Hydroelectric generator

(mechanical engineering) An electric rotating machine that transforms mechanical power from a hydraulic turbine or water wheel into electric power.

A low-speed generator driven by water turbines. Hydrogenerators may have a horizontal or vertical shaft. The horizontal units are usually small with speeds of 300–1200 revolutions per minute (rpm). The vertical units are usually larger and more easily adapted to small hydraulic heads. The rotor diameters range from 2 to 62 ft (0.6 to 19 m) and capacities from 50 to 900,000 kVA. The generators are rated in kVA (kilovolts times amperes). The kilowatt output is the product of kVA and power factor. The normal power-factor rating of small synchronous generators is between 0.8 and 1.0 with 0.9 being common. For large generators a rating of 0.9–0.95 is common with the machines able to operate up to 1.0 when the load requires. The generators may also supply reactive power. See also Alternating current; Electric power measurement; Volt-ampere.

The turbine shown in the illustration has an adjustable blade propeller, typical of large, low-head units that are common on large river power plants. The water enters the turbine spiral scroll casing, falls down through the turbine, causing rotation, and empties into the river. The shaft transmits the rotation to the generator spider or hub and thence to the rotor rim and poles. The magnetic field of the rotor poles transmits the torque to the stator and changes the mechanical power to electrical power. See also Hydraulic turbine; Turbine.

Large hydroelectric generator. (Westinghouse Electric Corp.)

The poles are spaced around the rotor rim and are magnetized by direct current flowing in the turns of the field coil around each pole. The magnetic field, or flux, crosses the air gap between rotor and stator, flows radially through the stator teeth and thence to the area one pole pitch away, and back to the adjacent pole on the rotor. The magnetic flux is stationary with respect to the rotor poles but sweeps around the stator at the peripheral rotor speed. Coils are installed in the stator slots between the teeth. Thus there is an ever-changing flux linking stator coils, which causes an induced electromotive force in the coils according to Faraday's law. See also Alternating-current generator; Magnetism.