Alternators and generators

By Norman Nock

Originally published in the Austin-Healey Magazine, April-May 1994


There are advantages for both types of charging devices. The original generator will always keep your battery fully charged providing all units, battery, wiring and ancillary equipment are in good working order.


Many times when car owners have a no-crank problem, they replace the battery, solving the present dilemma only to find that after a few days driving they have the same old problem - no crank. This haphazardly replacing of parts goes on until the faulty part is replaced or repaired. Who gets the bad name for this? Lucas. The problem is that the diagnosis was not done correctly.


The Generator


Most Lucas generators produce 20 amps to 25 amps at about 1800 RPM. The engine, when running, spends most of its in-use time above 1800 RPM. It will produce its maximum charge rate when called upon by having to charge a low battery, with lights, wipers, and heater working. When the battery becomes fully charged, the output of the generator will drop to near zero but will always be on the plus side of charge at normal driving speeds.


However, if you start with a flat battery and most of your driving is within well-traveled downtown areas, it will take a long time to bring your battery back to full charge. You should always get your battery fully charged up to correct specific gravity before using your car. This will allow the generator to keep the battery fully charged.


If you use your car in the winter when temperatures are in the 20's and lower, and you do a lot of slow speed driving, your battery will not be able to reach a full-charge condition. The battery, if only 75% charged at a low winter temperature, will not start your car.


When the brushes wear they can easily be replaced, but the commutator should be turned to make it round again, best done by a generator rebuilder mechanic. If the fan belt is pulled too tight the bearings or end covers will become damaged and the armature will hit the field windings and damage the armature. The regulator can be adjusted very easily but if done incorrectly the generator will produce too much electricity and damage the armature inside the generator.


The low speed output of the generator can be increased by fitting a smaller diameter pulley. This will produce the same amount of electricity but sooner, i.e. output will reach its maximum at lower engine speeds. But, with the armature turning faster the windings within the armature will bethrown out due to higher centrifugal force, damaging the armature. Generators maintained correctly will give very few problems. If they do cause a problem usually some kind of get-you-home emergency repair can be made.


The Alternator


Alternators provide high outputs at low speeds and will bring a battery back to fully charged condition quickly, even if you use all your electrical equipment.


The bracket to modify your Austin-Healey is available to install an alternator. The armature in a generator is the part that spins, but in the alternator the armature is now stationary and the field windings spin, eliminating the problem of too high RPMs.


The weight of early generators was 30 pounds but today's alternators are about 10 pounds, with much higher outputs. Alternators provide high outputs at low speeds and will bring a battery back to fully charged condition quickly, even if you use all your electrical equipment. Your battery will be receiving a charge even at idle speeds. The pulley on an alternator is much smaller than a generator. The rotor will travel much faster than the generator armature and will not damage itself by too high a rotor speed. It can go up to 12,500 RPM without problems and still produce maxi-mum amps. Alternators are available up to 80 amps and more. You could run all your equipment high output radio, air conditioner with a flat battery and still have a reserve of electrical power. The alternator belt can be pulled tighter than a generator without the fear of damage to the end bearings. Pickup brushes carry a very small amount of electricity so there is very little wear when compared to generator bushes that carry high amps and wear the commutator. Today's alternators have their own voltage regulation circuits built-in, eliminating the need for an external regulator. The original regulator can be left on the firewall for looks only and as a junction box.


Aside from higher amperage levels, there's one other key difference between an alternator and a generator: Generators will give you a warning when they are going to fail, like the ignition warning light will be slow going out. But the alternator will stop charging without any warning. Rebuilding alternators is not a job for the average mechanic. If your alternator goes faulty, you have to replace it.


If you do have an old battery and you are continually cleaning away white corrosion from the battery area, the only way to stop it is to replace the battery and check charge rate. If you have a weak old battery the alternator will continue pushing high amps into it. The acid inside will boil causing lots of corrosion. The generator will do the same thing but the output is not as great. If you do have an old battery and you are continually cleaning away white corrosion from the battery area, the only way to stop it is to replace the battery and check charge rate. You can run your car without a battery using the output of the generator but if you have an alternator you can damage it running without a battery.


Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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