periodically for evidence of loose or fractured bars and
equipment and proper cleaning practices to preserve the
localized overheating.
integrity of its insulation. Periodic testing and
inspection of the various electrical components of
Wound rotors (fig. 7-26) consist of wound coils
rotating electrical machinery will help to prevent
insulated from each other and laid in slots in the rotor
catastrophic failure of the equipment and leads to
core. These coils are wye-connected and terminate at
reduced overall maintenance cost of the equipment.
three slip rings.
AC MOTORS
Wound rotors, like other windings, require periodic
inspections, tests, and cleaning. The insulation
AC motors comprise the majority of rotating
resistance determines if grounds are present. An open
equipment to be maintained by the EM aboard ship. Of
circuit in a wound rotor may cause reduced torque
the three general classes of ac motors in use today
accompanied by a growling noise, or failure to start
(polyphase induction, polyphase synchronous, and
under load. Besides reduced torque, a short circuit in
single-phase) the polyphase induction motor is, by far,
the rotor windings may cause excessive vibration,
the most common used aboard ship. These motors are
sparking at the brushes, and uneven collector ring wear.
well suited to shipboard use and can be counted on to
With the brushes removed from the collector rings, a
operate for very long periods of time if maintained
continuity check of the rotor coils will reveal the
according to prescribed PMS procedures.
presence of a faulty coil.
Rotors
Stator Coils
Basically, the rotor of an ac motor is a squirrel-cage
rotor or a wound rotor. The squirrel-cage rotor usually
The ac stator windings require the same careful
has heavy copper or aluminum bars fitted into slots in
attention as other electrical windings. For a machine to
the rotor frame. These bars are connected to
function properly, the stator windings must be free from
short-circuiting end rings by being cast, brazed, or
grounds, short circuits, and open circuits.
welded together (fig. 7-25). In many cases, the cage
A short circuit in the stator of an ac machine will
rotor is manufactured by die-casting the rotor bars, end
produce smoke, flame, or an odor of charred insulation.
rings, and cooling fans into one piece. The cage rotor
The machine must be secured immediately and tests
requires less attention than the wound rotor since it is
conducted to find the reason for the abnormal condition.
less susceptible to damage. The cage rotor, however,
should be kept clean, and the rotor bars must be checked
The first and easiest test that you should conduct is
to test the insulation resistance of the winding. This test
is made with a megohmmeter or similar
resistance-measuring instrument. Connect one
instrument lead to ground and the other to each motor
lead; crank the meter handle and read the scale on the
meter face. If the insulation resistance is within the
Figure 7-26.--Wound rotor.
Figure 7-25.--Squirrel-cage rotor.
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