Topic: Generator Warning Light

[proplover]

Ok people, anyone come accross this;

Engine running at 1000rpm, gen light out but low volts lamp flashing - increase rpm to 1300rpm and its the same - increase to 1400rpm and the gen warning light starts to glimmer and the low volts lamp carries on flashing. This last effect stays constant even if you go upto 2000rpm. The moment you drop to 1300rpm the gen light goes out and stays out until you drop to idle when it comes on again as a glimmer.

Had something similar to this before some 2 years ago and the cause was a duff battery but not so this time - welcome any thoughts especially if you've experianced something similar. Just in case your thinking the obvious, the generator brushes are fine and I have had no cause to doubt the generators performance as yet.

This is delaying the Test Flight quite severely now and Im missing lots of good flying weather

[Bob]

Have you checked the regulator and the connections?

[Dick Gower]

Here is my tuppence worth on the subject.

The generator lamp has a parallel resistor/rectifier combination in series and all of this is connected across the following items starting at the G+ side of fuse 9:

fuse 9, terminal 4 on the cutout, the two series coils and contacts within the cut-out, terminal A of the cutout, terminals 2 & 3 of the genertor test switch, any of the busbar-to-fuse connections 1,2, 4-8 in the fuse box, and terminal BB to FL on the master switch.

A high resistance anywhere in this series circuit will produce the symptoms you describe.  In rough order of probability the usual suspects would be:

the cutout contacts in the v/reg & cutout
fuse 9
the contacts between 3 and 2 of the generator test switch.

What appears to be happening is that, at idle RPM, the lamp is glowing because the battery voltage is higher than the generator output therefore there is a current flow from battery throught the resistor in the resistance/rectifier unit back through the generator.  In other words normal operation.

As RPM increases the gen. voltage rises closer to the battery voltage therefore the lamp dims until finally extinguishing as the gen. and battery approach equal voltage.

As the RPM further increase the gen. voltage rises above the battery voltage and, at this point, the cut-out would normally close (extinguishing the light) and the gen. will charge the battery. 

In this case however something is open circuit in the charging circuit listed above so that now the light is powered (via the recrtifier) by the forward voltag drop across whatever this item is.

Because you say that the lamp glimmers rather than comes on brightly at increased RPM, it looks like a voltage drop rather than an complete open circuit so the first place I woud check would be fuse 9 (and its holder) and the test switch.  Both are accessible by opening the electrical panel.  Also look for softened fuse clips on the bus-bar side of the fuses listed above.  The bakelite panel can shrink which takes the tension off the copper rivets linking the fuse clips and busbar.  As a result the copper clips anneal and soften which reduces their tesnion.  This becomes a viscous circle and it all turns to worms.

Let me know how you get on and good luck!

Regards,
Dick Gower 

[proplover]

Hi Dick,
Sorry for long wait on my reply!
I think your analysis of my situation was completly correct. However before I could start checking anything I had to move the aircraft down to our other hanger as the lighting and space were better there. Guess what? The Gen light did as you would expect it to do. It fact over the morning I started the aircraft up some 8 times and each time once the rpm went above 800-900 the Gen light went out and stayed out! Typical!
We've been flying it on a regular basis over the year so far and the Gen light has behaved correctly each time therefore I suspect that it was a sticking contact in the regulator.
Thanks for your clarification on the system
pw

[Dick Gower]

Pleasure PW!

Don't you hate it when they do that!

Regards,
Dick