Alternator- can someone enlighten me?

lazza

HI Forum

I'm trying to get to grips with how alternators work as we are having to install batteries and inverters on boats. I thought I had a basic understanding after previous posts, but things dont seem so clear in reality.

In the following photo, I wonder if anyone could explain to me what each terminal is for?

When in operation the top 2 terminals were +ve and the bottom -ve when i measured the voltage.

I dont understand what is happening with the currents... they all showed current, one 20A, the other 6A and the bottom one (i think) 10A (for my sins i cant remember the current directions).... it all seems very confusing as to what is happening and what each terminal is for?

Any pointers greatly appreciated

Larry

waynefromnscanada

Re: Alternator- can someone enlighten me?

Well, how alternators work in general, is rather simple. A rotor, magnetized either by a perm magnet or a current carrying coil, spins within a stator wound with coils which produce electricity as the moving magnet fields cut through them. The alternating currents produced, are, in battery charging units, fed to a rectifier which changes the AC to DC which is then fed into the battery / loads.
How the specific alternator shown in your photo operates in it's own unique way, in detail, will depend on what it's manufacturer had in mind when designing it.
For example, it may, or may not have a built in regulator, it may or may not be controlled by the engine's computer (if the engine has one), it may or may not have a winding tap used to indicate the alternator is not charging when indeed it should be doing so, and the list can go on.
Without a schematic of you're specific alternator, or unless someone is familiar with this exact model of alternator, any further info we could give would be only guessing.
My guessing: The top wires, blue and gray, are probably + output, because they appear connected at a red insulator.
The green wire appears to be connected at a black insulator, so it may or may not feed one of the brushes feeding a wound rotor.
The brown and black twin wires appear to be connected directly to the metal frame of the alternator, so it may, or may not be "ground".
The black item to the right of the rear bearing, LOOKS like a thermal disconnect, although I've never seen such a thing on an alternator.

BB.

Re: Alternator- can someone enlighten me?

Many vehicle alternators use the frame of the alternator/engine for the return lead. So, if the Green with Yellow stripe is really a ground/earth return wire, it may be sharing a current path with the engine too (AC to DC rectifiers are typically installed inside the alternator to use the metal and moving air for cooling).

You might try measuring the current flow back into the battery and see if you can find your "lost" return current there (less any power needed to run the ignition/vehicle systems).

Vehicle alternators do use field windings. Many alternators in US cars use internal voltage regulators, so you could not directly access the field winding current. It appears that your unit may use an external regulator, and it is possible that your green/yellow wire is supplying field winding current (~3-6 amps would seem reasonable as a first guess).

Here is a good link on alternator basics.

-Bill

lazza

Re: Alternator- can someone enlighten me?

Gonna have to think about this, and of course, as i guessed depends on the manufacturer and type of alternator.

Do all alternators therefore have an electromagnet in the rotor and NOT permanent magnets?

waynefromnscanada

Re: Alternator- can someone enlighten me?

lazza wrote: »

Do all alternators therefore have an electromagnet in the rotor and NOT permanent magnets?

Most do, but not all. Friend of mine has an electric motor repair shop and rebuilds a lot of alternators. Some, especially marine types have at least part of the magnetism supplied by a magnet, and are "self exciting". A few I've checked out, you can definitely feel the "cogging" of the magnet as you turn it's shaft while it's sitting on the bench, not connected to anything. The last two motor bikes I had, came with PM alternators, as do most, or all snowmobiles. No brushes to wear out.

BB.

Re: Alternator- can someone enlighten me?

For small alternators (motorcycles, bicycle alternators, etc.), they will usually have permanent magnets in the rotor.

For larger alternators (cars, trucks, etc.), they will almost all have slip rings. There is a little residual magnetism in the rotor to "boot strap" the voltage regulation in the alternator (on purpose). The adjustable field is an advantage too. For a 100-150 amp alternator, there is only about an average of 10 amps or so needed to run the car during daylight hours--so reducing the field current helps reduce over charging of the battery.

PM alternators are going to be found more in off-grid stuff because they are, very roughly, 10% more efficient (no field current). Plus less maintenance (no slip rings).

The "cogging" is the magnetism of the rotor lining up with the metal in the stator (i.e., poles of rotor are attracted to closest steel parts).

If your alternator lasts for 150,000 miles in your car, you can use ~35 mph as the average speed to convert to hours of use:

• 150,000 miles / 35 mph = 4,286 hours

That is, roughly, about 1/2 a year of Run Time (~8,766 hours per year). So, having to take a wind turbine down once a year for maintenance is not too far off from normal lifetime of bearings/brushes, etc...

-Bill