Can a geared brushless motor become a generator?

[tmho]

I have a rear wheel kit installed on my bike. The motor is with gear (low, middle and high) an brushless. Since it is winter time and not suitable for outdoor riding, I would like to use it indoor as a generator. But when I check the output voltage (while turning the motor) from the haul wires, I can only get less than 1 volt and it changes from positive to negative. Will there be a way to make the motor become a generator?

 

[Jeremy Harris]

Provided that you can really turn the motor (i.e. that it doesn't have an internal freewheel between the gears and the motor as a lot of internally geared hub motors do) then the answer is yes, you can turn it into an alternator.

The phase wires that normally connect to the controller (not the Hall sensor wires) will output three phase AC at a frequency that is dependent on the rpm you turn the motor. The motor efficiency as an alternator will be pretty much exactly the same as its efficiency as a motor. The voltage you get out will be the same as the voltage that you apply to the motor at a particular speed, so if you're top road speed is 25mph and you run at 36V and draw 20A to get this speed, then you will still get 20A at 36V if you drive the hub motor to this speed and with enough power to deliver the current.

If you want to use the electricity generated then you need a way to turn it into DC, either by using a three phase bridge rectifier or by making use of the regen capability that some controllers have.

In practice, if you put the bike on a stand and just pedal away as hard as you can, you're not likely to get more than around 200W, probably less. An average person can sustain around 100 to 120W when pedalling, so a fair bit less than you normally use when riding the bike as an ebike.

Jeremy

 

[dogman dan]

Most likely your motor is a planetary gear motor, and therfore has an internal freewheel. If so, then you can't make it into a generator unless you open it up, and do something to lock the freewheel permanently.

I suspect your three gears are actually three selections of your controller functions.

Direct drive motors can do regen, provided the controller supports it.

 

[tmho]

Thank you for Jeremy and Dogman's comments and information. I think the motor is with free wheel. Therefore, I can only leave this project to be done later when I have time to see if I can open the motor to lock the free wheel.

Yes, dogman, the 3 gears are controlled through a switch connected to the controller. Also, the factory claims to have regen function and there is a wire connection for regen.

 

[amberwolf]

There is no need to disable the freewheel to use it as a generator; just spin it backwards and it should work. :wink:

People trying to generate power on a bicycle (like regen braking) wouldn't be able to do that, becuase the bike is rolling forward, but it won't matter if you've got the motor mounted to something else off the ground.


BTW, is your motor by Fusin? If so, that's the same kind Dogman and I have both used (mine came from him, actually), so we have some experience with them as bike motors. I ask because Fusin originally sold their regen-capable controllers with freewheeling hub motors that can't actually do regen, and later made non-freewheeling gearsets and motors available to make their advertised regen braking actually work.

 

[tmho]

There is no need to disable the freewheel to use it as a generator; just spin it backwards and it should work. :wink:

People trying to generate power on a bicycle (like regen braking) wouldn't be able to do that, becuase the bike is rolling forward, but it won't matter if you've got the motor mounted to something else off the ground.


BTW, is your motor by Fusin? If so, that's the same kind Dogman and I have both used (mine came from him, actually), so we have some experience with them as bike motors. I ask because Fusin originally sold their regen-capable controllers with freewheeling hub motors that can't actually do regen, and later made non-freewheeling gearsets and motors available to make their advertised regen braking actually work.

Hi, glad to hear that it is possible to do so without opening the motor to disable the freewheel. I am using an old style bike trainer to support the bike. I cannot think of any simple way of spinning the wheel backward by peddling. Could you please give me some suggestions on this?

Yes, the motor is from Fusin. Could you also tell me how to how the voltage out from the motor? From the phase wires? Do I need to connect to the controller? Should the wires for regen be connected?

 

[amberwolf]

I am using an old style bike trainer to support the bike. I cannot think of any simple way of spinning the wheel backward by peddling. Could you please give me some suggestions on this?

Unless you have no freewheel on your gear cluster, there isn't any easy way without modifying things. You'd end up having to disable the freewheel in the gear cluster, or replace it with a non-freewheeling gear cluster.

The simplest way I can think of without involves using a second regular bicycle on a second trainer, facing the opposite direction from the first. Place both so the rear tires are in direct contact, with the trainers clamped together to keep friction force strong between the tires. Pedal the regular bike, and the tire on the Fusin wheel will be spun backwards. You'd also want to take the chain off the Fusin wheel, so it doesn't spin the pedals on that bike, wasting a little bit of the power you're putting into it.

Could you also tell me how to how the voltage out from the motor? From the phase wires? Do I need to connect to the controller? Should the wires for regen be connected?

I don't understand the first sentence, but the answers to the last two are yes, if you want to use the controller to rectify the AC out of the Fusin into simple DC. You will probably have to have the battery hooked up to it to make it work properly, but I have never tried it without, so it might still work (but it is easier to use the battery as the load than come up with something else).

Otherwise, you'll need to build your own three-phase rectifier and voltage regulator. Doing that would probably be more efficient than using a bike controller, but a lot more complicated.

 

[ic3wall]

First of all sorry for bringing back this old thread ... but I'm trying to achieve something quite similar.

I bought a used bafang 8fun motor that I'm trying to use as a generator. As mentionned previously, the only way to produce electricity is to spin it backward due to the clutch in the gearbox, which is not a problem for I want to do.

I noticed however that spinning it backward (no load) is quite difficult and I was wondering if this was due to the gearbox or to the fact that it is generating ?
I measured current and voltage when giving it a spin by hand... max voltage was about 5V, which is not bad, but current is like 20mA. I guess connecting a load would increase current ?

Could somebody explain what would happen if I connect a load ?

Thanks

 

[ic3wall]

Anybody?

Ideally, I would like to use the motor without the gear reduction ...

 

[d8veh]

Anybody?

Ideally, I would like to use the motor without the gear reduction ...

It's 3 phase AC, so you need something like this:
http://www.ebay.co.uk/itm/75-A-AMP-3-PHASE-Bridge-Rectifier-AC-to-DC-for-WIND-/150384824969?pt=LH_DefaultDomain_0&hash=item2303a25289

You have to use the reduction system because it connects the motor to the hub. You can weld the clutch to make it work in both directions. The resistance you felt is normal and five times as much as a DD motor because of the gearing, but you get more power at low rpm. You can bolt a sprocket to it and use indirect drive to it with a chain to get it to spin at whatever speed you want. How had you planned to drive it?