Blowing the field fuse

[RV7ator]

My 60 amp Plane-Power alternator's field fuse occasionally is blown during engine start using a Sky-Tec inline starter. P-P is at a loss to explain why.

This never happens if the field circuit is open during start - no surprise there - but if powered even a 10 amp fuse is occasionally blown. The alternator field excitation power is pulled off the main fuse block, but no other circuit's fuses are ever blown, likely because nothing else is turned on during cranking. But the battery buss would be exposed to any nasty spikes that kill the field fuse, and all those circuits (like fuel pump, ignitions, clock, keep-alives) survive.

I'm open for guesses and hunches to explore where this killer current is coming from.

John Siebold

[chaskuss]

John,
Hook up a volt meter to monitor system voltage during cranking. Permanent magnet starters tend to draw more power than the older, electromagnet field style starters. What is probably happening is that the system voltage drops low enough to require an increase in field circuit amps which is beyond the 10 amp fuse.
Remember that if system voltage drops, the amps must increase, as each devise uses the same number of watts [volts X amps] to achieve it's mission. [Cranking the engine].
Charlie

[RV7ator]

That's a good clue, Charlie. I'll run it past Plane-Power and Sky-Tec, too. This is the first in-line starter I've used. It's supposed to draw a bit less current than the compact lightweights of which I've had five other ships worth of experience. Stranger things have happened.

John Siebold

[DanH]

John, I had my PP field on a fuse and it did the same thing. Rather than chase it I just rewired the field line for a breaker....solved.

[hgerhardt]

PP alternators have a crowbar overvoltage device built in which shorts out the field circuit and blows the fuse/breaker when it senses an overvoltage condition. Fuses blow faster than breakers and probably accounts for why a fuse blows but not a breaker, if the overvoltage effect is only for a second or so when engaging the starter.

[az_gila]

Try a Slo-Blo fuse....

[kevinh]

Originally Posted by az_gila

Try a Slo-Blo fuse....

Exactly. Similar to a breaker this will allow very brief high current draws.