AI-2
DRUM-TYPE ARMATURE—An efficient, popular type of armature designed so that the entire length
of the winding is cutting the field at all times. Most wound armatures are of this type.
EDDY CURRENTS—Currents induced in the body of a conducting mass by a variation in magnetic
flux.
FIELD—The electromagnet that furnishes the magnetic field that interacts with the armature in motors
and generators.
FIELD EXCITATION—The creation of a steady magnetic field within the field windings by applying a
dc voltage either from the generator itself or from an external source.
GENERATOR—A machine that converts mechanical energy to electrical energy by applying the
principal of magnetic induction. A machine that produces ac or dc voltage, depending on the original
design.
GRAMME-RING ARMATURE—An inefficient type of armature winding in which many of the turns
are shielded from the field by its own iron ring.
INDUCTION MOTOR—A simple, rugged, ac motor with desirable characteristics. The rotor is
energized by transformer action (induction) from the stator. More induction motors are used than any
other type.
INTERPOLES—Small auxiliary poles placed between main field poles, whose magnetic field opposes
the armature field and cancels armature reaction. Interpoles accomplish the same thing as
compensating windings.
LAP WINDING—An armature winding in which opposite ends of each coil are connected to adjoining
segments of the commutator so that the windings overlap.
LEFT-HAND RULE FOR GENERATORS—A representation of the relationships between motion,
magnetic force, and resultant current in the generation of a voltage. The thumb, forefinger, and
middle finger of the left hand are extended at right angles to each other. The thumb should point in
the direction the conductor moves. The forefinger should point in the direction of magnetic flux from
north to south. The middle finger will then point in the direction the generated voltage forces current
to flow. Any of three quantities may be found if the other two are known.
MAGNETIC INDUCTION—The generation of a voltage in a circuit by causing relative motion
between a magnetic field and the circuit. The relative motion can be the result of physical movement
or the rise and fall of a magnetic field created by a changing current.
MOTOR—A machine that converts electrical energy to mechanical energy. It is activated by ac or dc
voltage, depending on the design.
MOTOR LOAD—Any device driven by a motor. Typical loads are drills, saws, water pumps, rotating
antennas, generators, etc. The speed and power capabilities of a motor must be matched to the speed
and power requirements of the motor load.
MOTOR REACTION—The force created by generator armature current that tends to oppose normal
rotation of the armature.
MOTOR STARTERS—Large resistive devices placed in series with dc motor armatures to prevent the
armature from drawing excessive current until armature speed develops counter emf. The resistance
is gradually removed from the circuit either automatically or manually as motor speed increases.
