On Aug 16, 2009, at 6:45 PM, Stephen A. Lawrence wrote:
What was bothering me is that if the actual *resistance* varied with
RPM, that would *look* *like* variation in the induced back EMF, which
would confuse any attempt at understanding the relationship between
the
"useful" power draw and RPM.
This is true. It's also why I say the fact this thing is so darn
inefficient makes sorting out what is going on difficult.
That said, right when the current is applied at the beginning of a
run, the resistance of the motor should be a known because it hasn't
had time to heat up, aside from the arcing vs not arcing (and also
the conducting vs not conducting that must happen some too, even
though there are lots of balls.) Before I put the bearings through 4
washes up to acetone, 4 bearings in series would not conduct well
enough to move, although the problem was mostly with the stainless
steel bearings.
BTW, I've gone way beyond my original objective, which was merely to
show the motor runs on magnetic effects and not heat. I'm well aware
that I need to refine things a lot to get good quantitative results.
I especially need a good reliable low resistance current sense
resistor separate from the main resistor that keeps the battery from
self destructing. A lower temperature main resistor is also
desirable, so I've been thinking of liquid immersion. I probably
need some voltage averaging to keep the readings smooth - because the
voltages drive DVMs wild. A capacitor across the motor would
probably smooth the DC load and improve performance at the same
time. A better structure than ball bearings is probably a good idea
too. I'm not sure I want to go to much trouble on this without some
good evidence of something truly anomalous.
Assuming your model is correct, as you say somewhere, the back EMF
should be proportional to i^2.
No, I think I said the torque or force (and thus power at some fixed
rpm) in some range of operation should be proportional to i^2. The
torque is proportional to i L B, but B is roughly proportional to i
at a specific rpm, provided saturation does not occur. The
complexity is driven by the fact that at some optimum rpm any
increase or decrease in rpms will reduce the B for a given i. The i
has to be positioned right in the crest of a wave of magnetization
that depends on the magnetization time and latency with respect to
time, the hysteresis of the material. Adding power that changes the
rpms from the optimum point results in less power out from power in
(I think.) One obvious extreme point is when v = 0. Infinite power
in results in no torque, no mechanical power out. Even without
saturation I think the motor reaches a point where more power in
results in no more power out, and the lighter the load on the motor
the sooner that point is reached, because added power merely results
in higher rpms.
On the other hand, "back EMF" due to
resistance goes as i,
I don't think i is in the equation for back emf. Only v, L and B.
Current is only there to the extent it affects v and B.
so it should be easy to sort them out: Spin the
shaft at constant RPM, and apply varying (relatively low) currents to
it. (Seems a lot easier than the experiments you've been doing...)
I have a couple 0-50 ohm 200 W variable resistors that might be
useful for that. The voltages would be very low. I think noise would
be a problem.
Doing this right requires a lot of equipment. I should add a
numerically controlled variable speed motor, rpm transducer, and
given all that, might as well add a dynamometer to the shaft. Really
should go all out in computer instrumentation. An FEA model would be
handy to compare predicted values to actual.
You said, later on in the email,
The resistance is not a function of rpm. It is a function of
temperature, but the back emf effects start out when the motor is
cold
and continue when it is hot.
Of course it will be a function of temp, but my thought was that it
could also be a function of whether the shaft is spinning. In
particular, if you're getting arcing in the bearings when the thing is
in operation and spinning, it's a safe bet that the current's
following
a path when it's spinning that it's not taking when the motor is
stationary, and maybe that's a higher resistance path.
I thought this too. However, the voltage drop traces at constant
current looked pretty smooth from 0 to low rpms. I may have been
thrown off by the fact I have a digital scope, and was looking at a 1
sec/division trace. Maybe checking things out on a high speed trace
would show something regarding the noise.
(But maybe that
doesn't make sense, either, and it should actually be a lower
resistance
path, via the arcs?)
But any difference would probably be an abrupt, on/off thing that
showed
up when it started to spin, and your scope traces for the "stopped
runs"
seem to show the back EMF dropping smoothly as the RPMS dropped (if I
didn't totally misunderstand them). And I suspect I'm barking up the
wrong tree anyway.
I agree these kinds of things should be checked out to do a thorough
job. Problem is I'm not sure I want to invest much more on this kind
of motor. I carry a low personal expectation for finding magnetic
free energy motors. A personal bias true, but when it comes to
shelling out cash and time I need a lot of motivation - like Kyle
mentioned.
However, with this motor, B varies with v, and not linearly. B
drops if
v is too high or too low. It likely is the case the motor is running
(especially when v is stabilized) in a range where B drops with
increased rpms, thus as v grows B drops nearly proportionately.
This,
if solidly confirmed, is good confirmation of my hysteresis model
of the
motor's performance.
This is my mental model of the thing anyway.
It makes sense, and your diagrams, on pp 15-16 of HullMotor.pdf, made
its (assumed) operation pretty clear.
Gasp!
and on the face of it that word
"hysteresis" suggests the EMF and current may not be in phase.
It's DC, voltage and current have to be in phase because there is
only
one phase.
Yeah, I finally got that bit :-[
Sorry for the repetition.
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.)
The main thing that varies is the resistance of the nichrome
resistors
as they heat up.
2) Is there any way to measure the *resistance* of the running
motor?
On page 12 of:
http://www.mtaonline.net/~hheffner/HullMotor.pdf
You can see the voltage drop of the single nichrome resistor
configuration was 0.063 ohms.
More recent experiments are documented at:
http://www.mtaonline.net/%7Ehheffner/HullMotor2.pdf
Note especially the experiment on page 6, and the wiring diagram
for it
in Fig. 6. This experiment still used the 0.063 ohm cold resistor at
that time.
Here are the traces for the motor being stopped:
http://www.mtaonline.net/%7Ehheffner/HullVAmotorStop.jpg
The current through the resistor starts out at 5 V. The current
is thus
(5 V)/(0.063 ohms) = 79 A, The voltage drop through the motor starts
out at about 2.8 V, so the resistance Rmotor = (2.8 V)/(79 A) =
0.036 ohms.
I added some shunts to the nichrome resistor to increase current,
at the
risk of blowing the battery. Here are traces from a regular run, and
here are traces from the stopped run using the new resistance:
http://www.mtaonline.net/~hheffner/HullShuntRun1.jpg
http://www.mtaonline.net/~hheffner/HullShuntStop1.jpg
I'm not sure I understand what I'm seeing in HullShutStop1.jpg.
Voltage
drop across the motor goes from about 3.5 volts down to about 1.6
volts
before you pull the plug -- is that during *braking* of the shaft, so
that it slows from full speed down to 0 RPM? That's what I think
you're
saying; but is that correct?
No. The motor is fully stopped. The current drops because the
nichrome resistor heats up to a glowing orange, and the resistance
goes up. I turned the motor off to keep the resistor from burning
out. Note that the resistor doesn't runaway like that in
HullShuntHighRPM2.jpg noted below. I think that is because the back
emf prevents enough current from getting through the circuit for the
resistor to burn up. That reduces the amount of the motor i*R voltage
drop attributable to its native resistance R.
I then jacked the starting speed way up and got this:
http://www.mtaonline.net/~hheffner/HullShuntHighRPM2.jpg
I'm wondering if it's the same as when it's stopped,
I don't think the resistance goes up very much because the motor
doesn't
heat up a lot like the nichrome wires.
Eh -- good point. It's dropping around 3 volts, at 100 amps or
more, it
should be getting pretty hot if that's going into heat.
Running 4 seconds gets a bearing warm to the touch but not hot.
Running 12 seconds gets them uncomfortable to touch.
But wait, let us think this through.
Stopped voltage drop looks like about 1 to 1.5 volts on the scope,
running drop looks like about 3 volts,
difference is 1.5 to 2 volts. So
that's the back EMF, right?
Yes, provided the current is the same as in the stopped run. The
back emf drops the current though. Dropping the current drops the
voltage attributable to the native resistance. It could be the true
back emf is 2.5 volts or more if the current drops. The back emf in
HullShuntHighRPM2.jpg limits the current to the point the filaments
stabilize temperature, which doesn't happen in the stopped run.
Around 1.5 to 2 volts?
If that's *NOT* going into heat, then it's going into mechanical
energy
(or RFI...). If you're putting, what, 140 amps through this, I think
you said in the paper, then that's in the ballpark of 250 watts, or
about a third of a horsepower.
A third of a horsepower -- going into an unloaded shaft? Hmmm.... I
think this had better be showing up as heat somewhere, if the energy
books are to balance!
Well, there is always the approach of doing calorimetry to sort
things out! Yuk!
There are indeed some mysteries about this motor, especially in my
mind. Much of the mystery is probably the result of the quick and
dirty experimental approach. With a lot of work all the mysteries
can probably be solved one by one and resolved as not anomalous. The
question I have at the moment is whether it is worthwhile for me to
pursue them, given I have a lot of other things that are of more
interest. I set out to show the effects are magnetic, and I have
certainly done that to my own satisfaction, and replication by others
should add confidence in that. I'm not sure I want to go any further.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
