Some typos corrected.
EXPERIMENT REPORTS
The Fig. 1 resistance R1 was increased by adding 4 nichrome shunts,
as shown in Photo 1 below. Using the Fig. 1 circuit the motor moving
(Photo2) and stopped (Photo3) runs were made again, with a few
minutes cooling time in between.
CH1
o
|
------(-)battery(+)--o---SW----Motor----
| |
| LED 4.7 k ohms |
| ----|<|---R2-------------- |
| | | |
-----o-----------R1-----------o---------
| ?? ohms |
o o
CH2 Ground
Fig. 1 - circuit to measure motor voltage drop
The results show a clear back emf effect. The resistors reach a
resistance plateau in 2-3 seconds when and as the motor runs (See
Photo2), and not when the motor is stopped (See Photo3). Two of the
filaments glowed, the old large blackened one, third filament from
the top in Photo1, and the new one with fewest turns in it, second
filament from the top in Photo1.
The stopped motor current stabilizes at 1.5 V across it or less, the
running motor stabilizes at about 2.7 V, giving a back emf of 1.2 V
when running.
I don't know why the back emf isn't higher than for the prior run,
which had a stopped voltage across the motor of 0.7 V (due to lower
current), and running 2.1 V, giving a back emf of 1.4 V. Perhaps the
reason is in the prior run the manual start put the motor at a higher
rpm than where it stabilizes, but the motor didn't get a chance to
stabilize speed because I had to cut it off due to the filament
overheating. I don't see how it might have affected this, but I
recharged the battery before taking this last set of data.
I'm pretty happy with the performance of the little motorcycle battery.
Photo1: New probe configuration and shunts added:
http://www.mtaonline.net/~hheffner/HullShunt1.jpg
Photo2: Traces with motor running:
http://www.mtaonline.net/~hheffner/HullShuntRun1.jpg
Photo3: Traces with motor stopped:
http://www.mtaonline.net/~hheffner/HullShuntStop1.jpg
I thought one way to validate a back emf is to drive the motor to a
higher rpm and look for an increase in the back emf measured. I
stuck a half inch buffing pad on my Dremel tool and stuck it into the
partly exposed 1/2" shaft hole in the pulley and revved the thing up
to at least twice normal speed. I expected the back emf to double
and that trurning on the power would slow down the motor. It didn't
slow down when power was turned on. If anything it just ran faster
when I threw the switch than where the Dremel tool took the rpm. It
appeared to take much longer for the filaments to heat up though, and
the Channel 2 trace in Photo4 below bears this out, showing the
voltage across the current resistor R1 is almost flat at -7 V
throughout the run. The voltage drop across the motor, shown in
Channel 1 is nearly flat also at about 2.8 V. The prior run
stabilized at about 2.7 V, with the stopped motor voltage drop at 1.5
V. This means the back emf only increased by about 0.1 V over the
run in Photo2, even though the rpm doubled, and the motor power
output apparently doubled with no increase in overall current.
From my hysteresis model, I expected torque to increase with RPMs to
an optimum point where the magnetized material migrates into the
current i such that i * M is at peak strength, and then to decline as
RPMs increase beyond that point because the material doesn't have
time to be magnetized. What I would not expect is that the back emf
would not change significantly at all even though the RPMs doubled.
It also appears *superficially* that the motor power doubled and the
heating of the current resistor dropped significantly, even though
the voltage across the resistor is measured at pk-pk 7.20 V, not too
different from the 8.8 V for the stopped motor.
Weird. By starting at a higher RPM, the motor runs faster, system
current is less, yet back emf is unchanged. If the motor were not so
darned inefficient this would be a monumental discovery. The
inefficiency and quirky behavior of the hysteresis effect make
quantifying individual variables difficult.
Photo4: High rpm current start:
http://www.mtaonline.net/~hheffner/HullShuntHighRPM2.jpg
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
