----- Original Message ----- From: Horace Heffner <[email protected]> Date: Friday, August 14, 2009 4:55 pm Subject: [Vo]:BB motor - surprising experiments
> > 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 The value of efficiency should not diminish the value of the discovery. The sentiment that nature does everything efficiently (and we should too) arose in the 18th century. I think the idea has come to hinder rather than help discovery. Harry
