0.0631 ohms cold.
So, at startup the running resistor heating power Prun and stopped
power Pstop are:
Prun = (7 V)^2/(0.0631 ohms) = 777 watts
Pstop = (6 V)^2/(0.0631 ohms) = 571 watts
The ratio is 1.36, with the running motor circuit initially producing
more heating in the current sense resistor by a factor of
Prun/Pstop = 777 W / 571 W = 1.36 !??
This is not what we would expect in that overall the resistor turns
orange much faster when the motor is stopped. By the time of orange
glow, at resistor voltage and current equilibrium, we don't know the
resistance, but the power ratio appears (assuming identical
resistance at similar temperature) to be:
Prun/Pstop = (9.6 V)^2 / (10.8 V)^2 = 0.79
The equilibrium numbers make some sense in that 0.79 * (8 sec) = 6.3
sec, though it is off quantitatively a bit in that the orange
temperature was reached in 5 seconds.
The initial power numbers made no sense to me in terms of the way the
resistor acted though, and that has nothing to do with the
performance of the motor. The resistor should heat according to the
energy applied to it.
Finally it dawned on me. The resistor was per-heated in the second
run. It started out with a higher resistance, but heated to orange
faster, i.e. with less energy. I ran a quick test. Starting out
cold it took 8 seconds to heat the resistor to orange. Doing it
again, a few seconds later, it took only 3 seconds. The resistor
apparently takes a while to cool down even after it is no longer red
or orange.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
