110427-1442 EDT
Depending upon how the pole faces are formed (shape) the output waveform of an alternator can be modified. See
http://en.wikipedia.org/wiki/Alternator
Automotive alternators
Alternators have the great advantage over direct-current generators of not using a commutator, which makes them simpler, lighter, less costly, and more rugged than a DC generator, and the slip rings allow for greatly extended brush life. The stronger construction of automotive alternators allows them to use a smaller pulley so as to turn faster than the engine, improving output when the engine is idling. The availability of low-cost solid-state diodes from 1960 onward allowed car manufacturers to substitute alternators for DC generators (major American manufacturers had made the transition to alternators by 1962, for example). Automotive alternators use a set of rectifiers (diode bridge) to convert AC to DC. To provide direct current with low ripple, automotive alternators have a three-phase winding. In addition, the pole-pieces of the rotor are shaped (claw-pole) so as to produce a voltage waveform closer to a square wave that, when rectified by the diodes, produces even less ripple than the rectification of three-phase sinusoidal voltages.
Typical passenger vehicle and light truck alternators use Lundell or claw-pole field construction, where the field north and south poles are all energized by a single winding, with the poles looking rather like fingers of two hands interlocked with each other. Larger vehicles may have salient-pole alternators similar to larger machines. The automotive alternator is usually belt driven at 2-3 times the engine crankshaft speed. Automotive alternators are not restricted to a certain RPM because the alternating current is rectified to direct current and need not be any constant frequency.
The fundamental reason an automotive alternator can provide charging voltage at low RPM is that it is run at a higher RPM than a DC generator. This can be done because of mechanical factors. At lower RPMs a DC generator could provide a higher voltage, but for the same current capacity it would be larger. For a given physical size higher RPM means greater power output.
For a fixed diode configuration if you square up the input waveform you will reduce the ripple current.
Alternators were a vast improvement over generators. Generators rely on residual magnetism to start charging. That means if the battery is taken out and replaced you have to 'spark' two terminals on the generator together before it will charge. That is known as 'polarizing'.
This description needs clarification. I will describe the operation from the perspective of a Ford generator and regulator system.
The Ford regulator consists of three relays:
A voltage regulator.
A current limiter.
A cutout.
The cutout relay is a normally open type and has two coils. A voltage coil from the generator, and a current coil in series with the lead to the battery. The regulator I have is a 6 V unit from about 1950. The cutout relay pulls in at about 6.8 V. For this system to work the generator has to produce voltage before the battery is ever connected to the generator to provide field excitation.
If fact the generator is self exciting as a result of the residual magnetism of the field.
A typical Ford generator had two terminals, the plus armature, and the plus field. The negative side of the generator armature and field are connected to the generator frame and in turn to the engine and car chassis.
The positive field terminal on the generator is connected to the field terminal of the regulator. The field terminal is connected thru a series pair of normally closed contacts on the voltage and current regulating relays to the armature. The armature terminal of the generator is connected to the armature terminal of the regulator. From here the armature current flows thru the current regulator and the cutout relay to the battery terminal of the regulator assembly.
The armature voltage is applied to the voltage regulator coil and the the cutout coil.
The current require to operate the cutout relay at 6.8 volts was about 1.1 A. This current is distributed between two voltage coils and a series string of resistors used to dampen arcing of the voltage regulator contacts.
It should be noted that the voltage regulator contacts oscillate at a high rate. Quite possibly above 10 Hz. In the engineering labs on this product many experiments were run on various contact materials to improve contact life. In those days 1000 hours of operation for the regulator life was a goal. At 60 MPH that is 60,000 miles.
So a DC generator is self-exciting. It only needs two terminals plus the chassis ground. It will produce full output voltage to the vehicle without a battery.
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