I looked this over, and two questions occurred to me. 1) What's the power factor when it's running? It's got a back EMF, due apparently to a hysteresis effect, and on the face of it that word "hysteresis" suggests the EMF and current may not be in phase. In particular, if the power factor varies as the RPM changes that could have odd effects. (You may have mentioned that somewhere; if so, apologies, I overlooked it.)
2) Is there any way to measure the *resistance* of the running motor? I'm wondering if it's the same as when it's stopped, but off hand I don't see how to tease apart the effects of the back EMF from the effects of the (possibly varying) running resistance. Not sure why resistance would be a function of RPM, of course, but on the other hand I'm not sure it wouldn't be, either! Horace Heffner wrote: > > 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/ > > > >
