E-Bike dynamo top up.

[benjy]

I am designing an electric trike running on two 500w 36v hub motors at the back and a small wheel at the front with a matching ping battery. This will be my first e-bike I have built.

To make the trike look like it is pedal powered (it won't be) I will build an ordinary bike pedal system that directly drives a dynamo (not the wheels) to help top up the battery. Willing to pay upto about £50

Could you give advice on- How the dynamo would connect to the controller
What to use as a dynamo (link?)
Where to get one (link?)

Thanks,
Benjy

 

[Jeremy Harris]

Any PM motor will work just as well as a generator. If you use a brushless motor then you just need to add a simple three phase rectifier (6 diodes) to it to get raw DC that would be OK for charging. You need to match the shaft RPM to the pedal cranks to get enough voltage, so look for a motor with then lowest Kv you can find. Power rating isn't super critical and you're only going to be able to put a few hundred watts peak, maybe 120 watts average, into the pedals. The biggest problem will be gearing up the pedal crank to a fast enough rpm. If you pedal at, say, 80 rpm, and you use, say, a 75kV motor, then to get enough volts to charge the battery a little bit (around 42 V or more for a 36V Ping) then you would need get the generator shaft speed up to about 3150 rpm, which is a big step up from 80 rpm. If you can find a really low Kv motor, maybe a hub motor with a Kv of around 12, then the required generator shaft speed comes down to around 504 rpm, still a big step up, but possibly manageable with a single stage and some custom sprockets, maybe using small pitch (#25 or maybe 6mm) chain.

You can possibly reduce the required generator rpm by using a controller in regen mode as the rectifier, it would complicate things electrically, but would allow a higher voltage from a given shaft speed.

 

[benjy]

Thanks for the reply Jeremy.

So it looks like using a brushed motor is the best option, I have had a look around for the right thing but I am finding it very hard to find websites that give the rpm (to calculate the kv) letalone finding one with something like 12kv.

Could you give me a link to something of the type so I can see what I am looking for?

Would it be possible to use the same (regen enabled) controller as the rest of the bike instead of having to buy a new one?

Thanks Benjy

 

[Jeremy Harris]

Thanks for the reply Jeremy.

So it looks like using a brushed motor is the best option, I have had a look around for the right thing but I am finding it very hard to find websites that give the rpm (to calculate the kv) letalone finding one with something like 12kv.

Could you give me a link to something of the type so I can see what I am looking for?

Would it be possible to use the same (regen enabled) controller as the rest of the bike instead of having to buy a new one?

Thanks Benjy

I'd not go for a brushed motor, really because it's hard to find a good one. Even a cheap brushless motor is likely to be more reliable, and probably lighter, than the brushed motors that are available.

I can't help on links, you'll need to do some serious searching and be prepared to do some work on whatever you manage to find. If it were me, then I'd try and see if I could find a cheap direct drive, small hub motor and modify it. I believe Conhismotor do a small direct drive hub that has a Kv of about 9 or 10, which would mean spinning it at around 420 to 450 rpm to effectively charge a 36 V nominal (more like 42 V typical on low charge) battery pack. You'd need to chain or belt drive it from the pedal crank with a ratio of around 5.5 or 6:1 to get it to spin fast enough, plus add a three phase rectifier (just six diodes) to turn the AC to DC.

You can't use the same controller as you use to drive the bike motor.

 

[amberwolf]

FWIW, at least two people here on ES have done this (pedals run generator instead of bike wheel). Lowracer was one, and I have forgotten the other just now (and I feel bad because I knew it when I started typing!). There should be pics and maybe even videos of them here on ES, as well as some build info about them.

SOmeone else also recently posted an "exercycle" that runs a generator.

 

[Punx0r]

Treadmill motors seem popular with people who build wind generators?

IIRC they're usually rated at 1800rpm at 180VDC, giving a KV of 10.

 

[lbz5mc12]

I had an idea about something like this. I was thinking why not use the wheels to power a generator? Use a battery pack and the cranks to get the bike up to speed. Have a toggle switch for two power sources. Once you get up to speed, switch over to the generator and the bike/trike will power itself. If you notice the speed dropping just add a little pedal power and you're good to go. You'd have to have a pretty big extra gear attached to one of the wheels to spin the generator or make the front wheel into the generator. Then you could use the battery for stop signs, streetlights, intersections etc.

 

[Jeremy Harris]

I had an idea about something like this. I was thinking why not use the wheels to power a generator? Use a battery pack and the cranks to get the bike up to speed. Have a toggle switch for two power sources. Once you get up to speed, switch over to the generator and the bike/trike will power itself. If you notice the speed dropping just add a little pedal power and you're good to go. You'd have to have a pretty big extra gear attached to one of the wheels to spin the generator or make the front wheel into the generator. Then you could use the battery for stop signs, streetlights, intersections etc.

Hmmm........

This is getting well into the territory of a perpetual motion machine, or even, maybe, an over-unity machine.

Sadly it breaks the laws of physics and won't work, as the losses in the power conversions mean that it stops pretty much instantly when you stop pedalling.

If you want to know more, then do a web search for the law of conservation of energy.

 

[Chesta]

I was also thinking of a similar idea. I know according to the second law of thermodynamics energy has to be lost in a cyclical process, but that doesn't mean that you will not get usable energy from the generator portion of the hub. It will only extend the range of the battery but you should get some useable energy to reuse. Eventually the battery will die. I Am toying with the idea of using two hub motors, one on the front wheel and one on the rear. That way regardless if I pedal or use th motor to move at least one hub will be dedicated strictly to charging the batteries. Like I said before, a lot of energy will be lost, but you can get a little back as the other hub/generator is spinning the batteries will be charged to some degree depending on the wattage output of the generator. If I'm not mistaken this is the same way a alternator/stator keeps the battery charged in a car. Without an alternator the car would go less than a mile. The alternator creates usable electricity to charge the battery as the engine spins the wheels. This is the same concept on the bike but my legs is the engine! Also recouping a lot of energy on downhills and by the other hub moving the bike. They already make bike generators that put out low voltage for bikes to power bike lights. A generator/hub I think puts out more. You might only extend your range 5 miles at least but you can recharge by pedaling and regenerative braking. I'm really not sure what the power output would be but I am in the process of putting it together.

 

[amberwolf]

It will only extend the range of the battery but you should get some useable energy to reuse. Eventually the battery will die.

Due to losses in each stage of the power conversions, it will not extend the range of the battery. It will shorten it. Try it sometime.

If I'm not mistaken this is the same way a alternator/stator keeps the battery charged in a car. Without an alternator the car would go less than a mile.

Nope. Except for running lights, computers, etc., not very much electrical power is actually used by a car's engine. If you had it running just the spark plugs on an older engine without electronics, it'd go quite a long ways.



You do get power back with regnerative braking, but it's very little. On my 2.5-mile flat-road commute to work with at least a dozen full stops, regen braking nets me at most a couple of percent, and sometimes less. It could be made more efficient, but even so it is very different from using that regeneration *while moving*.

If you do it while braking you're just recovering a little energy that would otherwise be lost as heat.

If you do it while moving, you are actively wasting power in the multiple conversion processes from battery to controller to motor to wheel to ground to wheel to generator to charger to battery, and you're not going to recover enough out of that to be worth it--it will most definitely shorten your range, not lengthen it.

 

[Jeremy Harris]

If I'm not mistaken this is the same way a alternator/stator keeps the battery charged in a car. Without an alternator the car would go less than a mile.

Nope. Except for running lights, computers, etc., not very much electrical power is actually used by a car's engine. If you had it running just the spark plugs on an older engine without electronics, it'd go quite a long ways.

I used to run my road going race car for a whole day of racing with the alternator belt taken off (to give a tiny bit more power) and the whole car electrical system running from a small motorcycle battery (so I could save the weight of the big car battery). The little motorcycle battery would run the ignition system for hours just fine, and even had enough power for one, or maybe two, emergency engine starts on the grid if I happened to stall it.

 

[Chesta]

I understand what your saying amberwolf, did you ever try it. There are a lot of contact and moving parts where energy can be lost as heat, but I thought the hub would put out enough power to regain some energy. It still doesn't make sense to me that all power output by the generator would have all it's power lost n the process. I guess I'll have to give it a try and see how everything turns out!

 

[amberwolf]

It isn't that all the generator's output will be lost.

It is that it can't output enough power to make up for all the losses in the transfer of power from motor to generator, plus the motor is also trying to move the bike forward at the same time as trying to drive the generator.

If you really want to see how well it works, disconnect the battery from everything, then pedal up to speed with the controllers turned off, but connected to each other (so the generator can power the motor).

Turn them on, stop pedalling, hit full throttle, and see how far you go on *only* energy converted from motive power. Depending on efficiency of everything involved, you probably wont go more than a few feet to a few dozen yards.


It's the laws of physics in operation, and lots of people have tried this sort of thing over the last hundred-plus years, with many variations. Some have even tried to market plans for such devices, but they cannot work in a practical fashion, so while they might make a lot of money off of people that do not understand basic highschool physics, they aren't going to make a usable vehicle from it.



Some of the variations include using a windmill-generator to regain power from the wind (a Chinese man was trying to market such a thing to his country recently, and it has been tried many times before), or extra coils on the motor to "pull power" from the magnets as they go by, using a generator on a different wheel to "pull power" from the road as the vehicle goes along it, etc. They all do the same thing, though--use up more power than they can get back, and thus shorten the range of the vehicle if it is battery (or gas-engine/generator) powered, or make it not really work at all if it is supposed to be powered off of the windmill/other-wheel/etc.'s generator.


I would love to see it work, and it is a great exercise in physics to try it, so I wish you well in the endeavor.

 

[Jeremy Harris]

Let's look at an ebike with the option of an alternator and chuck some typical figures in:

An ebike motor might be around 80% efficient at converting electrical power to mechanical power, only losing 20% as heat

The battery, wiring and controller will be around 95% efficient, only losing about 5% as heat

An alternator will be similar to a motor, about 80% efficient at converting mechanical power to electrical power and only losing about 20% as heat

A rectifier and associated wiring might be around 98% efficient, with only 2% being lost as heat.

All of the power for the closed system comes from the battery, there is no other stored power source.



Let's look at the first case of the ebike running without the alternator connected, and see how much power from the battery is lost:

The total power lost from the battery as heat will be 5% from the battery, controller and wiring losses, plus 20% from the motor loss, giving about 25% loss and hence about 75% of the useful energy from the battery being available to run the ebike.


Now let's look at what happens when we add an alternator:

The total power lost from the battery is now the 25% from the above case, plus the extra 20% from the alternator heat loss and 2% from the rectifier, so the total system power loss has risen from 25% to around 47%, meaning that the battery can no only deliver about 53% of it's useful energy into actually propelling the ebike.

Once power is lost as heat there's no easy way to get it back.

 

[Chalo]

This discussion reminds me that over the years I have been fooling around with e-bikes, probably the most common question has been whether I could charge the battery by pedaling. From an engineering standpoint, it seems like an absurd thing to do, but on reflection it begins to look quite modern and culturally relevant.

For instance, people go to work to earn money to pay off (with interest) the amount they borrowed to buy a car. They own a car so they can drive to work.

Some of them drive their cars a few miles from their homes to the gym, so they can get some exercise.

Others go to lengths to visit casinos, so they can spend a lot of money in the hopes of winning some money.

Still others drink light beer, so they can buy and drink more beers to achieve the same effect as a smaller number of regular beers.

In light of the above behaviors and others, it seems to me that an e-bike powered by the rider pedaling to run a generator and charge a battery would be exactly the kind of bike for people who don't understand why you'd ride a bike for transportation. Transportation is too purposeful, and a normal bike too simple and efficient to suit the predilections of a real purebred American consumer. In fact, a nice heavy expensive e-bike powered by a battery that's charged from a pedal-powered generator should probably be mounted on a trainer stand, so that the power from the motor can be used to drive a fan or-- better-- a magnetic resistance unit. And the whole thing should of course be placed in a really cold air-conditioned space, because a bike rider who isn't moving through the air can quickly overheat.

Wait-- maybe instead of a magnetic resistance unit, the rear wheel of the bike should drive a generator by means of a friction roller. And then the generator's output could power a heating pad, to help warm any part of the rider's body that gets too cold from the air conditioning.

Chalo

 

[docnjoj]

Chalo that is simply wonderful. You made my afternoon with giggles!
otherDoc