Help with an engineering idea for a DC motor safety device

You would need one about the size of railroad boxcar.

Luhan

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 numbers into your calculator.

Paul Burke

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

Paul Burke wrote:

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

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.

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

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?

"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.