motor as generator, where does current go?

[[email protected]]

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?

 

[[email protected]]

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?

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

 

[[email protected]]

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.

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?

 

[[email protected]]

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?

dissipated as heat to the surroundings

 

[[email protected]]

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

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

Michael

 

[Jasen Betts]

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?

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)

 

[Greegor]

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)

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? <grin>

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

 

[Rich Grise]

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? <grin>

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

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

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