Generating energy and releasing energy through motor

[barrett189]

Hello everybody,

I'd like to ask for help with a circuit for a school project.

The goal of the project is to let a small car roll down a slope, store the potential energy, and release it to go back up the slope.
I will be using a small 5W motor, and a PWM circuit to release the energy gradually. I already built the PWM, but now I do not know how I can hook everything up.
I guess I'll use diodes between the motor connections, and the PWM input (which has a capacitor to store the energy). Now i'll need to do something about the output of the PWM.
Only when the motor supplies 0V, can there be a current between the PWM output and the motor.


Help would be very much appreciated! Thanks in advance,


Alex

 

[chuckey]

I think all you need a one capacitor. Connect it across the motor, it will charge going down the slope and maintain the car for a short distance after. The secret is to optimise the capacitor, too small it will get the max voltage too soon, so the after run will be also too short. Too big and it will not charge to any useable voltage.
Frank

 

[abaz]

hmm...
You haven't described the physics behind it but I beleive if you hooked a cap directly to the motor, assuming the motor will produce a dc voltage that is within the cap's specs, it may very well not have enough wieght to roll down the hill as a discharged cap will be an essential short circuit load accross the motor effectivly acting as a brake. You may have to use the pwm to regulate power into the cap from the motor so that the load presented to the motor is sufficiently low enough to allow the car to roll down the hill

 

[FvM]

You may have to use the pwm to regulate power into the cap from the motor so that the load presented to the motor is sufficiently low enough to allow the car to roll down the hill.

That's how recuperation in hybrid cars works.

The goal of the project is to let a small car roll down a slope, store the potential energy, and release it to go back up the slope.

You should think about efficiency. It's effectively impossible to make the project car go fully back up the slope. Half the way without supplying additional energy won't be bad.

 

[abaz]

You should think about efficiency. It's effectively impossible to make the project car go fully back up the slope. Half the way without supplying additional energy won't be bad.

Makes you wonder, if you left out all the electrics and just put some lead in it would the momentum get it further up the hill than the motor... A study in the conservation of energy?

 

[barrett189]

Hello everyone, happy new year!

Sorry i haven't replied for so long.

The reason why we are using a pwm, is because we think that the wheels might spin through. Using a resistor would not be as efficient as a pwm. (And yes, the motor produces a DC voltage )
To visualize my question, i'd like to show you the below picture.


Thanks a lot in advance,

Alex

 

[FvM]

A standard recuperation setup, as e.g implemented in hybrid cars, would feed the energy through the PWM bridge both in generatoric and motoric mode. It allows to convert the motor emf to the battery voltage with maximum efficiency at any motor speed and also connects/disconnects the motor from battery.

 

[BradtheRad]

I have been experimenting with several small motors from my junk box. I spun the shaft as I watched volt and current output for the highest reading.

I find a wide variation in power output. The best one puts out 5 V at 50mA. That's .25 W. I'm not sure any of them could produce enough power.

I hooked the motors up to a 160,000 uF capacitor.
Once the capacitor acquires a charge, it assists the motor in running in the same direction.
This means you will have to reverse the polarity in order to make the motor push the car in the opposite direction (uphill).
You will need to either (a) detect when the car stops going downhill, or (b) brake the car to a stop at some arbitrary time.

You may need gearing in order to spin the motor fast enough to charge the capacitor.
The same gearing may or may not be suitable for the motor to spin the wheels later.

Your wheels will need to be as friction-free as you can get them.

 

[FvM]

You may need gearing in order to spin the motor fast enough to charge the capacitor.
The same gearing may or may not be suitable for the motor to spin the wheels later.

A PWM bridge can basically provide the "gearing", convert motor emf to actual capacitor voltage as long as it's lower than the capacitor voltage.

 

[barrett189]

Hi,

I don't want to be a dick, but nobody seems to be answering my question

Let me reword it:
I'm looking for an electric component, that allows a current between two poles to flow, if and only if, the voltage between two other poles is 0.

Thank you,

Alex

 

[FvM]

I think, you just don't see how the contributions are related to your question.

To suggest a shortcut, you are apparently imagining a circuit that closes a dual-pole switch when the motor voltage drops to zero, and holds it closed afterwards. This sounds like a simple circuit. It would basically make the car drive backwards up the slope, as requested, but not achieve maximum efficiency.

 

[chuckey]

the first thing to do is to re-draw your circuit with a common "earth" line. Then you want a relay with a single pole change over contact, that will direct the motors voltage into the capacitor when the relay is at rest. Then you want to sense the level of the voltage coming from the motor, energising the relay when the motor voltage is really low NOT capacitor voltage(car stationary) so the output of the PWM to DC generator goes to the motor. You also need some circuit to sort the ambiguity of Vmotor =0V, charge capacitor from the motor, and Vmotor = 0V switch motor to output of PWM generator.
Frank