More on alternators

[Midnight]

Years ago I swopped my standard Beta 90amp alternator for a 110amp Prestolite. There's been no problems even after 12 years but I suspect it will eventually need replacing. When I fitted the Prestolite I used a wiring diagram supplied by Adverc which included soldering another wire to the negative brush, by-passing the Beta alternator wiring and using a relay to energise the alternator from the ignition switch. There's obviously two (at least) types of alternator so can anyone explain the difference and why one type needs energising. It would also be a big help to have suggestions for a 110amp replacement which would use the original wiring and the Adverc.

[Tracy D'arth]

S

"Standard" 9 diode alternators have no residual magnetism in the rotor and so need energising through the brushes.

Alternators with a magnet in the rotor have 6 diodes only and will self energise.

[nicknorman]

11 minutes ago, Tracy D'arth said:

S

"Standard" 9 diode alternators have no residual magnetism in the rotor and so need energising through the brushes.

Alternators with a magnet in the rotor have 6 diodes only and will self energise.

No, absolutely not.

 

6 and 9 diode machines have no residual magnetism, the rotor is an electromagnet. Current through the rotor (via the brushes) is controlled via the regulator to regulate the output voltage. Neither has a permanent magnet.

 

The difference is the source of the rotor current. A 9 diode machine gets its rotor current from 3 field diodes which rectify the alternator 3 phase ac output. Before the alternator is working properly, it needs some small initial current into the rotor to generate a bit of magnetic field. This is supplied by the warning light circuit. Once the alternator is working, the field diode supply is at the same voltage as the supply on the other side of the warning light, so the light extinguishes. If the warning light circuit is incomplete (bulb blow, wire off etc) the alternator will not start to work.

 

When the engine is stopped, there is no rotor supply from the field diodes and when the ignition is switched off, the supply to the warning light is removed and thus no current draw.

 

A 6 diode machine gets its field current supply direct from the B+/battery. Of course when the engine is stopped, the regulator would be trying to maximise output and so the rotor current, coming from the battery, would be maximised at 4A or so. Battery would go flat! So to prevent this, some electronics inside the alternator needs to know when the engine is shut down. Typically there will be an IGN connection, to the ignition switch,  which is used to tell the alternator to start working, or not. And a separate warning light connection, to operate the warning light when the IGN terminal says “start working” but there is no alternator output. The warning light thus plays no part in getting the alternator to work initially.

[ditchcrawler]

The first 24 volt alternator I fitted would happily excite its self once the revs built up. so I could start the engine on tickover and no charge, wind the revs up and in it would come. I never bothered to wire a warning light to it. When I changed alternators then I had to add the warning light.

[Craig Shelley]

Just to add a bit of confusion to the conversation, the design of the main rectifier varies somewhat too.

 

The traditional design for a 3 phase rectifier uses 6 diodes.

 

However, the waveform shape at the input to the rectifier doesn't look anything like the sine waves we see in the textbooks. As a result the neutral point at the centre of the Y doesn't stay nicely centered at a mid point voltage. Instead it bounces around all over the place, and at times exceeds the supply rail voltages.

So the designers capitalised on this by adding a 4th pair of diodes connected to the neutral.

This small tweak can apparently recover up to an additional 10% more power. This scheme sometimes referred to as "3rd harmonic" rectification, presumably because the neutral bounces around at 3 times the fundamental frequency.

 

Interestingly, the last time i replaced the diode pack on our alternator, was because this 4th pair of diodes had decided to phase change to the 4th state of matter. We only happened to notice the blackened remains of this diode pair when we had the alternator stripped down for an entirely unrelated reason. It had probably been like that for years.

 

--

Craig