Paul,
While I agree that the principal reason you might wish to switch off the
alternator is because of an overvoltage malfunction, which would be taken
care of by the crowbar circuit if you fit this, it does serve also to
isolate the fuel pump that is supplied from the alternator circuit when you
put the battery master on. This means that you are not driving this pump
when the engine is not running, e.g. during pre-start checks, programming
the GPS, etc. It also provides an easy means of checking that the
battery-supplied pump provides pressure prior to starting.
I note that the latest Z-16 diagram for a Rotax 912/91 circuit from Bob
Nuckolls AeroElectric Connection shows his crowbar circuit driving a relay
to disconnect one of the yellow leads connecting the raw alternator output
to the regulator. The purpose of this may be to stop the alternator from
frying a faulty regulator.
Mike Gregory
Europa Club Safety Officer
-----Original Message-----
[mailto:owner-europa-list-server@matronics.com] On Behalf Of nigel charles
Sent: 26 July 2006 19:10
>I have another couple of questions relating to wiring:
>1. The wiring diagram in the build manual has a alternator output
>switch (capable of taking 25A). Does the OV crowbar mod take the
>place of this switch?
Effectively yes. The main purpose of the switch is to be able to isolate
the PM generator (alternator) from the rest of the system in case of a
problem. It should normally be left connected to ensure that some load
is created as no load can apparently cause a problem for the regulator.
>2. In a 914 fit, one of the fuel pumps is wired directly to the
>alternator/regulator output. In the event of a regulator failure, the
>OV mod prevents the runaway voltage frying electrics/battery. Why
>would it not fry the fuel pump and if it would, why not wire the
>essential fuel pump directly off the battery - would you not then
>have a working pump to get the aircraft down.
The fuel pump load is normally sufficient to keep the regulator output
within limits. If the regulator goes totally out of control it needs
isolating from the rest of the system. Although this would mean one of
the electrical pumps going off line the other will still provide
pressure. Conversely should the battery or associated switches or
busbars go offline the generator will always have a direct connection to
one of the pumps. Thus any one failure should mean that either the
battery or the generator should be able to power at least one pump. Your
suggestion does not cover the case of total battery failure.
Nigel Charles
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