What Are Synchronous Generators?

Synchronous generators are alternating-current electrical machines that convert mechanical power into electrical power, with their rotor field rotating at a speed that maintains exact frequency synchronism with the AC network they feed. Because the output frequency is determined entirely by rotor speed, a synchronous generator connected to a large grid must be brought to synchronous speed before it can be paralleled, and the grid then holds the machine's speed constant thereafter. Synchronous generators are the primary source of grid-level electrical energy worldwide, installed in power stations driven by steam, water, gas, and wind turbines. The operational and modeling standards for these machines are codified in resources such as the IEEE guide for synchronous generator modeling practices, which defines the equivalent-circuit parameters used in stability and short-circuit analyses.

The class of machines collectively called synchronous generators includes turbogenerators (cylindrical-rotor designs for high-speed steam and gas turbine drive), hydrogenerators (salient-pole designs for low-speed hydraulic turbine drive), and diesel-driven standby generators. All share the same electromagnetic operating principle but differ substantially in physical construction, cooling method, and speed-torque characteristics.

Alternators and Their Relationship to Synchronous Generators

The terms "alternator" and "synchronous generator" describe the same class of machine, though usage differs by context. In automotive and small-engine applications, a compact permanent-magnet or wound-field alternator charges batteries and supplies vehicle electrical loads. In utility-scale power engineering, the term synchronous generator is standard. Both produce alternating current through the relative rotation of a magnetic field and a set of stator armature conductors. Large utility alternators are wound-field machines with separate excitation systems rather than permanent magnets, because the field current must be controllable to regulate terminal voltage and reactive-power output across a wide operating range. The construction principles behind both types are outlined in Iowa State University's power systems engineering course notes on synchronous generator modeling.

Rotor Types and Mechanical Design

Cylindrical (round) rotors are machined from solid forged steel and carry the field winding in longitudinal slots distributed around the periphery. They are used in two-pole and four-pole machines running at 3,000 or 1,500 rpm (on 50 Hz systems) or 3,600/1,800 rpm (on 60 Hz systems), sizes common in thermal and nuclear plants. Salient-pole rotors have individually shaped pole pieces bolted to a central hub and are characteristic of hydroelectric generators that run at 60 to 500 rpm with many pole pairs. A ten-pole salient-pole machine on a 50 Hz system runs at 600 rpm, for example. The choice of rotor type influences the synchronous and transient reactances, which in turn govern the machine's voltage regulation and its contribution to fault current.

Output Parameters and Ratings

Synchronous generator ratings encompass apparent power in volt-amperes (VA or MVA), power factor, terminal voltage, short-circuit ratio, and efficiency. Power factor is typically 0.85 lagging, meaning the machine is designed to supply reactive power to inductive loads on the system. The short-circuit ratio (SCR) is inversely proportional to the direct-axis synchronous reactance Xd and indicates how sensitive terminal voltage is to changes in load. Efficiency for large utility generators exceeds 98 percent, as described in reference materials from the University of Chicago's power generation engineering texts.

Applications

Synchronous generators have applications across a wide range of sectors, including:

• Base-load thermal, hydro, and nuclear power stations
• Gas turbine peaking plants and combined-cycle facilities
• Wind farms using doubly-fed induction and full-conversion permanent-magnet variants
• Emergency and standby generation for hospitals, data centers, and industrial plants
• Shipboard and offshore platform power generation