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Alternator 101

Posted: 13 Feb 2018 09:32
by bogeyman
There is a lot of talk about alternators on different pages on the forum with a lot of assumptions and some untruths.
Here I will share what I know about 12v automotive alternators and how they function. If my facts are wrong , please correct them with factual proof.

An alternator consists of a rotor , stator , rectifier and voltage regulator and a set of brushes.
Mechanical components are two end plates , each housing a bearing. At the drive end is a pulley with cooling fan.

The rotor is a fast spinning electro magnet that is powered with an energising dc current from the regulator. There should be resistance between the two slip rings and the wind should be completely isolated from the shaft body.

The stator has three long isolated copper wires wound inside a soft metal housing fitting tightly around the rotor. At one end , the wires are connected together(starpoint) and at the other end (output) it is connected to the rectifier. When the rotor (magnet)turns inside the winding , alternating current is generated.
There should be equal resistance between the three open ends of the wind and completely isolated from the metal core.

The rectifier has three positive and three negative diodes mounted on a positive and a negative plate. It converts the alternating current from the stator to direct current suitable for charging a battery. The diodes get damaged with incorrect jump starting or a bad connection on the earth/ positive wire to the battery. On most modern alternators , the diodes can not be separately tested and the complete plate is replaced if faulty.

The voltage regulator measures the system (battery) voltage and then increases or decreases the energising current to the rotor through the brushes and slip rings. This changes the intensity of the electromagnet and as a result , the output current. It is usually energised by the small current from the dash warning light at start up and then draws current from the output when operational. The light goes out because the energising point becomes a positive when the alternator is in operation. Pre set parameters will cause the alternator to produce maximum amperes when the voltage is at a low limit (11v) and gradually reduce output until the system maximum voltage (14.5v) is reached. These are sealed electronic units and are replaced when suspect. Different voltage parameters are available , but most units are set to standard.

The alternator base must be properly connected to the negative pole of the battery through the engine bonding cable.
The positive output terminal must be directly connected to the positive terminal of the battery. No current should run when the engine is turned off.
The warning light (energising) wire becomes active when ignition is turned on.

To properly test function , a dc voltmeter and a dc amp meter is needed. The charge rate amps must be close to max output when battery is depleted from starting or winch activity and should gradually decrease as the battery becomes charged. When volts reach battery maximum , only 2 to 4 amps output is correct.
As components in the vehicle requires energy , the alternator will react and match the draw with equal recharge.

Re: Alternator 101

Posted: 13 Feb 2018 09:37
by Tinus lotz
Dankie mr goeie info

Re: Alternator 101

Posted: 13 Feb 2018 09:43
by bogeyman
Upgrading an alternator usually only benefits the shop where you buy it.

Unless you have components in your vehicle that draws more than 50 % of the max output of the fitted alternator , no benefit is to be had from "upgrading.

Most high ampere components like starter motors and winches that use more power than the alternator can produce , will only be operational for short periods.
The alternator has the benefit of continuous operation and can replace the charge over an extended period with a slow charge rate. (better for the battery).

Re: Alternator 101

Posted: 13 Feb 2018 15:09
by davidvdm
Very nice write up, thanks...

Anybody got any ideas where/who can supply water cooled alternators. I have seen a few offroaders do this conversion in Aussie, and the alternators seem to be nice high output units of around 120-190A. Main reason actually seems to save yourself the problem of water/mud eating up the traditional air cooled type, that is open to the elements.

Re: Alternator 101

Posted: 13 Feb 2018 15:24
by davidvdm
No worries, scrap that idea at the price...... ... alternator

Re: Alternator 101

Posted: 13 Feb 2018 15:41
by hugejp
davidvdm wrote:
13 Feb 2018 15:24
No worries, scrap that idea at the price...... ... alternator



Re: Alternator 101

Posted: 13 Feb 2018 15:48
by davidvdm
Tell me about it... I really like the idea. With the cooling it is possible to get a whole lot more out of the alternator without the heat problems. Seems VW and BMW use it extensively, but at that price :confused: Gees, I can buy about 10 alternators for my 2.7TD for R28K without shipping and import tax. And to do this conversion, I'll probably end up spending another R5k to install an electric vacuum pump for the brakes.

scrap that idea :doh: