Alternator Synchronous Generator And The Types of Alternators

What is an Alternator?

Where is an Alternator Used?

An automotive alternator supplies vehicle loads and charges the battery while the engine is running. It replaced the commutator-based DC generators used in many earlier vehicles. Modern alternators can deliver useful output across a wide engine-speed range and package a regulator and rectifier into a compact unit.

Most conventional vehicle loads and the battery use direct current. The alternator’s stator produces multiphase AC, and a diode rectifier converts it to the DC output supplied to the vehicle electrical system.

Mechanical commutation is not required for the main output because the AC is rectified electronically. A voltage regulator controls rotor field current so the charging voltage remains within the range required by the vehicle.

This engine-driven machine is called an automotive alternator. Its claw-pole rotor, stator windings, rectifier, regulator and cooling arrangement are described in the guide to how an alternator is constructed.

Another use of alternators is diesel-electric traction. The diesel engine is the prime mover: it drives a generator or alternator, and the resulting electrical power supplies the traction system.

Some diesel-electric locomotives rectify generator output through a silicon diode bridge for DC traction motors. Modern designs may instead use an intermediate DC link and inverters to supply AC traction motors. The traction motors drive the axles through gearing.

Marine alternators range from engine-mounted battery-charging units to ship-service and propulsion generators. Their enclosure, cooling, insulation and corrosion protection must suit the vessel’s environmental and classification requirements.

Small craft commonly use nominal 12 V or 24 V charging systems. Large ships use higher-voltage AC generation and multiple generator sets to meet service, redundancy and propulsion demands, so 12 V or 24 V is not a general marine-alternator rating.

In a small-vessel charging system, rectified alternator output can charge the engine-starting battery and a separate house battery bank. The regulator and charging arrangement must match the battery chemistry and vessel wiring.

Large synchronous generators produce bulk AC power in thermal, hydroelectric, nuclear and gas-turbine plants. The turbine supplies shaft power, while the generator converts that mechanical input into electrical output at a frequency set by rotor speed and pole count.

Types of Alternators

Alternators or synchronous generators can be classified by application, prime mover, rotor construction, cooling method, excitation system and output arrangement.

The following legacy list gives five different types of alternators by application. It mixes current machine classes with the historical radio alternator and is not a complete engineering taxonomy.

• Automotive alternators – used in modern automobiles.
• Diesel-electric locomotive alternators – used in diesel-electric multiple units.
• Marine alternators – used in marine applications.
• Brushless alternators – used in electrical power generation plants as the main source of power.
• Radio alternators – used for low band radio frequency transmission.

For large synchronous machines, one useful rotor-design distinction is between salient-pole and smooth cylindrical rotors.

1. Salient Pole Type
2. Smooth Cylindrical Type
   

Salient Pole Type

A salient-pole rotor has projecting magnetic poles fixed to a rotor rim or spider. It is suited to many low- and medium-speed machines because a larger pole count can produce the required electrical frequency at a lower mechanical speed.

Salient-pole generators often have a large diameter and short axial length. This construction is common with low-speed hydraulic turbines, although the final arrangement depends on turbine speed, rating and mechanical design.

Smooth Cylindrical Type

A smooth cylindrical rotor is common in high-speed generators driven by steam or combustion-gas turbines. The forged steel rotor has axial slots for the field winding, while the unslotted pole regions carry magnetic flux and withstand high centrifugal stress.

At 60 Hz, a two-pole synchronous generator runs at 3,600 rpm and a four-pole unit runs at 1,800 rpm. At 50 Hz, the corresponding speeds are 3,000 rpm and 1,500 rpm. The original 36,000 rpm figure is a factor-of-ten error.

History of the Alternator

The history of the alternator begins with Michael Faraday’s discovery of electromagnetic induction in 1831 and Hippolyte Pixii’s magneto generator in 1832. Faraday demonstrated that relative motion between a conductor and a magnetic field can induce an electrical current.

Generator design advanced rapidly during the late nineteenth century as engineers developed stronger field systems, practical AC machines and polyphase power networks. Specific claims about a single first practical alternator or universal 100 to 300 Hz machines depend on how the machine and application are defined.

Polyphase alternators and AC systems emerged through work by several inventors and manufacturers during the 1880s and 1890s. High-frequency alternators also appeared for specialised lighting and radio work, but modern utility generators normally operate at the grid frequency rather than 15 kHz.