Motor / Battery Recharge

[ERev]

I am about to make a system which includes a dc motor, a generator, and a battery. Basically when the dc motor turns it will also turn a generator and that will provide a current to recharge a small battery. That same small battery would then be used to power the motor.

(Motor is a 3hp dc brushed)

My questions are:

1) Can the generator simply be a small motor?

2) If the efficiency of the generator is 70% (what would it
approx be?) how long would the battery last (we'll say its a 12 volt car battery)

3)Would the current provided by the generator be suitable
to recharge the battery (does it even matter the current / voltage) (* would a toy motor spinning 2x faster than a motor twice its size generate equal current?)

4)If the motor is hooked up to a pedalling system for instance, would the force of the pedalling motion make up for the losses of the generator making the battery always in power?

5) How long would the car battery last under this constant usage & recharge cycle? It would operate for 1-2 hours per day on average with a load of 160 pounds.

General Question:
If a motor is fed a specific voltage, does it naturally want to spin at a certain speed? And if theres a load causing it do drag by 50% what would be the effects on the current etc?



Any comments, tips, and suggestions are greatly appreciated. Thank you.

 

[Skogsgurra]

Are you trying to build a perpetuum mobile machine?

Or is there any other, more useful, purpose?

 

[itsmoked]

skogsgurra is really asking if you are trying to make a:

>perpetual motion machine<


Are you?

 

[ERev]

Yes I am, this will be installed onto a bicycle.

 

[IRstuff]

And by that, does that mean you understand that it can't be done?

TTFN

 

[ERev]

Well if you took the time to read my post, it's not a perpetual motion machine, however its simple a system that gives back most of the energy so it can be resused.

 

[aolalde]

ERev:

For the weight of a human body, the 160 Lb weight, the battery, air friction plus the energy required to charge the battery you will exhaust in few minutes the stronger person capable of pedalling the bicycle.

If you initially charge the batery it will last some minutes depending on the speed and battery size but after you ehaust the original charge it will be almost imposible to recharge it and at the same time move the bicycle with all the stuff.Another burden are the motor and generator losses.
Remember that the energy converted to heat by friction and electric resistances will dissipate in the atmosphere.

 

[ERev]

The total weight is 160 pounds, including everything.

Why would it be impossible to charge it, since most magnetos have 60-80% efficiency, and a 12 volt car battery under the load would last 10-20 minutes i'd imagine. Now if 75% of the energy is being put back into the battery continuously it would last much much longer, but just how much longer? What is the equasion for calculating this?

*Please view my first post for the information I need, thanks for replies.

 

[Skogsgurra]

There is an "equasion" that says that it cannot be done.

"Oh, another stupid old fart" you probably think. OK. But this old fart actually knows a few things about energy.

What you are trying to do has been tried by the same kind of people that tried binding wings to their arms and jumped out of windows to see if they couldn't fly like birds after all. And killed themselves.

Even if I gave you the equations, you would probably 1) not believe they were true, or 2) not understand them.

Sorry to be harsh, but I think that it is what you need.

 

[ERev]

Clearly there is a misunderstanding, I am not attempting to build a perpetual motion machine, as it is not stated anywhere in my post.

 

[IRstuff]

You answered in the affirmative:

Yes I am, this will be installed onto a bicycle

Seemed pretty unambiguous to me.

But, just for giggles, let say you have a 12-V 100-Ahr battery. That's 1200W*hr. According to:

http://hypertextbook.com/physics/mechanics/power/,

a cyclist puts out about 400W. That means assuiming 100% conversion efficiency, you'd take 3 hrs minimum to charge the battery up. When you factor in all the efficiencies, you're talking about 9 hrs to charge the battery.

TTFN

 

[Skogsgurra]

Well then. Go on assembling a bike with a generator and a motor. That will teach you a lesson.

But, if you really want to know: Let's start with the motor. You assume an effiency about 70 percent for the generator and I would say that it applies to the motor as well. So, for 1 HP (assuming that you do not need all 3 HP available) you need to input the equivalent of 1 HP/(motor efficiency) = about 1.4 HP electric power. To produce that amount of power you need to input 1.4/(generator efficiency) = about 2 HP. So, to get 1 HP you have to provide 2 HP. Not a very godd ROI.

I think that your problem is that you do not understand that you need to input power to a generator to produce electric output. So you do not feed back energy. You just waste it. Putting a battery and an electric motor on a bike will give you an electro-assisted bike. They have been around since 1920-30. They are fun and quite sensible vehicles. The generator will only waste energy - an electric brake, if you want.

 

[ERev]

We'll that makes alot more sense to me, since I am only an amature in electronics. However for curiosity if a tiny motor was hooked up to the large motor (tiny motor would be a simple toy motor) and it was geared to go 50x faster than the large motor, would'nt it provide a generous amount of current to the battery? Would spinning even a motor this small really add noticable resistance since it is so smooth and light weight?

Anyways thanks for the replies, I do appreciate it.
And btw if you have any suggestions on the best method for an alternative power to a bike without the use of a gasoline engine that would be helpful. Thanks again.

 

[IRstuff]

Power is conserved. Does not matter what the gear ratio is. You cannot get more power out than what you put in.

TTFN

 

[itsmoked]

No ERev the tiny motor won't work.

The efficiencies we've been talking about never mentioned the sizes of the generators/motors so those "problems" don't go away or get alleviated in any significant way by choosing different size motors.

The only hope an electric bike has for anything like you want is the ability to use any energy normally wasted in braking to instead put charge back into the battery.

The easy way to do it is to hook up a separate generator that you can engage instead of using the brakes.

The downside is EVERY OUNCE of weight you add to an electric vehicle drastically reduces its efficiency and the distance it can go on a charge!

The energy you would recoop would not be worth it.

SO! The better alternative is to use a motor that can be a generator too. Unfortunately the electronics required to do this are non-trivial and far beyond the scope of any of these posts.

Research the web for E bikes and buy one!

 

[rmw]

ERev, I am curious to know if you are actually are an engineer? We may be using terminology too technical for you.

rmw

 

[ScottyUK]

ERev,

The system you are describing would work IF there were periods where the load dropped below the prime mover power limit allowing the available excess to recharge the battery. Something like an automobile engine on a hybrid vehicle. Study how these work, and if you apply the losses to your situation you will see that there is a lot of wasted energy - more than a human can reasonably sustain while also pedalling a bicycle laden with motors, dynamos, batteries etc. Overall you will put in more energy to the system you are proposing than you would if you just pedalled the unladen bicycle.

Is there some reason why you haven't considered using the DC machine as both the motor and generator? This saves you one set of conversion losses.




----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Skogsgurra]

ERev, I feel that I should apologize for being quite rude to you last night. I thought that you were one of these guys that never listen to what other people say. Just because you know so much better.

I am glad that I was wrong. You hvae actually listened. And I think that you will eventually understand as well. It is all about conservation of energy - it cannot be created and it cannot be destroyed. Only changed from one form to another. And the change from mechanic or elecric energy to heat (i.e. losses) is not reversible. That means that every motor or generator will reduce the available (non-heat)energy.

But, Scotty has a point. By charging the battery downhill (instead of turning all energy into heat in your brakes) you can reuse part of that energy.

Let's use the 70 percent efficiency for the combined motor/generator and assume a good battery with 80 percent energy efficiency: The energy produced downhill is converted to 70 oercent electric energy and stored in the battery. From there you get 80 percent back (80 percent of 70 percent is 56 percent. And if you turn that into "bike energy" again (with 70 percent efficiency) you will have 56 x 70 = about 40 percent back.

I would say that it is hardly worth the effort since it is ONLY THE DOWNHILL ENERGY THAT IS REUSED. The extra gear will make your bike heavier and also add friction, which means that you have to pedal more in normal use. It is only in extremely hilly landscapes that such an arrangement would be of any use at all.

 

[mc5w]

If what you are trying to build is an electric moped for going up and down hills you could get some energy back when going downhill. For the amount of weight you would need to be using an adult tricycle. However, the rate at which you can put energy back into a battery without gassing it or overheating it is proportional to how nuch it is discharged.

The main bottleneck with electric and hybrid vehicles is that a battery bank that can rapidly charge and discharge with good efficiency is very heavy. You also have a problem that in warn climates such a Florida car batteries live for only a year.

Vienna, Austria at one time had battery buses that towed a trailer of batteries for motive power. These would be dropped off at recharging stations and replaced with a recharged trailer. A similar concept is use with electric forklift trucks by changing the battery using a floor mounted bridge crane. You generally have 3 or 4 batteries per forklift truck depending on how hard you are using them.

Theoretically, the lightest weight practical battery is based on the aluminum bromine reaction. This would need to operate at about 450 degrees Fahrenheit to keep the aluminum bromide electrolyte liquid or use a solvent and catalyst in place of the heat. 6 inches of glass fiber insulation would be enough to keep self discharge ( due to electric heaters ) down to around 15% per day which is comparable to nickel-iron batteries. There are some other practical considerations such as a strong enough pressure vessel for the cell to keep the bromine liquid at say 550 degrees Fahrenheit when the bsattery heats up due to internal resistance and heavy usage.

There are a lot of interesting concepts out there but building something that is practical is a different story. With the price of gasoline electric cars might come back and we might even replace the streetcars that got ripped out during the 1950s. A catenary wire read long extension cord is still one of the most practical electric vehicle concepts.