BACKGROUND: I am an intern with a local company. We have a powerful 60 KW Engine that runs throughout the day. We have a two Machineries that are powered off the Engine directly (using clutches and gears of cause).

The first Machinery runs a feed continuously, while the second Machinery is periodically engaged to cut strips of sheet metal (Cutter)


  • The cutter is engaged to the engine;
  • It spins at a very high speed
  • cuts a strip briefly
  • and disengaged

--> After it is disengaged, I've noticed the cutter spins for almost 40secs before coming to a halt.

--> I also estimated an initial Idling Torque 20 - 30 Nm ...@3000+ rpm (rough calculations from data I retrieved from as-built documents)

PROBLEM: We want to install a brake for the cutter; And am working with a staff on the brake design; ** I feel I can propose installation of a little Generator to utilize the free-spin energy. I believe this can act as a Soft brake. (using clutches still)

Obviously, only a small amount of current can be produced for a few seconds...


  • I want to know if I can store electricity that is generated for a few seconds repeatedly (non-constant intervals). If so how?
    • I know quite a bit on Capacitors and Batteries, but is there any battery that can be charged under these conditions? ...I have done lots of fruitless Google searches
  • Some staff have told me its a fruitle idea with no convincing reasons...(@least to me)
  • Please give me further directions on what to read

NB: I am an Undergraduate Mechanical Engineering student

  • \$\begingroup\$ It's likely to be a 3 phase induction motor if that's any help. These do not contain magnets so they don't naturally produce a significant output voltage when turned into generators meaning, I suspect you are not going to extract many useful joules when the motor is turned off. Hope I'm wrong. \$\endgroup\$ – Andy aka Dec 1 '14 at 9:54
  • \$\begingroup\$ @Andyaka, thanks. I don't think I can extract much Energy either.. But I believe the little extracted, can be stored in a capacitor, and when capacitor is full, the energy is flushed into a battery... But how efficient is this? Thanks \$\endgroup\$ – WhiZTiM Dec 1 '14 at 10:01
  • 1
    \$\begingroup\$ Whiz, you will probably not harvest enough energy to make the whole project worthwhile. It's fixed-installation equipment I guess, so it's not like access to grid energy is an issue. The amount of energy saved and re-injected to the system would be very low, and at the cost of power I highly doubt there would EVER be return on investment for the project costs. In this case, I suggest merely doing mechanical braking.. for a mobile vehicle/device, or battery powered systems in general, yes regenerative systems are beneficial. \$\endgroup\$ – KyranF Dec 1 '14 at 10:20
  • \$\begingroup\$ If you got a, say, 12V battery charged this way, what would you do with the stored energy? Would you want to somehow feed it back to the machinery, or just power the TV set in the supervisor's office? - For most cases you will need some more (power) electronics to get useful output, a 230V inverter at least for example. \$\endgroup\$ – JimmyB Dec 1 '14 at 11:02
  • \$\begingroup\$ If the brake is required for safety, it would be a bad idea to compromise its effectiveness in any way (and that includes its simplicity) just to recover a little power. How much is your little finger worth? \$\endgroup\$ – Brian Drummond Dec 1 '14 at 11:26

Storing energy in a capacitor is fine providing that the energy requirements are not that big. Consider that the energy stored is: -

Energy = \$\dfrac{C\cdot V^2}{2}\$ where C is capacitance and V is voltage across terminals of the capacitor.

Note that when storing energy into a capacitor, storing more volts as opposed to storing the same voltage on a bigger capacitor is more effective for energy.

Say you used a 100uF, 2000 volt capacitor (you might have to build one out of smaller values of course) and, charged it to 1000 volts DC. The energy stored is 50 joules and you could probably extract 80% of that energy and dump it into a battery.

Ask your self how much energy you'd like to extract from the braking process.

A better method might be to use a buck regulator that fed the battery and forget about the intermediate capacitor as an option. If you were using a capacitor, you'd need some form of high voltage buck regulator anyway to down size the 1000 volts to a battery.

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