DIY Electric Car Forums banner

1 - 11 of 11 Posts

·
Registered
Joined
·
61 Posts
Discussion Starter #1 (Edited)
Can anyone tell me how much spinny resistance (tech word) an alternator has when it has no electrical load connected?

if it is the same as a permanent magnet motor being used as a generator, it shouldn't have much if any resistance.

i've looked for this idea on the forums here, and didn't find much, but would hooking up an alternator to the 2nd drive shaft of the DC motor have any effect?
an electrical load would only be applied when the brakes are being pressed -the ciruit controlling the break lights can also control a relay which connects the alternator to an electric blanket (winter) or 12v aircon (summer) or what about a 12-120VDC converter for adding that extra 5% of power back into the battery pack?

i suppose this idea is scrap if the alternator always has physical resistance even with no electrical load.

edit: i'm talking about this because it is such a pain in the but to employ electronic brakes on a series wound DC motor.
also, perhaps no-one has done this because a good way to wreck an alternator is to run it without a load? i dunno
perhaps a motor that is designed to be a generator would be better, but my conversion is on a budget, and I already have a 90amp alternator (that equates to about 1kw!)
 

·
Registered
Joined
·
117 Posts
Makes sense that it would work.

I like how he wired it for higher voltage... he didn't mention but higher voltage windings will produce significantly less heat, and so the alternator can actually produce more watts that originally.


As for load questions, alternators will provide 0 resistance if they have no load and the stator is not damaged. Essentially without the load the magnetic fields will 'float' on each other.

HOWEVER: I have heard from racers that cheap alternators will heat up, possibly from bearings, at rpm's beyond their normal stock operation, though most DC motors won't be spinning faster than that (the belt is generally moving faster than the crank is due to the larger belt output.. damnit forgot the word for it the pully belt thingy, but in high rpm engines that can be the reverse... someone told me, and this is unverified, the Honda S2000 output is fairly reduced because it would burn up the compressor and alternator at max rpm (which is also max electrical load for ignition components)
 

·
Registered
Joined
·
61 Posts
Discussion Starter #4
i really like the idea of using an AC (aircon) clutch to engage the alternator, but what i'm really wondering now is why isn't this more popular?
one of the disadvantages that DC has, compared to AC systems, is the lack of simple regen/electronic braking. but this seems pretty streight forward, if you can find a way to use 12v from the alternator, then rewinding it shouldn't be required.


perhaps i am missing something? maybe the benefit is not worth the hours put in.
 

·
Registered
Joined
·
1,760 Posts
perhaps i am missing something? maybe the benefit is not worth the hours put in.
I live in a very non-hilly place. If I drive carefully I simply don't have to brake much so regen won't give THAT much for me. Ie, not really worth the bucks for me. If I build an AC-system of course I will add regen (since it's more or less "free"), but messing around with an extra alternator and stuff? Sounds very far away from my KISS-rule for the first EV. ;)
 

·
SPAM Cop
Joined
·
1,499 Posts
I have contemplated something similar to this as a regen option. I.E. use the rear shaft of the traction motor to turn a generator that can be used in a charging circuit like a regen braking system.

The complexity of such kind of boggles my mind, and I'm uncertain what the drag would be on the system when not "charging", and also the fact that the controller "ramps down" relatively slowly when you let off the go-pedal, so I'm uncertain if the benefit return would be worth the time and money invested.

I'm less interested in using it to slow the vehicle than I am to regain a small percentage of the stored kinetic energy of the vehicle. If you can regain 5% of the energy you put into making the vehicle move, then that should translate to a 5% improvement in range... but at the same time, the extra weight of components may result in a 5% degradation of the range, so it would be a break-even with no real benefit... maybe.

Like I said, kinda boggles my mind, but I'd love to have regen with the simplicity of the DC system.
 

·
Registered
Joined
·
1,025 Posts
I'm sorta toying with trying to design an automotive alternator regen system for my series wound DC powered car too. I suspect that if idling all the time the alternator (and belt drive, presumably) could easily sap 50-100 watts even under no electrical load (friction, windage, vibration, extra weight) but that is a SWAG.

The thing I wanted to be able to do before even trying it is find a way to use an automotive electromagnetic air conditioning clutch on the tailshaft of the motor to completely disengage the alternator and belt when not providing regen. This would nearly or completley eliminate all friction drag of the belt and alternator at the expense of a bit more weight, and the expense of needing to power the electromagnet to engage the clutch. Problems I see with this would be how to manage engaging the clutch at high speed since the alternator and belts would provide much more inertia than the original A/C compressor did and this might overstress the clutch. But you could manually engage it at low speed to avoid this when you knew you were going to start coasting down hill perhaps.

I probably won't try any of this due to the fact that what I could get back out of it probably would not justify the 30-40 pounds of weight (another SWAG) I would have to carry to implement the system. Another traction battery weighs about 60lbs so I am most of the way there and the additional battery would be simpler and guarantee another 5% or so of range assuming you had about 20 batteries to begin with.

It is worth noting that shunt wound DC motors DO make good generators, some DC golf carts implement regen using their shunt wound motors. A compound wound motor would work too. However motors like these and compatible controllers are much harder to find than series motors for full size conversions.

Is there enough room inside a series wound DC motors field housing to add a secondary shunt field winding for use in regen?

Interesting thought excersize though.
 

·
SPAM Cop
Joined
·
1,499 Posts
Actually, I think Otmar talked about that in his regen article he wrote back in '98. There's a link to it in the wiki under regen.
 

·
Registered
Joined
·
117 Posts
Problems I see with this would be how to manage engaging the clutch at high speed since the alternator and belts would provide much more inertia than the original A/C compressor did and this might overstress the clutch. But you could manually engage it at low speed to avoid this when you knew you were going to start coasting down hill perhaps.
Setup your circuit so the clutch engages while the alternator is under no load, then engage the load separately.

A loadless alternator has way less of an inertial absorbtion than a good sedan compressor on that initial engagement, then you engage the load and the clutch and alternator are already spinning.
 
1 - 11 of 11 Posts
Top