3 Phase Star Point

[Steamhead]

Many of Piper Twins (e.g. PA 31 Navajo and PA34 Seneca ) use
Prestolite alternators of both 12 and 24 volts.
These alternators are three phase, full wave rectification with the
six diodes internal to the alternator.
The alternator has two output terminals :-
1) Marked + which is the main output terminal for the 60 amp d.c.
output.
2) Marked Aux. which is the Star Point of the three phase windings.This terminal is never used.

Question:- What is terminal 2) for ?

Regards

[Wubble U]

Now I'm racking my brains and going back a looooong way here, but I seem to remember that the centre or star point of a 3 phase star arrangement is effectively the earth of the system. If the Aux terminal is never used then it may be that the DC electrical systems use the airframe itself as the return current route rather than add extra weight by wiring everything back to the Aux point. I could be way off base here (I'm sure you'll find a better answer in the Private Flying or engineers forums) but its been quite some time since I studied aircraft electrics and even longer since I did advanced electrical engineering!

[Flash2001]

WU

AKKK!

Don't ground the star point if you're feeding a grounded DC supply. That's a short circuit. Rectifiers exit stage left in smoke.

[Steamhead]

On a Three Phase transformer supplying my local housing estate
the star point is both the earth and neutral.
Thus on the U.K. systems phase to phase is 420 to 440 volt and
phase to neutral is 240 to 250 volts.
This is all standard Three Phase Technology.
However this AUX terminal on Prestolite Alternators when the system is d.c. airframe return is a total unknown.
I suppose the next move is to try e/mailing Prestolite.

Regards

[bluskis]

Steamhead is correct, three phase (ac) can be configured delta or star. Delta is where the outputs from the three phases come off the corners of what can be visualised as a triangle, each side of the triangle being one of the three phase windings.
In a star configuration each of the three phase windings are connected at one central point, the other ends of the phases being visualised as radials, the ends of which are not connected together, but from which outputs can be connected. This resulting in 440Vac nominal from any two end points in conventional UK distribution. An output can also be connected to the centre point and the voltage from this point to any outer end point will be 220 Vac nominal in the UK.

However the advice is don't connect anything to anything unless you fully understand the whole installation, and certainly not with an aircraft (DC) installation

[timzsta]

Sitting here with me ATPL notes on STAR/DELTA generators, trying to make sense of it again, just as well it didn't come up in the exam the other day!

From what I have it appears that a STAR has four outputs, the centre one being the one that is earther to the airframe (your Aux it would appear). Being three phase (hence the six diodes) there should be three outputs, one for each phase. Two should be at Line voltage and one a phase voltage, line voltage = 1.73 phase voltage.

I may have got it all wrong, but you may have another 2 outputs somewhere. The advantage of the star system for an airliner is that there are to voltages available, which may result in fewer transformers being needed.

[Steamhead]

Timzsta
You are correct for large aircraft , they do tend to use various a.c.
voltages for different services as well as TRU's (transformer rectifier units ) and inverters .
In the case of the Pretolite Alternator, the full wave rectification is internal to the alternator and the two teminals on the back are as stated ie
1) 60 amps d.c. output
2) AUX terminal connected to the star point.

Why the Star Point which is A.C. when you only have access to one side of the A.C. circuit?

By the way there are two more terminals connected to the two brushes for the slip rings of the Rotor. One teminal is connected to Earth and the other is connected to the Regulator which supplies the d.c. excitation current / voltage the level of which is controlled to vary the d.c. current output at terminal 1) depending on battery condition.

Regards.

[Bus429]

Your alternator is part of a single-wire system, therefore the alternator has an internal ground. The star-point (aux) could be for systems fitted with fault protection.
Also, don't forget that a lot of components fitted to GA aircraft have other industrial/automotive applications.

[Steamhead]

Hello Bus429
Sorry, some delay in replying , Drive "C" suddenly developed some bad sectors.Had to re-format and re-install Windows etc., what a pain, took all afternoon.
You are probable correct, if the AUX terminal was connected to a a.c. voltmeter or an oscillascope and the other side to earth, then if either a winding shorted to earth or a diode short circuited then a reading of a.c. volts would be obtained.
It is another test that could be applied to a suspect faulty alternator.

Regards

[Bus429]

Steamhead,

I was thinking more of integrated fault protection.
All the best,
Bus

[BigJETS]

I found an alternator with an "R" lead, aside from the voltage regulator lead. It goes to a relay and an indicator light...I THINK something to do with over voltage protection. Does that sound familiar to anyone?

[timzsta]

BigJETS - the R lead might be the reverse current cut out. This stops the battery trying to charge the alternator should there be a problem with the alternator.

[BigJETS]

Id like to amend my previous comment about overvoltage, and insert 'output relay' & indicator. If its what I am thinking....normal output charges coil, circuit open - indicator off. low output, circuit closed- indicator indicates. Not sure if it applies to steamhead though.

[Steamhead]

HI Everybody
I think I have at last solved the problem.
The Prestolite alternators are now overhauled by Electrosystems,Inc. and their blerb states that the AUX. terminal is for connection to an electronic tachometer.
This makes sense as I believe this is the system used by a number of vehicle manufacturers.
Just requires an electronic division circuit for pulley ratio and number of phases to output r.p.m.

Been an interesting exercise, thanks for everybodies input.

Regards

[EchoTango]

The Star point is the Neutral, and is Ground BUT ONLY when there is a balanced load over the three phases.

Any imbalance over the three phases will cause a current to flow to that Neutral.

So DO NOT ASSUME THAT NEUTRAL = GROUND, especially when associated with DC networks. You need only tiny current flows thru that AC neutral to play havoc with DC instrumentation and control equipment.

ET

[411A]

Last year fitted 100 amp Prestolite alternators to my aeroplane (the Westinghouse units removed suffered from poor parts availability and are/were difficult to balance) and the aux terminal is used for output fail indication, at least on this particular installation with transistorized voltage regulators.
My GPS unit fitted shows bus voltage, and the output from these alternator/regulator combination is very stable...28.0 volts precisely, adjustable as desired.