Alternator Genorator

The brushes on the commutator of the generator connect to the windings as they pass the fixed poles and so takes out the current as the same polarity all the time. So the commutator acts as a mechanical rectifier to put out DC from the AC that is in the output windings.

In the alternator, solid state rectifiers are used.

Since the field rotates in the alternator, it can be compact and gets its power through slip rings in most cases though they can be gotten around.

It is possible in the generator to mount slip rings and rectifiers on the spinning armature an so get DC out through the slip rings saving the complexity of the commutator. Its not common to do it that way.

Its possible in the alternator to mount a separate excitation generator with those diodes on the back but on the same shaft and dispense with slip rings. So far slip rings and brushes are far cheaper than that added little generator so its not commonly done, though there is a Delco part number for such an assembly, probably for several versions.

Since the output voltage depends on the field strength and the speed the output winding cuts that field, voltage varies practically as a direct function of shaft speed in either the alternator or the generator. So for good battery and lamp life, both need a voltage regulator which can internal or external, can be an electromechanical device with vibrating contacts, or a carbon pile variable resistor, or made with transistors.

A three brush generator can get by without a voltage regulator (but still needs a cutout) because the third brush causes current limiting so its more of a constant current device and the battery sets the voltage. Its characteristic of such systems that one needs to monitor the battery charge and divert it when the battery is charged by turning on the headlights of car, truck, or tractor.

Generators generally depend on a relay type cutout for reverse current, a part of the voltage regulator but it allows more than a little reverse current discharging the battery at low generator speeds. The modern diodes of the alternator (or my own regulator design for that 12 volt generator on a 6 volt system) eliminate that reverse current and improve on the charging efficiency at the cost of greater energy loss in the diodes themselves.

Automotive alternators have a bit more robust rotor construction than the automotive generator so can be spun faster without the rotor flying apart. That allows the alternator to be designed to kick in at a lower speed though I've use a 12 volt generator on a 6 volt system with great results and I've seen American Bosch heavy duty generators made to withstand a fast shaft speed too.

There are many variables in the design of generator or alternator, and many have to be traded off with cost and volume when designing either for production use on car, truck, tractor, or utility power plant.

Gerald J. Electrical Engineer

Hurst,

Here are two points of clarification. First, some alternators have external regulators and some have internal regulators. Although the regulators for alternators and generators are different from one another, that is not what distinguishes an alternator from a generator. The distinctions are exactly as stated in Allen's post.

Second, although an alternator does indeed produce an alternating current voltage, alternators have rectifiers built into them that convert the AC to DC, so the output from the alternator is a DC voltage.

Good luck,

Tom in TN

An alternator is constructed backwards from a generator in that the field windings rotate inducing current into the stationary stator windings producing an "alternating (A/C)" current at that stator.

A generator spins an armature inside of and in which current is induced from a stationary field thereby producing an immediate DC voltage at that armature.

In other words, the electromagnets (field current if you will)spin in the alternator and are standing still in a generator.

The upshot is that an alternator can therefore produce more electricity at a slower rotational speed but of course, it's output has to be converted to DC voltage before it can be used.

Make any sense? I've got myself confused. :>)

Allan