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Master Fuse


Paul Willett
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I seem to have been having a bad run with blowing the 100a master fuses of late.

 

Thought that the battery might have been the problem, so replaced with a new Odyssey.

 

Today, though, during third start of the day, after about a 30 minute shutdown - snap - away it went again.

 

Anyone else had this problem?

 

This is the third fuse I have blown. I am wondering if my battery is being charged correctly - I do not have an Ammeter fitted - yet.

 

 

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Paul,

 

 

From experience, a fuse blows because there is a fault (short circuit perhaps) in the electrical system somewhere. Especially when it's a 100A fuse ! It takes a very low resistance (~ 0.1 ohm) to blow a 100A fuse at 12 volts. Nothing in the aircraft would be that low resistance except the starter motor. The power supply to the starter motor is not fused because it drags very large currents.

 

 

Batteries themselves don’t blow fuses. It’s just the amount of current you try to drag out of them that will.

 

 

My concern would be that it will blow in mid flight. I'm not familiar with the wiring of a Technam so don't know if you would loose spark in such an event but you would certainly loose everything else. I would start the engine, then remove the fuse and see if it still continues to run. At least you will know the worst scenario should it happen after you've left the ground.

 

 

I would be checking the wiring everywhere I could get access. With that amount of current draw there is very likely burn marks where the short is occurring. Pay particular attention where wires pass through the airframe. Vibration of the insulation against sharp metal airframe parts will soon turn into a short. There is a possibility that an instrument (gauge, radio, gps, relay etc) could have an internal fault but this is less likely than a direct short to the airframe somewhere.

 

 

Does it always blow when the engine is being started ?. If so I would be looking around the starter solenoid/starter switch area. Electric trim motors and their wiring would also be high on the list. You may need to get in behind the panel and check there if nothing else is obvious.

 

 

 

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Be careful, Paul. If you remove the fuse while the engine is running you could damage the alternator.

 

Alternators do not like running without a 'load'. I don't know your setup but for your own sake please check with someone who does.

 

The starter motor should not be supplied through the fuse...typically they draw 200A plus during start-up and should be wired directly to the battery. If the starter is wired correctly then I suggest you have an intermittent short circuit. Look for broken insulation on all wiring.

 

The starter solenoid should be wired through the fuse and would be worth a check...and the starter switch...and etc. etc.

 

Bruce

 

 

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Thanks. No I don't intend to run the engine without the master fuse.

 

Anyone who has had to change one of these will know it is quite difficult access - something I am working to change.

 

I will check on the starter motor connection to the battery, but I am pretty sure everything goes through the master fuse.

 

 

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The circuit diagram indicates that the starter motor draws its current through the 100A fuse...I'm buggered if I know why. I love Italian design, but when it comes to simple things like engine electrics, I wonder what they are thinking. Why they did not accept the Rotax starter circuit I have no idea.

 

If the aircraft was mine and I was allowed to modify the wiring I would wire the starter directly from the battery without a fuse and put a substantial master switch in the earth lead from the battery.

 

Fire in an aircraft really worries me...even more than an an engine failure. Wing frames without fabric or aluminium skin are a bit short on aerodynamic features!

 

Bruce

 

 

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The Battery is located aft - for reasons of ballast I presume - so the reason for going through the master fuse is probably because of weight of running an extra lead - however, this could have been overcome by placing the fuse fwd - even in the cockpit would have been a good idea(!) - and then running a direct lead to the starter motor from there.

 

Without wishing to sound stupid, what function exactly does the Starter Motor Relay perform?

 

 

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The starter motor relay is a low current solenoid switch for closing the high current circuit to the starter motor (a series wound electric motor). Without the solenoid switch the high current wiring would be unacceptably long (with associated voltage drop) and the starter switch in the cockpit would need to carry 200A plus. Ideally the battery should be as close as possible to the starter. Keep the fuse, or master isolation switch, as close as possible to the battery to reduce the risk of fire due to a short circuit.

 

Bruce

 

 

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I would consider installing the battery just underneath the fuel drain part of the engine, (substantial amount of room).It is constructed with a large plate with brackets installed on the firewall coming through the cab. Fortunate for me, this was already done when I bought my P92 but is so beneficial of having it near the engine/starter motor. Not to mention visible with Pre-flights etc. I have seen a few Tecnam's with battery fitted like this.

 

 

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Fuse in starter circuit

 

Don't think I would remove the fuse from the starter circuit. Had an instance where an engine had to be shut down on the ground, post start-up, and it continued to turn over , due to a stuck starter solenoid ( points welded ). Had the shutdown not been required, the engaged starter would not have been noticed, & it would have eventually flew to pieces/ melted. shorting out some very heavy wiring. Outcome, Complete electrical failure or fire in-flight

 

Not a good idea. N......

 

 

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Hiperlight.

 

I am not really up to explaining the theory, but as I understand it. If the earth is poor the resistance goes up, meaning Volts go down. As the starter has a certain wattage, to achieve this Wattage the Amps go up, due to Watts equalling Volts multiplied by Amps. The high amperage is what does the damage.

 

Ian Borg

 

 

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Yenn (the older),

 

I=E/R...the higher the voltage for a given resistance the higher the current. The higher the resistance for a given voltage, the lower the current. (I=current; E=electromotive force or voltage; R=resistance.)

 

W=EI...if the voltage or current is lower the wattage is lower.

 

Bruce (the younger)

 

 

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Guest AusDarren

Series motors have very low resistance, and regulate the current by back EMF produced by magnetic forces that occur when rotation is occurring. So the motor being too tight or hard to turn over could stall the starter producing very high currents. bypassing the fuse would likely allow the starter to burn out. Do you pull the prop through a couple of times prior to starting? just to free it up? EXTREME CAUTION REQUIRED.

 

(my background is an electrical one not a ROTAX one)

 

Regards,

 

AusDarren

 

 

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  • 2 weeks later...

Hmm - problem continues today (1st chance in ages to get up to the plane).

 

No signs of arcing anywhere along the master leads or around the starter motor.

 

I suspect my (new) battery was also low - so, will a low amperagte battery alone cause the master fuse to blow when there is high demand - i.e. starter cranking?

 

I have a suspicion of inadequate charging in flight.

 

 

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Guest bateo

G'day Paul,

 

That sounds much like the cause of poor charge in flight.. I would just be careful trying to crank the engine with a Low battery.. (especially with smaller starter motor) as it does damage the starter motor and the Sprag clutch internally very quickly if cranked long enough.. ( it happened to ours unfortunately within 2 flights on cold morning, with the same situation of a low battery (not an odyssey battery at the time) and poor electrical current)..

 

I know it maybe a different problem, and a bit off topic but I'd hate to see you do damage while trying to figure out another problem..

 

Keep me posted as I would be very interested into what the cause of your problem is???

 

 

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'so, will a low amperagte battery alone cause the master fuse to blow when there is high demand - i.e. starter cranking?'

 

The short answer is NO! All things being equal the exact opposite should happen.

 

Last week I was talking to the CFI at Mid Murray Flying Club at Swan Hill. They have a Tecnam 92S Echo on line. He told me he has had a 100 amp fuse blow just once.

 

I'm not privy to the wiring setup on all aircraft but I haven't heard of the battery to starter cable being fused. Why 'fuse' a cable capable of carrying 400-500 amps with little voltage drop? I really don't understand. The way the Tecnam is wired, if the 100 amp fuse blows, you lose the lot...as you know.

 

The standard starter on a Rotax 912 is one kilowatt and in theory should draw less than 85 amps. However if your battery is supplying full voltage and you hit the starter the initial current draw could be 150 to 200 amps.

 

Are you using an OEM fuse? (which may be designed to carry an initial current surge.)

 

Of course if you have other electrical equipment switched on during starting that could contribute to the problem.

 

I'll do some more research and I'll let you know if I come up with a brilliant solution.

 

Bruce

 

 

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Guest Seabug

Starter Motor

 

You might have found the tecnam circuit diagram a bit misleading regarding the starter motor battery supply. I built my tecnam from a kit and the starter is supplied directly from the battery through the starter solenoid .

 

It looks like the 100amp fuse is protecting the starter solenoid which could be at fault.

 

 

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Hmm. OK.

 

Not quite sure where this leaves me, but checking the battery after removing it from the aircraft, the charge appears to be low. So, definitely suspect something to do with the problem.

 

The 100a fuse is replacement, but identical to OEM.

 

 

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Paul,

 

If your aircraft is wired as shown on the circuit diagram supplied by 'Seabug' then all electrical systems are protected by the 25A fuse (or circuit breaker) on the battery side of the master switch (and other individual lower current fuses) including the solenoid side of the starter relay.

 

So if the 100A fuse is the correct one it looks as though the starter could have an internal short or excessive load...like stiff or partly seized bearings, bent shaft etc. or the Rotax 912 is hard to turn (as 'AusDarren' suggested).

 

If I were you I'd whip the starter off and have it checked.

 

Also if the starter is too small for the job it would draw excessive current. But if you fit a larger starter you would probably need a larger fuse.

 

I have done some research and find that starters on large diesel engines used in mining have overload protection to 'interrupt current flow before the starter motor or cable are damaged from overcurrent conditions' to reduce the risk of fire in mines. The initial current draw by the starter in these cases is often in excess of 1200A.

 

Bruce

 

 

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Agreed. Most likely culprit appears to be a short somewhere in the electrics of the starter motor, as it is only happening during start cycle.

 

Doesn't explain the loss in the battery of course, but may be unrelated matter.

 

We are going to remove the starter motor for inspection tomorrow.

 

Thanks for the input from everyone - this is a superb resource.

 

 

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