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-   -   Blown Starter Fuse (https://vansairforce.net/community/showthread.php?t=174879)

sglynn 09-02-2019 07:57 PM

Blown Starter Fuse
 
What would cause a starter fuse to blow? It is 7.5 amps in the Advanced Control Module used with a Quick Build panel. It is the older model before they went to logical breakers. I flew yesterday, no problem. But today it wouldn't turn over. Turns out the starter fuse was blown. But why? I examined it and it looks kind of cheap and flimsy. I put in a new one and everything is working fine. Would excessive cranking to start a hot fuel injected engine blow the fuse? Does 7.5 amps sound right for the starter motor? I was thinking it would be a 10 or 20 amp fuse. What else might cause the fuse to blow. I have only 38 hours on the airplane, RV-7A. Thanks

BobTurner 09-02-2019 08:01 PM

The starter motor draws a huge current. The fuse in question most likely is for the starter solenoid (relay), and no, it should not blow from excessive cranking.

sglynn 09-02-2019 08:12 PM

starter contactor fuse
 
Bob
Thanks for your quick reply. Yes, I pulled the manual just now, it is the starter contactor fuse. That's interesting to think about. What would cause it to blow? Could it just be an old weak fuse?

Mich48041 09-02-2019 08:23 PM

It could just be an old weak fuse. A bad fuse holder connection can generate heat which will blow a fuse.

Yen 09-03-2019 01:22 AM

I seem to remember that when the voltage drops in a circuit, the amperage goes up.
You hit the starter and the volts drop way down because the battery is not 100% charged, that may cause the amperage of the starter solenoid circuit to rise enough to blow the fuse.
Replace the fuse, charge the battery and it may be fixed.

sglynn 09-03-2019 08:35 AM

starter contactor fuse
 
Yen
interesting idea, because just before this happened I was running the panel and lights for awhile with the engine off. I was loading new database and trouble shooting a tail light that burned out. So I may have run down the battery just before trying to start it. Fuse is changed. I'll see how it goes.

BobTurner 09-03-2019 11:17 AM

Quote:

Originally Posted by Yen (Post 1371157)
I seem to remember that when the voltage drops in a circuit, the amperage goes up.
.

This may be true some times, but for a dc relay, no.

gasman 09-04-2019 11:19 AM

Quote:

Originally Posted by BobTurner (Post 1371245)
This may be true some times, but for a dc relay, no.

OHM'S LAW only applies sometimes??......

BobTurner 09-04-2019 12:46 PM

There are lots of non-ohms law machines. e.g., motors often rely on induction to limit the current. If the voltage is low and the motor fails to turn, it can draw a large current.

snopercod 09-04-2019 01:19 PM

You should have a kickback diode or MOV between the contactor coil and ground to quench the voltage spike produced when the coil is de-energized.


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