Using a DC motor as a generator

It's easy if you're using a brushed permanent-magnet motor--just spin the shaft and a voltage will appear across the terminals. If it's a larger motor that has stator coils, it's a little more difficult--you need to supply the excitation current.

Cheers,

Phil Hobbs

What sort of motor? Specification?

Hmm well I haven't bought a motor yet. But to give you an idea I'm looking for around 10kW generator that can be mounted on the front wheel/fork of a motorcycle in order to capture kinetic energy in a regenerative braking system. So obviously this needs to be a high RPM motor/generator.

10kW? That's a pretty big motor/generator! That would BE the front wheel!!

10 Kw? That's 12 Hp. High RPM? How fast will you be going? It seems to me motorcycle wheels don't spin all that fast, compared to common electric motors. Assuming 60 MPH, that's 5280 Feet/Minute. If the tire is 15" diameter (total W. A. guess) that has a circumference of ~47". so 5280 *12/47 = 1348 RPM. I'm sure you can find a motor/generator that will run optimally at that speed (if that is your desired speed) but it is going to be a big honker! If you use a belt drive you could get the M/G spinning much faster, and thereby use a smaller frame motor. A run-of-the-mill 12 Hp DC brush motor is going to weigh about 100 Lbs., I'd guess. I can give a coule of data points. I have a jet engine starter motor that is rated for 11.2 KW continuous (400 A, 28 V). It has a surge rating of 1300 A for one minute, so that is 36 KW. I'm sure this surge rating adds to the weight. Its speed range is 3000 - 8000 RPM. It uses a wound field so it can be regulated. That also adds to the weight, which runs about 90 Lbs. This is a VERY lightly constructed motor/generator, as it was for early jet aircraft use, where weight was insanely restricted. I suppose with rare-earth magnets, belt drive to allow higher shaft speed, and using a peak rating of 10 Kw instead of a continuous rating, a servo motor in the 50 Lb range might be able to do this.

I have real doubts you will find anything that really does this job and also physically fits the space available. So,e motors just have a lot of drag, too, which might defeat the whole purpose.

How will you connect the wheel to the motor? It takes a pretty hefty belt to transmit 12 Hp!

Jon

doesnt sound like a very good idea to me at all, a 10kw motor will drmatically increase the unsprung weight of the front wheel and so make the suspension inefective at providing good grip, not so good for braking especialy with the front wheel.

why not use the rear wheel wich is already connected via chain/belt to the unsprung mass of the bike, its also geared up wich means the motor will spin faster and so be smaller.

I suspect however the extra energy needed to push the extra weight around will more than counteract any energy recovered from braking, you need to store the energy too, or is it an electric bike anyway, if so just use the main motor.

Colin =^.^=

thanks for the replies! I see what you mean about 10kW being too much...what power do you think would be appropriate for such an application?

By the way the wheel is app. 26" and the speed will be ~70mph max