charging a capacitor with a small voltage 5

[wazi210]

Hi,
I’ve got a flywheel driving the rotor of a dc brushless dc motor, and I want to store the generated electrical energy to a reservoir capacitor.
The question is that how can I take out all the energy out from the rotary energy of the flywheel. As we know, while charging up the capacitor, the rotational speed of the flywheel reduces, hence the induced voltage at the generator output reduces, and the current cannot flow into the capacitor.
Are there any solutions can conquer this problem?
Thanks.

 

[MacGyverS2000]

I have to ask for my own curiosity... what's the application here?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[djs]

You will need to use a buck/boost converter.

 

[IRstuff]

Not sure what you're asking. If you "take out all the energy out from the rotary energy of the flywheel" then you will cause the flywheel to stop; that's the whole point of conservation of energy.

TTFN

FAQ731-376

 

[wazi210]

Hi IRstuff,

You are albsolutely right. My ideal goal is to take energy out from the flywheel until it stops. However, the problem is that when the rotor reduces to a very slow speed, the generated voltage will be very small compared to the voltage stored in the capacitor. So I need to figure out a way to force the current flow into the capacitor, thus the speed of the rotor is further reduced.

 

[wazi210]

Hi djs,

A step-up converter is the current solution in my mind. However, do you think will it work very well? If the voltage generated is far more less than the capacitor holding voltage, will it still work?

Are there any other better alternatives?

 

[IRstuff]

You should be able to do your own research:

http://para.maxim-ic.com/en/search.mvp?fam=stepup&tree=powersupplies

TTFN

FAQ731-376

 

[jimkirk]

How much energy are you talking about and how fast do you want to transfer it? Voltage ranges, currents...

One thing to keep in mind is that in typical switch mode power supply chip specs, the minimum voltage is limited by the control circuitry needing some voltage to operate.

With FET switches and synchronous rectifiers that have essentially zero forward voltage, the power switching circuitry can operate down to very low voltages. Component parasitics become the limiting factor. So if you can, have a low power supply powering the control circuitry and provide enough gate voltage for the FETs.

Some simple SMPS chips might even let you separate the voltage supply from the power section (which would be your motor) and the control section, which would require several volts, but at milliamp type currents.

 

[VEBill]

Maybe a wee little transmission so you could shift gears to maintain a more-constant generator speed as the flywheel slows down.

 

[roydm]

At the high speed end there will be a higher current therefore more breaking force.
What would happen if you were to use a current regulator between the motor and capacitor, would that even out the breaking force?
The other problem as the motor slows, it's voltage drops but meanwhile the capacitor voltage is getting higher.
Perhaps you need some sort of DC/DC converter to boost the voltage to a much higher level or with a constant current Input or output.
Thats my 2c worth
Roy

 

[wazi210]

Hi jimkrik,

The stored electrical energy in the capacitor is used to power up an electric circuit, which consumes less than 100mw, therefore this is not an application for high power. The power rating of the generator is chosen at 1w, and I expect to transfer the energy stored in the flywheel as fast as possible to the capacitor and prepare for powering the electric circuit.

The external force exerted on the flywheel is not continuous, so I wish to finish transferring and storing the energy at the interval of force acting activities.

wazi

 

[wazi210]

Hi roydm,

Thank you for your input, and I also consider a boost DC/DC converter as one of the possible solutions.

However, are there any other ways of dealing with it?

wazi

 

[KnoItAll]

Use a boost converter

http://en.wikipedia.org/wiki/Boost_converter

The motor inductance can be used as all or part of the boost converter inductance. Generate a square wave of a suitable frequency and use it to drive a transistor that shorts out the motor. When the transistor opens, the diode in the boost converter will act as a typical flyback or snubber diode, thus protecting the transistor from over-voltage by clamping the voltage across it to the capacitor voltage.

 

[jimkirk]

Hi wazi,

Good, the energy levels are quite manageable. If you were dealing with microwatts or kilowatts, things would be much trickier.

My main point is that if, as your original post states, you want to convert ALL the energy (less efficiency losses, of course), then avoid transistors and diodes, they have a forward voltage that will limit the lowest voltage they can deal with.

Silicon diodes have about 0.6 volts drop, Schottky gets it down to about 0.4 volts, and germanium down to 0.2 volts or so. A few years ago, there was some talk of Schottky germanium rectifiers, but I haven't heard anything about that for a while.

But a properly switched FET can work down to zero volts. They do require control circuitry which will need a few volts at a few mA to operate, so if that's available you'll be able to eek out every last erg, and do it more efficiently as well. All those forward biased junction drops will hit your efficiency, especially with a low input voltage.

 

[Skogsgurra]

Hello wazi,

This seems to be a case where "perfect" is enemy of "good enough".

The energy in the flywheel is proportional to speed squared. If you try to squeeze out the last few percents of speed, you will have very little energy advantage. If, for instance, you try and get energy out of the flywheel down to 10 % of maximum speed, you will recover 99 % of the energy. If you use only a 100 - 20 % speed range, you will get 96 %. Question: Is the extra effort really worth the 3 % energy gained?

Even if you use a modest 100 - 50 % speed range, you will get 75 % of the stored energy out of the flywheel. That may be all you need. Especially since your circuitry probably will be more efficient than one that needs to work over a broader voltage range.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I like your approach and attitude, Gunnar. LPS
Good enough may be better than perfect in the long run.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sreid]

Properly implemented, the problem solves itself. First, you have to have an "Inverter" to spin the motor-flywheel up to get the energy in in the first place. Use an inverter that controls the motor phase currents [not the phase voltages]. When you decelerate the motor by reversing the phase currents, the energy in the flywheel is automatically sent to the B+ supply [regeneration]. (Lots of details left out so the basic concept is clear).