Classic Car Automotive Electrical Systems - Part 2: How Generators and Alternators Work
GENERATORS AND ALTERNATORS
Last month's article dealt with the principles of DC electricity and how your car's battery functions. Now we can go on to how that battery gets charged. In older cars (before about 1964) this was done with a generator. After that time all cars switched to alternators and the reasons for the change will become clear. Let's see how each works. First, the generator:
The basic principle at work here is that electricity produces magnetism. Conversely, magnetism produces electricity. If a current-carrying coil of wire is placed around a bar of steel, the bar will become magnetized. The more turns of wire and the stronger the current, the more powerful the magnet. By placing a soft iron core within the coil, the magnetic force lines are concentrated and strengthened. As there is less electrical resistance (remember resistance?) in the iron than in the surrounding air, the force lines will follow the core.
The two pole shoes of a generator are constructed in this way. Rather than use magnets - which are heavy and expensive - many turns of wire are wound around the pole shoes. When a current passes through these windings the pole shoes become electromagnets, called FIELD COILS. These two field coils are connected in series (current passes through one and then through the other) and wound so that one becomes the north pole and the other the south pole of the magnetic field.
Inside the generator is a spinning central shaft which is supported in bearings at each end. Loops of wire (armature windings) are wound on a special laminated holder called the ARMATURE. The armature is turned by placing a pulley on one end of the shaft and driving it with a V-belt from the engine's crankshaft, as seen in the figure.
Attached to the armature are electrical contact segments, called the COMMUTATOR. These segments are electrically insulated from the armature — and each other — but each is soldered to one of the armature windings. It is the commutator which distributes electricity to the armature in an on-off manner, creating a magnetic field around the armature. Riding over the spinning commutator segments are carbon "brushes". These brushes are held in spring-loaded brackets and that pressure holds them against the commutator. It is the brushes which wear out over time and require replacement.
How Everything Works
When the generator armature first begins to spin, there is a weak residual magnetic field in the iron pole shoes. As the armature spins, it begins to build voltage. Some of this voltage is impressed on the field windings through the generator regulator (commonly called the VOLTAGE REGULATOR, explained in the next article). This impressed voltage builds up a stronger winding current, increasing the strength of the magnetic field. The increased field produces more voltage in the armature. This, in turn, builds more current in the field windings, with a resultant higher armature voltage. This voltage could, of course, continue to increase indefinitely, but it is limited (by regulation) to a pre-set peak. At this point all this sounds like perpetual motion, doesn't it? Remember, though, that the energy driving all this is the engine's crankshaft!
Study the illustration and familiarize yourself with the generator's parts. It should be noted that the most common failure of a generator is the brushes. Second to that is bearing failure, especially the bearing next to the drive pulley (improper belt tension hastens the demise of this bearing!)
A major failure-mechanism in generators is improper installation of a new or rebuilt one. Mechanically, the installation is straightforward but electrically, things are more complex. When the generator stopped the last time, residual magnetism remained in the pole shoes. The polarity of the shoes at that time depended on the direction of current flow in the field coil windings. If — during testing and rebuilding — current is caused to flow in the opposite direction, the pole shoes will change polarity. If the generator is then run in the car, the reversed polarity will cause current to flow in the wrong direction, damaging the regulator and discharging the battery when the car is left overnight. Therefore, all generators must be polarized after installation and before running the car. This is done by holding one end of a wire on the "battery" terminal of the regulator and scratching the other end against the generator's output terminal (for externally-grounded generators). For internally-grounded generators the proper way to polarize is to disconnect the "field" lead from the regulator and scratch it on the "battery" terminal on the regulator.
Generators produce Direct Current. Alternators produce "alternating current", or AC. Alternators have the advantage of producing far more current at low speeds than generators, thus allowing more and more accessories in the car. In an alternator, the "field" windings are placed around the spinning central shaft rather than on "shoes" as in the generator. Two iron pole pieces — cast with "fingers" — are slid on the shaft, covering the field winding so that the fingers are interspaced. The fingers on one pole piece form the North poles and the fingers on the other form the South poles. This assembly is called the ROTOR. Surrounding the rotor are a series of windings around laminated iron rings, attached to the alternator's case. This assembly is called the STATOR. The engine's crankshaft spins the rotor.
Direct current from the battery is fed through into the rotor's field coil by using brushes rubbing against slip-rings. One end of the field coil is fastened to the insulated brush, while the other end is attached to the grounded brush. As the pole fields pass through the stator, current is electromagnetically produced (as in the generator) but since the rotor is composed of alternating North and South poles the current produced flows in opposite direction every 180-degrees of rotation. In other words, the current is "alternating".
Why is this more efficient? Well, the stator windings are made up of three separate windings. This produces what is known as three-phase AC. When only one winding is used, single-phase current results (like in a generator). In effect, the alternator produces three times the current of a generator for the same effort on the engine's part. Also, alternators are considerably lighter and smaller than generators.
There's a small problem with alternators, though. AC electricity doesn't work in a car! The car's electrical system — and battery — need DC. Therefore, the alternator's output is "rectified" into DC. This is done by passing the AC into silicon diodes. Diodes have a peculiar ability to allow current to flow readily in one direction only, stopping the flow if the direction reverses. Multiple diodes are arranged in alternators so that current will flow from the alternator to the battery (in one direction only, creating DC) but not from the battery to the alternator.
In actual operation, the voltage regulator senses the battery voltage and overall demand on the car's electrical system. When charging is needed, the regulator applies battery voltage to the stator's brushes and this creates the electrical field for charging. As the system's demand for charging decreases the voltage to the brushes cuts off. All of this occurs many times per minute, with the system turning on and off repeatedly to keep everything at optimum operating efficiency.
In our next installment we'll look at Voltage Regulators and how they work.