Alternators are designed to charge the battery of a motor vehicle
Like the dynamo, the alternator is designed to charge the battery of the vehicle. Alternators are a more recent design, made possible by the development of the silicone diode. Alternators started being fitted to vehicles in the 1960s.
The alternator, similar to the dynamo, has two main components; the rotor and the stator. The output is produced by the stator, the static component of the alternator. The field is produced by the rotor, which rotates within the stator and is usually driven by a belt from the engine. There is no commutator in an alternator as the output from the generator is converted into DC by the diode pack.
An alternator uses a voltage regulator, in the same way as a dynamo. Rather than using a voltage coil to measure the voltage produced by the unit and switch relay contacts, an alternator use electronics to achieve the same end. A Zener diode can be set to break down at a given voltage and this function can be used as a control signal to switch on a transistor, allowing current to flow from the battery through the voltage regulator and through the rotor coil.
The connection to the rotor coil is made through a slip ring set that contacts to carbon brushes. Slip rings are made of two rings of phosphor bronze, separated by an insulating material. A set of slip rings is mounted on the none-drive end of the rotor shaft.
There are a number of different designs of rotor, the most common are the Lundell (or claw) type and salient pole type.
Salient pole type
The salient pole type are the oldest design and don't tend to be used much now. The rotor itself is made using a similar construction to that of an armature, utilising lots of laminations stamped out and pressed together on a knurled shaft. The salient poles can also be made as separate lamination stacks and then bolted to the shaft. The coils are wound around each of the poles such that the coils are in magnetic opposition consecutively. When current is passed through the whole rotor a north pole is followed by a south pole, around an axis of 360 degrees.
Claw pole type
The claw type rotor is less efficient than the salient pole type but is cheaper to manufacture. There is only one coil which is sandwiched between pole irons. Pole irons are forged in the form of a star with the radial fingers bent through 90 degrees as to run parallel with the rotor axis. When assembled the pole irons are arranged so their respective fingers intertwine. This turns the axial bipolar field into a polypole radial field which will match the multi-pole stator winding.
The stator core is made of stamped laminations stacked on top of one another and welded or cast into aluminium to hold them together. The laminations are stamped in a pattern which produces a series of slots when the laminations are pressed together. The stator coils are wound into the slots. The number of slots vary depending on the alternator. Older units may have 12 or 24 slots but more modern units usually have 36 or 48 slots. The number of slots governs the number of poles and the frequency of the output. The rotor poles also have to match those of the stator.
The stator winding is made up of a series of coils that can be wire wound or wound with a series of pulled hair pin coils which are connected together. Most alternator stators are three phase, which means they have separate windings - displaced physically from one another by 120 electrical degrees. Each of the phases are connected to one another in either a star or delta configuration, depending on which characteristics are required from the unit.
How An Alternator Operates
As the rotor spins within the stator a current is allowed to flow through the voltage regulator and through the rotor coil. The rotor coil generates a magnetic field which causes a voltage to be induced into the stator coils, depending on:
- the speed of rotation
- the battery voltage being applied
- number of turns in the stator winding coils
Each of the three phases of the stator windings are being excited as the rotor spins but each phase is at a different stage in the cycle, depending on where the rotor pole is in relation to that phase.
As a pole of the rotor approaches a stator coil it starts to excite the stator coil. This process will produce a voltage that increases until the pole of the rotor is directly above it. The peak voltage of the stator coil will occur at this point. As the rotor pole spins away the voltage produced by the stator coil will start to fall until the rotor pole is completely out of the coils pitch. This will be the minimum output from that coil. As there is another stator coil displaced 120 electrical degrees from the first the rotor pole leaves the peak of the first coil and starts to excite the coils of the second phase, and so on with the third phase. Each phase is always producing.
The rising and falling output as the rotor spins is sinusoidal (or AC) but the battery requires DC in order to be charged. The AC output is rectified into a single DC output, suitable to the charging system, by the use of a series of silicone diodes in a three phase bridge configuration, known as a diode pack.
When the alternator output voltage increases above the voltage regulator's threshold, the transistor supplying the current to the rotor will be switched off. This will cause the magnetic field of the rotor coil to collapse, and although the rotor continues to spin it will not induce any voltage into the stator windings. The stator stops producing an output and the battery stops charging. At this point the voltage regulator threshold will not be reached so the transistor supplying current to the rotor coil will be switched on allowing current to flow through the rotor coil, establishing a magnetic field, and the whole cycle - which takes about 0.0001 seconds - starts again.
In more modern alternators, such as the AC203RA, the voltage regulators also have current regulators and timing functions. The voltage regulator shuts the alternator down for a few seconds to protect it if it detects voltage spikes.