flyrotary@lancaironline.net Mailing List Archive

Mike;

The first thing to try is a large capacitor at the alternator between the B terminal (output) and grnd (the case). Try about 10,000 microFarad in an electrolytic type. Be aware of polarity to connect it right.

The over-voltage (crowbar circuit) that I have I got from Aeroelectric Connection. I don’t think he sells stuff directly anymore but maybe you can get it from B&C. Connects across the field supply circuit breaker, and fails to short above something like 17-18 volts (IIRC) – tripping the breaker.

Don’t know about yours; but normally automotive alternators with internal regulators feed the current to the field internally directly from the regulator. The small excitation current required to get things started comes through the ‘idiot’ light circuit (one of the terminals on the alt). Over-voltage protection on that circuit won’t do anything. On my Mitsubishi alternator, I took it apart and undid the internal connection. I learned about this from an article in Contact! years ago (Issue 46, Sept 1998). Maybe you can find a circuit diagram for your alternator; or maybe it is made by Mitsubishi or Nippondenso, both of which are covered in the article. I can provide a copy if that helps.

Best,

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Mike Wills
Sent: Sunday, March 22, 2009 7:37 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Alternator (Off topic)

Slightly off topic, but does anyone have a suggestion for an off the shelf filter to kill/reduce alternator electrical noise? I have an alternator off of a Geo Metro/Chevy Sprint with internal regulator. I have a switch on the panel to energize the field. After the engine is running and I switch on the alternator I get a noticeable hum in the intercom. This is one of those nagging little issues I havent gotten around to fixing yet since first flight.

Also in a previous thread someone (maybe Al G.?) mentioned an over-voltage protection device for the alternator output that would be compatible with the internally regulated alternator. I'd like to add something like that as well - suggested sources?

Thanks,

Mike Wills

RV-4 N144MW

----- Original Message -----

From: James Maher

To: Rotary motors in aircraft

Sent: Sunday, March 22, 2009 7:45 AM

Subject: [FlyRotary] Re: Alternator (Off topic)

Tom,

You are right, of coarse, about the alternator field being either on or off.

I may have implied that the field current was a linear function but I was only trying to

simplify the explanation.

However, by switching the field on and off at a fairly fast rate and varying the pulse

width duration of the field current, a linear approximation can be achieved with a constant voltage as the result, within the limits of the system of course.

That is in essence how the voltage regulator achieves its function.

As for your "simple test" it may not give the expected results, as most modern car engines, those that are controlled by computers, will have the idle speed determined by the computer as well.

So increasing the load at idle, either mechanical or electrical, will cause the

computer to sense a decrease in RPM and if it goes below a predetermined value the computer will open the throttle (or idle speed valve) to increase the RPM to the correct idle speed.

Jim
--- On Sun, 3/22/09, thomas walter <roundrocktom@yahoo.com> wrote:

From: thomas walter <roundrocktom@yahoo.com>
Subject: [FlyRotary] Re: Alternator (Off topic)
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Date: Sunday, March 22, 2009, 10:03 AM

Jim,

The field is either on or off.

The regulator would create too much heat with a linear function, so solution is just a 'switch' function in the regulator IC.

Battery does the trick in smoothing the voltage.

George -- simple test is with the engine idling, kick on the lights/fan... note the rpm drop.
That is the load of the alternator turning on. Newer vehicles will have additional inputs to the alternator so it knows when the AC is on, or when you kick WOT (wide open throttle) as it will disconnect the alternator and A/C off for a brief amount of time. Also a simple battery test is to (in the car) kick on all loads at idle.... if the engine dies, battery reserve isn't there and time to replace 'em.

Tom

1) Re: Alternator (Off topic)
by James Maher <delta11xd@att.net>

George,

I'm sorry to disagree with you but you are quite mistaken in your belief about the

alternator function.

The alternator output is mostly a function of load because of the voltage regulator,

whose function is to keep the voltage constant with changes in load and RPM.

It performs this function by varying the field current in the alternator.

If fact this is one of the many advantages of the alternator over a generator.

The alternator's voltage output is also more or less constant regardless of engine RPM, also due to the regulator's function.

You said "The alternator charges the battery - which in turn runs the accessories".

This is not exactly true. If the battery voltage is below that of the alternator output

then yes the alternator will function to charge the battery, however if the battery is fully charged then the alternator will not be charging the battery but will provide the necessary power to drive any other loads connected to the system.

So reducing the electrical load on the system will indeed reduce the mechanical load on

the engine thereby using less overall energy.

Hope this clarifies your misconception.

Don't feel bad, many people do not fully understand even the most simple of electrical devices and the alternator/regulator charging system is fairly complex.

Jim