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Alternator question

357Bronco

Sr. Member
Joined
Oct 13, 2006
Messages
999
Loc.
New Hampsha
Can you spin an alternator too fast? I had a new one fail ( one wire 100amp ), and asked about shaft speed when I bought it, and was told I was fine. Now I'm told I'm spinning it too fast and that may void warrantee, go figure. I have a March crank pulley, I think about five inches in diameter, and an alternator pulley, about 2.5 inches. I buzz the motor to 6500, for a couple seconds at a time, maybe ten to fifteen times a weekend. I didn't see this as being a problem.The old crappy 65amp, Ford one never failed. Anyone with some experience with this? And I was told 6000 RPM was max on the shaft speed.
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,316
I don't know the answer but I can tell you that a 6K rpm max shaft speed is crap. An alt will spin about 3-4 times as fast as the crank and should be fine to at least 6K crankshaft rpms.
 

tasker

Contributor
all knowing of nothing
Joined
Jun 2, 2006
Messages
20,730
Loc.
NH
THAT looks like a home game for you...
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,345
Yes, you can spin an alternator too fast. But as you sort of figured, it's not likely that you're getting to that point. Unless your particular alt just didn't like getting up into the 13k range that is.

The way I understand it, each design would be diferent (different case sizes, heat dissipation properties, bearing diameters and designs, stator size, etc.) and have different specs. And perhaps more powerful alternators (with larger components and electromagnetic fields and more stress on the bearings and heat output?) can't handle quite as much as smaller ones? So perhaps some brands (whether O.E. new or aftermarket rebuilts) just can't handle as much as others can.

Still, at 16k for a 3G like RedLeg was saying, you'd be within it's limits with your 2to1 ratio and ok with some brands, but maybe getting close to other's limits.
For those reasons, you might want to verify your pullies exact size, just so you can determine actual shaft rpm at the alt, so that nobody can give you any grief when you try to warranty another one.

Might just be good info to know since you're competing too.
And now that I think about it, if you're racing, you'd want your alternator spinning slower than normal anyway. Higher ratios for normal driving put out more power to keep things charged and running, but take more HP to drive. Lower ratios for racing around keep the shaft slower and waste less HP.
Not only would you lower your alt's stress, but gain hp at the same time. Bonus!

In the old days, when we'd drag race our cars (or dad's cars) at the track, an old timey trick was to disconnect the alternator entirely. Some would pull the belt, others would just unplug the wires from the back.
Hey, anything that would help in a pinch! We were just learning and the aftermarket was playing catch-up.

Good luck with whatever you're doing.

Paul
 

RRRAAAYYY2

Bronco Guru
Joined
Nov 10, 2004
Messages
1,684
Loc.
Brantford, Ontario
Alternator speeds: for considering bearing failure, you look at the size of the bearings vs the weight of the rotor. Nothing else is in the equation when determining the rpm it can handle, other than the rpm. This varies from model to model, generally manufactures aim for 15,000 maximum rpm, with intermittent speeds of 18,000rpm.

Here are some examples you might draw from. Ford 3G's and GM CS144 large case alternators both have a pretty small back bearing. Both are really well know for this bearing failing, or in the 3G's case being done by the the time the slip rings are eaten through. (Which is consider a premature failure)

So spinning either of these alternators at high rpm is not recommended. We addressed this with our units, and 15,000 is a walk in the park. We have several seeing well over 20K.

GM first generation CS130 was notorious for rear bearing failure in the winter months, rectifiers in the summer. Its small back bearing was good for around 70K miles. Their front bearings would last last upto 3 months (for 6303, and 2 weeks for 6203) once the belt tensioner failed. When it failed it caused the alternator to be sort of whipped by the belt. So it would go from 6000rpm, then for 1/2 revolution it would see 28,000rpm speeds, and then back to 6000rpm.

As for disconnecting the alternator for more power, we are doing an article on that right now. I think a few people will be surprised with the outcome. Reducing the pulley size is a good idea as long as the alternator is on the top end of its charge curve at idle. If not you run into the yes it requires less HP because of the gear ratio, but requires double what was saved to turn the alternator in a less efficient phase of it's charge curve.

But at 2.5 to 1, most good alternators will be very happy at idle. Most factory units are 3 to 1 ratio.

I thinking something Dirtdonk was speaking about is in really high amp applications, you can actually have the rotor's field surpass the stator. i.e. the magnetic field it creates appears on the other side of the stator. If that happens the alterntor stops charging, as the stator no longer breaks the field. It is pretty weird, and I have only seen it 1/2 dozen times. Always when messing around with different rotor winds. You get some killer bottom end numbers, but then nothing in the top end.
 
OP
OP
357Bronco

357Bronco

Sr. Member
Joined
Oct 13, 2006
Messages
999
Loc.
New Hampsha
My alternator stopped charging, the bearings were fine. They said they'd warrantee this one. Even if I do spin the motor up to 6500, it's literally only for a couple seconds at a time.
 

Larry B

Full Member
Joined
Mar 4, 2009
Messages
305
Loc.
Medford
Even the old generators of yester year would live at 5k and Alt were soppos to be able to turn 3x them
 
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