Alternator

An alternating current generator is a special embodiment of an electrical generator is used to generate single-phase alternating current. Since in this type of generator, in contrast to the direct current generator there is no commutation, an alternating current is generated, whose frequency is proportional to rotor speed. For the generation of electricity on a large scale single-phase AC generators are now only of minor importance. You are now built only for small services (including emergency power supplies ) to about 2.2 kilowatts.

The most common alternator is the bicycle dynamo, which operates on the device built by Hippolyte Pixii generator principle.

History

At first alternator by Frenchman Hippolyte Pixii rotated a horseshoe magnet in front of two coils connected in series. The changing magnetic field induced in the coils of an AC voltage. But since you were not interested in AC, but at DC at the time, you built a commutator, which made a direct current from the AC. This principle of equality of power generation by means of a commutator and brushes is still used today in the DC generators.

Principle

The principle of an AC generator is extremely simple. Due to the rotation of a conductor loop in a magnetic field, the magnetic flux is constantly changing through the loop. Thereby a voltage is induced in it. We describe the position of the loop by the angle, so is the passing magnetic flux:

Rotating said loop to the angular speed ω, then

And the induced voltage

The induced by the rotation of the line voltage can be tapped through brushes on the slip rings. The conductor moved transversely to the magnetic field acts on him by current drain the Lorentz force. This inhibits the movement of the loop and thus mechanical work is to move the loop necessary. It just depends on the relative motion between the field and conductor - hence it does not matter whether the conductor in a stationary field, or the field is moved to the head rest.

Design and function

The alternator is comprised of a stator and a rotor. Today's standard alternators are designed as single-phase synchronous machines in Innenpolmaschinenbauweise, ie the rotor is magnetically excited by the excitation field and a voltage in the stator is induced. To excite different systems are used. Thus, the exciter field may be provided, for example, by permanent magnets. The advantage of this is that no excitation power to be supplied. Another method is the excitation by means of static excitation means, wherein the excitation power supplied via the slip rings. As a buffer to stabilize the excitation voltage is often a small accumulator. To start without battery may also include a small permanent magnetization of the rotor used. After the start of the process of self- excitation of the exciting current must be controlled in order to keep the output voltage constant.

The rotating magnetic field induced in the stator coils of a single-phase AC voltage, whose frequency is dependent on the speed of the driving machine and the number of pole pairs of the generator. In contrast to the direct current generator, but here rotates the energizing field, and thus induces a current in the arranged order to the rotating field stationary stator.

As alternators produce a single-phase AC voltage, they are only suitable for operation on the grid. So in Germany are about the generators to produce electricity for the German railway in this category. In the three-phase system can cause the supply of single-phase AC voltage to an asymmetry, so alternators otherwise be operated in small hydro power plants on the grid.

Another type of AC generators are asynchronous. A single-phase induction motor is connected to the mains voltage, is able to operate as a generator when rotated by the slip faster than the synchronous speed. If no network is available, is aided by the with capacitors in order to compensate for reactive power and provide the excitement. Such self-excited induction generators require but for the operation of a residual magnetization of the squirrel-cage rotor or a starting current pulse.

Areas of application

• Small hydropower plants
• Small wind turbines
• Emergency generators ( up to 2.2 kW )
• AC alternator on bicycles and some motorcycles

Legal regulations and other rules

• EN 60 034 Part 1 General requirements for rotating electrical machines
• DIN IEC 34 Part 7 types rotating electrical machines
• EN 60034-5 protection of rotating electrical machines
• EN 60034-6 types of cooling for rotating electrical machines