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Auto alternator used as motor

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Hero999

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I'm sure it wouldn't be hard to build an inverter.

Still, I don't see the point, it probably won't be as efficient as using a DC motor.
 

crutschow

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An typical auto alternator generates a current of about 50-70A so a 3-phase inverter to drive it would be large. That amounts to about a horsepower at 12V. Since alternators operate at high speed (2-3 times engine speed) their torque output would be relatively low as compared to a standard electric motor.
 

Dean Huster

Well-Known Member
Automotive GENERATORS used to be used on the International Cub Cadet lawn tractors of the mid-1960s as both a generator for battery charging and as the starter motor for the 7-10 hp Kohler engine.

I can't imagine trying (or wanting) to use an alternator as a motor because of the 3-phase problem.

Dean
 

Nigel Goodwin

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Automotive GENERATORS used to be used on the International Cub Cadet lawn tractors of the mid-1960s as both a generator for battery charging and as the starter motor for the 7-10 hp Kohler engine.
The Yamaha RD200 two-stroke twin did the same thing, it used a dynamo rather than an alternator, and used it backwards as a starter motor.

A friend of mine had one, and actually stalled it on his motor-bike test (big no no), but because it has no need to engage like a normal starter motor he restarted it instantly without the examiner noticing :D

Interestingly, three or four other friends borrowed his RD200 and passed their tests on it as well, it was a really nice bike to ride, and went like a rocket.
 

Chippie

Member
The Yamaha RD200 two-stroke twin did the same thing, it used a dynamo rather than an alternator, and used it backwards as a starter motor.
Actually it was preceeded by the Yamaha CS3 which had the same setup.

Yup, it was a shunt wound dynamo or a series wound starter motor...I had a P reg one,....The regulator doubled up as the starter contactor.

Tuned mine up and had just over 100mph on it! (on the clock of course..)
 
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Nigel Goodwin

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Tuned mine up and had just over 100mph on it! (on the clock of course..)
At the time I had a 350cc Suzuki, another two-stroke twin, and one night we left our local pub and went down to Bakewell (where the tarts/puddings come from) - we both had pillion passengers. Going into Bakewell there's a nice long flat road, and we were flying :p

As you reach Bakewell you hit a 40mph speed limit, so I slowed down as we entered it, but my mate Alan didn't - he flew past me, giving it so much thrash on his RD200 that both exhausts were on fire, with pieces of burning carbon bouncing down the road behind him. One way to de-coke the exhausts :D

RD200's really shifted, they certainly wouldn't be far off 100mph.

Incidently, he bought it on the recommendation of Mick Andrews (multiple world trials champion) who lived in the same village, and was a works Yamaha rider. Alan picked it up brand new, rode it home (about 15-20 miles), and took it to show Mick. Mick jumped on it, wheelied it the entire length of the main street and back again - on the back wheel all the time.
 

Hero999

Banned
They were two strokes though, yuck.

They might be fast but they're also noisy, dirty and thirsty. Get a 600cc four stroke, same performance if not better, cleaner, quieter and half the fuel cost.
 

Nigel Goodwin

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They were two strokes though, yuck.
Great performance from two-strokes! :D

They might be fast but they're also noisy, dirty and thirsty. Get a 600cc four stroke, same performance if not better, cleaner, quieter and half the fuel cost.
Certainly two-strokes are much more thirsty, that's where the power comes from - but they are also much lower revving.
 

Hero999

Banned
I thought they were higher revving?

I've never ridden a two stroke (thank goodness) so I don't know.
 

Hero999

Banned
Isn't the powerband right next to the redline though?

Four strokes have more power lower down.
 
Hi,

This thread appears to have digressed since it was first started but the original point was something I'm currently looking into now. I have looked briefly at how motors work and as I understand it a car alternator is very similar to a brushless DC motor. Correct me if I'm wrong but a BLDC works by applying a 3-phase high power signal and having a permanent magnet to generate the field. An alternator has an electromagnet field and 3-phase (output) which goes through a bridge / regulator to charge the battery. What's stopping someone disconnecting the bridge/regulator, applying a DC voltage to the field windings and a 3 phase signal to the 3-phase part, just like a BLDC motor?

I only ask because I have an alternator knocking around and am looking into powering a small electric bike / vehicle. Building most of it from scratch. I initially thought of getting a BLDC hub motor (from China) to get me started, then work on a controller to replace the one that comes with the hub.

See attached pic of the vehicle I am planning on motorising.

Any comments / recommendations?

Thanks,

Jules
 

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Reloadron

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Without even considering the torque. You would need 3 phase AC to run it. Yes, an alternator starts out a a 3 phase source and is rectified but you can't apply DC to a rectifier and get AC out. It just would not be at all practical to use an automotive alternator and modify it to push a bike. You would need three phase AC to run it and the shaft speed would be a function of the AC frequency.

Ron
 

alec_t

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What's stopping someone disconnecting the bridge/regulator, applying a DC voltage to the field windings and a 3 phase signal to the 3-phase part, just like a BLDC motor?
I don't see any theoretical problem with that; just the practical problem of having to start with DC and invert it (with associated inefficiencies) to 3-phase AC and control that. So simpler (and even perhaps cheaper overall) to start with a BLDC motor.
 
I'm sorry if I wasn't clear, I know I would need an inverter to run it and not just a reverse rectifier. Just to clarify, how is a BLDC motor driven compared to how an alternator would be driven. It seems the same to me. Is it that driving an alternator is unknown and a BLDC motor already has a controller. If I plan to build the controller myself anyway? I was comparing an alternator to a 1kW 48V hub motor, I figure an alternator is about 1kW but is only 12V so would be 80Amps+ to run it whereas the 48V motor would be 20Amps which would be easier to control. Would it be possible use a BLDC controller to run an alternator or is it more trial and error?

Thanks,

Jules
 

alec_t

Well-Known Member
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Your 20A and 80A values presumably are assumed average values, so peak current is likely to be much (40%+?) more.
 
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