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Alternator Speeds

jeffw - 9/4/12 at 07:07 AM

On a normal alternator (not a small motorsport one) what is the highest rpm you can spin them at before you damage them? My current pulley size puts peak alternator rpm at 18250 against a engine redline of 7800. I keep losing alternators and I wonder if over-speed is killing them. Thoughts?

Ben_Copeland - 9/4/12 at 07:20 AM

Hi Jeff, hows you?

Couple of websites say max rpm should be about 12000, so i'm thinking your killing yours by overspeed as you suggest....

Time to make your pulley bigger!

jeffw - 9/4/12 at 07:38 AM

I think so. I know the Brise one I have is rated at 18K but I don't think the ones on the supercharger frame are rated that highly.

snowy2 - 9/4/12 at 07:46 AM

heat is the usual killer of alternators......i have an ally heat shield added to my car because alternators would only last a few thousand miles, since i have only had to replace one (collapsed nose bearing)

rusty nuts - 9/4/12 at 07:58 AM

quote:
Originally posted by snowy2
heat is the usual killer of alternators......i have an ally heat shield added to my car because alternators would only last a few thousand miles, since i have only had to replace one (collapsed nose bearing)

jeffw - 9/4/12 at 09:09 AM

these are lasting less than 30 miles.

DIY Si - 9/4/12 at 09:45 AM

As said, most are rated at roughly 12K rpm, so at 18K, they won't last long at all!

Definitely time for a new pulley me thinks.

Ben_Copeland - 9/4/12 at 04:47 PM

quote:
Originally posted by jeffw
these are lasting less than 30 miles.

30 miles is unlikely to be heat, unless things are melting under that bonnet!

Perhaps you could change the pulley and stick a heat shield over it at the same time.

jeffw - 9/4/12 at 05:48 PM

I know my Brise one is rated to 18K so I did wonder if standard ones where. To fit a larger pulley will require a different belt as there is little scope of change with the Supercharger system.

jeffw - 10/4/12 at 06:04 PM

Turns out the alternator was repaired not replaced and it had the bearings failed. A new one is on its way to me as we speak.

loggyboy - 11/4/12 at 07:22 AM

could still be speed that killed the bearings? i thought it was common to under drive 'nators anyway as lil kit cars don't need much and there is a very small performance adv. to lessoning the force required to turn the 'nators.

[Edited on 11/4/12 by loggyboy]

jeffw - 11/4/12 at 07:58 AM

Don't disagree, a new one turned up this morning with a 40mm pulley (equates to 25K rpm at the redline !).