Another Knowledge Question

[Mithion]

If you hooked up an alternator to the output shaft of an electric motor would it work as a generator to at least recharge a battery a little bit? I was reading that the alternators on big trucks and buses can produce up to 300amps.

[Brammofan]

I expect protomech will be able to expand on this, but basically it sounds like you're looking at a regenerative braking system that would be separate from the motor itself. I think the drag that something like that would place on the motor would be counter-productive to the amount of energy produced.

[Mithion]

I see what your saying, maybe instead of a direct connection to the output shaft there could be an alternator connected freely to the wheel where the motion from the spinning wheel rotates the alternator. This way the electric motors are propelling the vehicle and that very rotating motion they create is spinning the alternator which is creating the recharge to the battery.

[EmpulseRider]

Brammofan is right. The first law of thermodynamics says that this would not work:

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

[Mithion]

Makes sense, but what if the system were externally cooled somehow to reduce the created heat?

[EmpulseRider]

In this case you are trying to harness kinetic energy, which would in turn slow down the bike, and on top of that a percentage of that returned energy would be lost as heat (battery charge and motor waste heat)... so you have diminishing returns at both ends, loss of kinetic energy and waste heat. Now if there was a way to harness that waste heat, then you may have something. Regenerative breaking works because you WANT to remove kinetic energy to slow down, even if it is not 100% efficient its better than just losing all that kinetic energy to brake friction which produces more waste heat.

A 100% efficient motor would produce 100% kenetic energy and no heat...

Now that I think of it, both the Zero and Empulse have simulated engine breaking... so, they already do this, even though its wasteful. Protmech or BrammoBrian can probably explain this much better.

[Mithion]

Well...lol... Guess I won't be the guy to figure out perpetual motion!!! :0)

[protomech]

Certainly an alternator could recharge the battery by converting kinetic energy back to electrical energy. In fact, both on the 2012 Zero bikes and the Empulse the main traction motor can do this by acting as an alternator. No need for the weight and complexity of a secondary alternator.

However, you will always, always get less energy out of the system than you put in, once you account for all inputs. For example, if you start at the top of a tall mountain you can recharge the battery somewhat through regen braking. At the bottom you'll likely have more charge than you started with ... so what happened? You lost the potential energy from having the bike at the top of the mountain ... and in practice you'll regain roughly 25% or so back into the battery pack .. so you can perhaps ride 4 miles down, then use that regained charge to ride 1 mile back up.

So where does the rest (roughly 75%) of the potential energy you started with go? It's lost as resistive heat through the motor, both in regen and power modes; heat in tires as they grip the road; heat in the forks and shocks; friction heat in the wheel ball bearings; sound; drag and turbulence through the air; the headlight and other onboard electronics; etc.

Similarly, you can slowly accelerate from a stop to say 25 mph, then slowly regen brake your way back to 0 mph. You won't regain > 100% of the energy used in acceleration, or even close to 100%; over a short distance (immediately braking once you hit 25 mph) you might see 50% regeneration.

[Mithion]

Ok, so basically when it's all said and done- the energy required to run the alternator included in the system is more than the energy the alternator would produce anyway so this not worth it.

[Richard230]

Just to toss in a little motorcycle history into the discussion, my first "full size" motorcycle was a 1963 Yamaha YD3 touring bike (complete with accessory legshields and 16" whitewall tires). It used a "dynamo" installed on the end of its crankshaft (the clutch was on the other end) that first acted as an electric starter when you pushed the starter button and when the engine started running, it reverted to a electric dynamo and generated electricity to recharge the battery and operate the ignition and lights.

[Mithion]

Kind like the hub dynamos used on bicycles

[protomech]

The older Honda Goldwings operated in a .. somewhat similar fashion. Separate starter motor and stator to operate the electrical system. Have to pull the gas engine to change the stator.. seems most people report the change interval is about 20k miles (stator gets caked up with oil, not sure what Honda was thinking there). Rather labor intensive.

Friend bought a 1984 aspencade, he's replacing the stator system with an external alternator. Seems like a lot of work, but oh man those bikes have a bunch of electrical gadgets on them.. even the old 1980s bikes.

@Mithion exactly like that.

And to bring it somewhat full circle.. you can basically use a beefed up motor to drive the rear wheel in the same way to turn a bike into an ebike.
http://www.electricbike.com/friction-drive/

[Mithion]

So would hub dynamos have any benefit on an electric motorcycle or are not even worth the power they would generate? I was picturing using Hun dynamos to power the lighting system so the batteries can be fully used to power the motor and ecu only

[protomech]

No. The batteries would still be powering the lights indirectly. Think about it this way: if the bike doesn't move, the dynamo doesn't turn. How does a pedal bike move? The human turns the pedals. How does an electric bike move? The battery turns a motor. Therefore the human (or battery) is indirectly powering the dynamo..

You can do this two ways:

Battery => DC/DC converter => 12V lighting system
Battery => motor controller => motor => dynamo => 12V lighting system

DC/DC converter is probably 90-95% efficient.

Motor controller is probably 97+% efficient.
Motor is 85-90% efficient typically.
Dynamo (kinetic energy to electric energy) is probably 60-70% efficient at low speeds, and gets much worse at high speeds unless you could build in some type of variable resistance.

Worst case vs best case, 90% vs 97% * 90% * 70% = 61%.

There's some debate about dynamo lights vs battery-operated lights in the pedal bike community as well.. though this may be done with the introduction of affordable LED headlamps. You could even have a hybrid battery/dynamo (or capacitor/dynamo) setup, as in a ICE motorcycle or car .. which would give some of the advantages of both along with increased weight.
http://nordicgroup.us/s78/experts.html

Here's a review of various hub dynos for a pedal bike, take a look at the power requirements (page 3) to operate a 3W (nominal) headlight:
http://www.bikequarterly.com/VBQgenerator.pdf

[Mithion]

All makes sense. So from what know/read/etc, what's does the industry seem to be looking into in terms of research for that seemingly unobtaibable perpetual motion machine? That electric system that powers and recharges itself from its own operation etc...