# Help with an engineering idea for a DC motor safety device

M

#### [email protected]

Jan 1, 1970
0
Hello all,

I have an idea to use energy stored in a capacitor to drive a small DC
motor in the event of power loss to the system.

Will something like this actually work? How would I determine the size
of capacitor needed?

I assume that I would charge the capacitor each time the motor runs
normally, so in the event of power loss after a loading cycle, I would
be able to then activate a switch, and used the stored energy to

Any ideas would be greatly appreciated.

Mike

L

#### Luhan

Jan 1, 1970
0
Hello all,

I have an idea to use energy stored in a capacitor to drive a small DC
motor in the event of power loss to the system.

Will something like this actually work? How would I determine the size
of capacitor needed?

Luhan

P

#### Paul Burke

Jan 1, 1970
0
I have an idea to use energy stored in a capacitor to drive a small DC
motor in the event of power loss to the system.

Will something like this actually work? How would I determine the size
of capacitor needed?

C = I*T/delta(V), assuming a constant current load will give you a
ballpark figure. So choose T in seconds that you want the motor to hold
up, I as the motor current, and delta(V) as the voltage droop that will
be acceptable (i.e. keep current constant-ish). I'll let you plug the

Paul Burke

M

#### [email protected]

Jan 1, 1970
0
Paul,

Thanks for the equation. So, if I understand it correctly, if I
measure the amount of current that the motor draws during an unload
cycle, as well as monitoring the voltage drop across the motor leads,
and the unload cycle time, I would then have all the info I need to
calc C?

Thanks

Mike

P

#### Paul Burke

Jan 1, 1970
0
Paul,

Thanks for the equation. So, if I understand it correctly, if I
measure the amount of current that the motor draws during an unload
cycle, as well as monitoring the voltage drop across the motor leads,
and the unload cycle time, I would then have all the info I need to
calc C?

Sorry, I was being a bit tongue in cheek. The capacitor is bound to be
BIG for any size of motor. Example, I have by me a tiny motor, about the
size of a phone vibrator, that takes 60mA at 6V. Say I want it to hold
up for a second. I estimate that the current won't have dropped off much
by 5.5V, so delta(V) is 0.5. The capacitor is thus:

C = 0.06 * 1 / 0.5 = 0.12F

You can get supercaps this size, but remember the 6V rating. Just about.
So you may be able to do it for a tiny motor, but it's impractical for
any sort of serious power.

Paul Burke

Z

#### [email protected]

Jan 1, 1970
0
I have an idea to use energy stored in a capacitor to drive a small DC
motor in the event of power loss to the system.

I have an idea that if you ever build this device you will find
yourself using a rechargeable battery instead.

Lead-acid cells are simple to float-charge and have a reasonably long
lifespan.

T

#### Tim Wescott

Jan 1, 1970
0
Hello all,

I have an idea to use energy stored in a capacitor to drive a small DC
motor in the event of power loss to the system.

Will something like this actually work? How would I determine the size
of capacitor needed?

I assume that I would charge the capacitor each time the motor runs
normally, so in the event of power loss after a loading cycle, I would
be able to then activate a switch, and used the stored energy to

Any ideas would be greatly appreciated.

Mike
I think you'll find that the amount of kinetic energy the motor can
store is way more than the amount of electrical energy a same-sized
capacitor (or capacitor bank) can store.

This is why computer UPS's use batteries.

And the amount of chemical energy that a given battery can store is
dwarfed by the amount of chemical energy in a same-sized jug of
gasoline. You need a generator set, though, which can't deliver as much
current for the size, but for long term power use it can't be beat.

This is why my neighbor has a generator for winter storms, instead of a
garage full of UPS's.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

L

#### Luhan

Jan 1, 1970
0
Tim said:
I think you'll find that the amount of kinetic energy the motor can
store is way more than the amount of electrical energy a same-sized
capacitor (or capacitor bank) can store.

This is why computer UPS's use batteries.

And the amount of chemical energy that a given battery can store is
dwarfed by the amount of chemical energy in a same-sized jug of
gasoline. You need a generator set, though, which can't deliver as much
current for the size, but for long term power use it can't be beat.

This is why my neighbor has a generator for winter storms, instead of a
garage full of UPS's.

--- or a shitload of gigantic capacitors...

Luhan

J

#### Jimbo

Jan 1, 1970
0
A flywheel may be an option for you on something like this. I'm not a
mechnical engineer but I do know they are used on large UPS systems.
See: http://www.genpropower.com/powerware_pf2_flywheel.htm

It really depends on what you mean by "small" motor. Designing on to
work with your DC motor may be very costly.

M

#### [email protected]

Jan 1, 1970
0
Luhan said:
--- or a shitload of gigantic capacitors...

Luhan

That reminds me... why does the National Ignition Facility use banks of
capacitors to store the energy for their lasers? Could they make the
building smaller by using rechargeable batteries instead?

http://www.llnl.gov/str/Powell.html

"The power conditioning system provides the energy for the flashlamps
with the highest-energy array of capacitors ever built. The system's
design is a collaboration among Sandia National Laboratories in
Albuquerque, Lawrence Livermore, and industry. Sandia is responsible
for designing the system, developing the switch, and testing the
integrated module at its dedicated facilities, while Livermore is
responsible for developing capacitors, power supplies, and other
components. Ultimately, Sandia will lead the assembly installation and
checkout of the NIF power-conditioning modules (Figure 7)."

T

#### Tim Wescott

Jan 1, 1970
0
That reminds me... why does the National Ignition Facility use banks of
capacitors to store the energy for their lasers? Could they make the
building smaller by using rechargeable batteries instead?

http://www.llnl.gov/str/Powell.html

"The power conditioning system provides the energy for the flashlamps
with the highest-energy array of capacitors ever built. The system's
design is a collaboration among Sandia National Laboratories in
Albuquerque, Lawrence Livermore, and industry. Sandia is responsible
for designing the system, developing the switch, and testing the
integrated module at its dedicated facilities, while Livermore is
responsible for developing capacitors, power supplies, and other
components. Ultimately, Sandia will lead the assembly installation and
checkout of the NIF power-conditioning modules (Figure 7)."
Probably because for getting the energy out _fast_ capacitors just can't
be beat.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

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