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Direct current is required for the electro-magnetic field. A single coil is wound in the alternator stator (the stationary
winding) to provide the current for the base field. This coil is entirely separate from the main winding and is at right
angles in mechanical position. The AC voltage generated in this coil is fed to a full wave silicon diode bridge to provide
rectified direct current for the base field. As the field is the rotating component, the current connection is accomplished
by the use of slip rings and brushes. The base field is connected to ring #1 (nearest to the bearing) and ring #3. The
base field is designed to provide magnetic lines of force required to generate rated voltage with no load on the alternator.
In order to maintain close voltage regulation as the load is varied, a control field is used. The control circuit consists of
an additional full wave bridge, in series with one load line, and a control winding on the field poles. The control field is
connected to ring #1 and ring #2. When a load is connected to the alternator, this current is rectified and fed through the
control field winding. By this means, the total strength of the field is varied in relation to the load.
The description of static excitation opened with the statement that "static" is-defined as without motion. In later
paragraphs the revolving field has been discussed. To avoid confusion, a word of explanation is in order. There must be
some relative motion between the coils which generate voltage and the magnetic field which causes the voltage to be
produced. In a revolving field generator, the winding that produces the voltage is stationary and the field poles revolve.
The reverse is true in the case of a revolving armature generator; the field is stationary and the voltage producing
winding rotates.
LOAD CONNECTION
Standard connection for 2000 watt, single voltage alternators use duplex grounding type receptacles. Special
applications, to be used as part of other equipment, may provide a plate to accept conduit fittings. Short leads are
brought out for connection.
Larger capacity machines are provided with an outlet box enclosing a terminal strip for connection. Optional panels are
available for capacities above 2000 watts. These are designed to be installed in the field. A voltage indicator, a circuit
breaker, 15 ampere duplex receptacle and a 50 ampere receptacle are included in the special panels. The voltage
indicator uses a color band of red and green in place of a numbered scale. Voltage and frequency are correct in the
green portion of the color band.
MAINTENANCE
Little maintenance is required other than routine inspection and cleaning. The bearings are pre-lubricated and will be a
long life item unless damage by accident or excessive driving belt tension.
The interior of the alternator should be clean and unobstructed. Slip rings and brushholders should be kept free from dirt,
oil and moisture. If compressed air is available it can be used effectively for cleaning.
