Revolving armature alternator

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In electrical engineering, an alternator is a machine that produces alternating current electric power when its shaft is turned by a prime mover such as a steam turbine. Most large alternators have a stationary winding, the armature, which has voltage induced in it by a rotating magnetic field produced by a rotor. However, some early machines, and some small machines even now, have a reverse arrangement with the armature rotating, and the field as a stationary structure.

A 60 Hz, three-phase revolving armature alternator was built by General Electric Company in 1895. It was designed by Elihu Thomson. Four were installed in the powerhouse at what is now the Folsom Powerhouse State Historic Park. Transmission 22 miles to Sacramento began 13 July 1895 at 2010 hp. These 750 kilowatt machines remained in service until 1952, when the new Folsom dam was being built and removed water flow from the old powerhouse.

File:21 April 2014 Rotating Armature Alternator.jpg
Folsom Powerhouse Alternator (armature phasing colors added)

These Folsom machines predated the Niagara Falls to Buffalo transmission and were the same power. The Niagara alternators were only two phase machines at 25 Hz. The Niagara transmission used the Scott-T transformer to get the third phase.


Weight: 57,877 pounds

Height overall: 10feet

Prime mover: John B. McCormick water turbine (1,260 hp), mixed flow, horizontal Dual runner, 1,360 cubic feet per second flow, supplied by S. Morgan Smith.

Shaft Rotation Direction: Clockwise, when facing the Nameplate.

Shaft Rotation Speed (Nominal): 300 RPM


Stator salient poles: 24

Dipoles: 12


Slots: 216

The winding is a progressive counterclockwise flattened-spiral wave, full pitch. Each of the six sub-phase wires circle the rotor three times.

There is a common ring whose diameter is approximately 5 feet. All six sub-phase low voltage sides (Cold) connect to this common ring near the rotor perimeter.

There is a wire connected at the common ring, routed to the back and connected to a single slip-ring. The single rear slip-ring output is routed to the back wall where it goes through a high impedance ground (a transformer primary and resistor on the secondary) to earth.

There are six bus bars that connect the armature perimeter inductors to the slip ring assembly.

The three bus bar pairs are not spaced equally (radial rotational symmetry) around the rotor perimeter.

Brush sets[edit]

Magnetic circuits[edit]

The pole faces alternate between North and South.
The magnets are powered by a separate Direct Current generator exciter at 12 to 16 Amperes at 600 volts.


There are no electromagnetic coils in the armature.

There is, however, mutual inductance between like sub-phases, that occupy the same slot.

Waveform: Three phase sinewave Voltage Output: 800v (Alternating Current)

Current per Output phase: 542 Amperes

The configuration is a double wye (star).

Form Factor: <math>\frac{\pi}{2\sqrt{2}} \approx 1.11072073</math>[1]

432 yard long rectangular inductors are laid in axial slots. They weave around the armature in a counterclockwise spiral wave, three times around the rotor.

Type: Synchronous

Load: three Westinghouse single phase, 11 Kv Water/Oil cooled Transformers, 22.7 miles of transmission line to Station A in Sacramento, then step-down transformers there, street arc-lamps and converted to Direct Current for trolleys.


The house General Electric Frequency Indicator was the rotation speed reference.[2]

A Lombard Water Wheel Governor provided negative feedback to the turbine.[3][4]

The governor was adjusted to set rotor shaft speed rpm to 300.

Connecting alternators in parallel was helped by using the Synchroscope.

Phases at rotor perimeter: 18 (voltage summed to six phases by overlapping the spiral wiring around the rotor circle, three times) [5]

Phase development[edit]

Each slot is occupied by two inductors of the same phase relationship but of different voltage, so the two occupants must be insulated from each other.



External links[edit]


  1. name="Jędrzejewski"
  2. Handbook for Electricity Metering (Ninth ed.). Edison Electric Institute, 701 Penn Avenue, NW. Washington, DC 20004-2696. 1992. p. 105. ISBN 0-931032-30-X. 
  3. Daniel Webster Mead. Water Power Engineering, The History of Water Power Development. 
  4. DeRoy, Benjamin E. (1966). "Control System Components". Automatic Control Theory. John Wiley & Sons, Inc. p. 73. ISBN 0 471 20371 8. 
  5. Hayt.Jr, William H.; Kemmerly, Jack E. (1978). "Polyphase Circuits". In Peter D. Nalle and Madelaine Eichberg. Engineering Circuit Analysis (Third ed.). Caledonia by Bi-Comp, Incorporated: McGraw-Hill, Inc. p. 365. ISBN 0-07-027393-6.