Alternator
An alternator is an electrical machine that converts mechanical shaft power into alternating-current electricity. In an engine generator, the engine drives the alternator. The two components have different roles: the engine supplies mechanical power, and the alternator converts that power into an electrical output. [1]
Rotor and stator
In a typical rotating-field synchronous alternator, the rotor provides the magnetic field and turns inside the stationary stator. The changing field induces voltage in the stator windings, from which the main electrical output is taken. The arrangement of windings and connections determines the available voltage and phase configuration. [1]
The term alternator can also describe the machine used to produce electricity in a vehicle. An engine-generator alternator should be understood in its own application, with its output voltage, frequency, winding design and rating conditions. A shared name does not make the two assemblies interchangeable.
Excitation and voltage control
A wound-field alternator needs excitation to establish its magnetic field. Many generator alternators use a brushless system with an exciter and rotating rectifiers. The automatic voltage regulator (AVR) senses output voltage and adjusts excitation to maintain the intended voltage. Some systems supply the AVR through a permanent-magnet generator or an auxiliary winding. [1]
Excitation arrangement affects the machine’s response to certain loads and faults. Voltage regulation is therefore a function of the complete excitation and control system, rather than a fixed property implied by the word alternator.
Speed and output frequency
For a synchronous alternator, electrical frequency is related to rotor speed and pole count: f = poles × rpm ÷ 120. A four-pole machine therefore operates at 1,800 rpm for 60 Hz output; a two-pole machine operates at 3,600 rpm. These are illustrative synchronous-speed relationships. [2][4]
The engine governor controls shaft speed, while the AVR controls excitation. A sudden load increase can affect both frequency and voltage during recovery. In an inverter generator, the final output frequency is instead established by the inverter electronics after intermediate conversion. [2][3]
Ratings and load behavior
An alternator’s kVA and current limits matter alongside the generator set’s kW limit. Low-power-factor loads can demand more current for the same real power. Starting motors impose a transient requirement, while nonlinear loads can influence voltage distortion. Alternator selection therefore considers load characteristics as well as continuous wattage. [4][5]
Cooling, operating environment and winding temperature also form part of the rating conditions. A nameplate rating should be read with the manufacturer’s data for the assembled generator set. An engine running normally does not by itself confirm that the alternator and its controls are delivering the required electrical output.
Sources and further reading
- STAMFORD | AvK — Excitation system and main alternator components ↗www.stamford-avk.com
- STAMFORD | AvK — Alternator operating speed ↗www.stamford-avk.com
- Honda — Sine-wave inverter control system ↗global.honda
- Cummins — Generator application manual, alternator and load characteristics ↗powersuite.cummins.com
- STAMFORD | AvK — Motor starting fundamentals ↗www.stamford-avk.com
Use the manual and nameplate for the exact model. Local requirements and site conditions take precedence over general examples.