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motor as generator, where does current go?

i have a DC motor driving a piston in a cylinder via a short "crankshaft" arm.

the cylinder is connected to a fixed, sealed, volume.

so as the motor turns and pulls the piston out, vacuum is generated. when the piston has reached the peak of its travel, it starts going back inside the cylinder, and the pressure rises until it has reached atmospheric again.

i noticed that when the piston is traveling back into the cylinder, the vacuum force is actually pulling the piston in, and it appears to be driving the motor as a generator. the voltage waveform across the motor actual increases above the power supply voltage of 12V, going up to about 20V! This causes my bench supply to start going into over voltage mode.

So I added a diode such that current can only go INTO the positive lead of the motor.

Now, the bench supply doesnt go into over voltage mode, and the voltage at the power supply remains steady. However, the voltage at the MOTOR still goes way above 12V during the return part of the cycle.

My question is this:

Where is the voltage "going"? Where does the current generated by the motor "go" since the diode is there?

Reply to
acannell
Loading thread data ...

Probably nowhere. It seems to be behaving just like a disconnected

12V battery just sitting there.

Your diode - does it get hot?

IANAEE - just my $0.02 worth

Michael

Reply to
mrdarrett

On a sunny day (Wed, 21 Jan 2009 10:48:55 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

Voltage does not 'go' anywhere, a voltage is generated by the motor. There is no load on that voltage because of the diode, so no current flows. Look at it from a _power_ POV. No power is delivered, this means the piston is sucked back in faster, then it would be if the motor was for example connected to a battery and putting back energy into the battery. In case of you lab supply, little or no current flows back into that either, so no power is delivered back either.

Reply to
Jan Panteltje

s.

en

er,

Hmmmmm...

Lets say nothing is connected electrically to the motor, except for a super high impedance voltmeter, say with an impedance of 10 gigohms. Now you drive the motor, and it generates a voltage. Why does the voltage return to 0 so quickly if you stop driving the motor? The output impedance of the motor must be in the single digit ohms, so its not just being absorbed by the voltmeter. Where does that energy go?

Reply to
acannell

ows.

then

ther,

dissipated as heat to the surroundings

Reply to
mrdarrett

On a sunny day (Wed, 21 Jan 2009 12:00:35 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

There is _no_ energy involved. Energy, or better power here, is current multiplied by voltage. there is no current, so no energy.

Reply to
Jan Panteltje

low=3D

th=3D

ith=3D

There is some energy - the piston is sucked back into the vacuum, remember. W =3D delta (P V), from what little I remember of thermodynamics

Michael

Reply to
mrdarrett

On a sunny day (Wed, 21 Jan 2009 12:26:25 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

Yes that is true, but I was under the impression he was talking about the electric energy. The induced voltage is depending on the magnetic field, and movement of the wires of the motor in it. No motion no voltage. This because he asked: 'Why does the voltage return to 0 so quickly if you stop driving the motor?'

Different from the question 'Why does the motor stop so quickly?' (Answer: because it has to pressurise air with the piston).

Whatever the OP's idea is...

Reply to
Jan Panteltje

there is no (electrical) energy transfer. (A tiny amount goes into the voltmeter but I assume you want to ignore that)

if you were to connect a load across the motor terminals the motor would become harder to turn and energy would be transferred into the load.

in your original experiment

connect a lamp (or other load) that's about the same size as the load the motor provides and then when the piston is turning the motor you see slower movement as the load absorbs the energy that your powersupply can't (or you could try using a 12V lead-acid battery as a power source instead of the lab supply)

Reply to
Jasen Betts

What was this motor crank pump apparatus originally designed for? Auto tire inflator? Breast pump?

You thought you had found a loophole and the basis for a perpetual motion machine or unconventional energy source, right?

Have you considered harnessing the power of water rushing down your rain gutter downspout?

Reply to
Greegor

That's what it sounds like to me. ;-)

A few months or years ago, somebody did the numbers on this, and came up with some number that simply wouldn't pay for the cost of the machinery.

Cheers! Rich

Reply to
Rich Grise

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