Quantcast Chapter 4 Electric Motors

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
4-1. Maintenance of electric motors.
goggles, gloves, aprons and respirators should be
This information is intended to aid personnel con-
worn when working with hazardous substances
cerned with the maintenance of electric motors. The
(chap 11).
data provided cover all DC and AC motors. The
c. Great care should be exercised in selecting sol-
recommendations are general in nature and nor-
vents to be used for a particular task.
mally can be applied to the type of motors found on
d. Adequate ventilation must be provided to
military installations. They are not intended to
avoid fire, explosion, and health hazards where
cover in detail the specialized applications occasion-
cleaning solvents are used.
ally encountered. For such cases, the manufac-
e. A metal nozzle used for spraying flammable
turer's instructions should be followed. Periodic in-
solvents should be bonded to the supply drum, and
spection and regularly scheduled preventive main-
to the equipment being sprayed.
tenance checks and services will enhance continu-
f. After tests have been made, discharge stored
ous operation of the equipment without undue
energy from windings by proper grounding before
breakdowns. Frequency of these inspections often
handling test leads.
depends upon elements such as the criticality of the
4-2. Alternating current (AC) motors.
service, hours equipment is normally in service, and
environment under which the equipment operates.
AC motors should, with reasonable care, give long
In order to ensure an accurate data base from which
continuous service. However, there is a tendency to
an effective maintenance program can be initiated,
neglect motor maintenance and, as a result, motor
a complete listing of machines in operation, the
failures are frequent and repairs may become a
functions they perform and past history of operation
continuous and costly process. It is therefore recom-
and maintenance services must be maintained. Mo-
mended that a preventive maintenance program be
tor inspection and scheduled maintenance in the Air
established to minimize emergency breakdowns.
Force is performed by the work center responsible
The program should be supported with an effective
for the system (HVAC, sewer plant, water plant,
spare parts stock to speed up any unscheduled out-
etc.). Preventive maintenance will generally involve
ages that may occur.
a. Squirrel-cage induction unit. This AC motor is
lubrication, cleaning and checking for sparking
brushes, vibration, loose belts, high temperature,
the most prevalent in use at military installations
(fig 4-l). The squirrel-cage motor is the most rug-
and unusual noises. Repair work on larger motors is
ged and the least expensive of all types of induction
normally limited to replacement or refinishing of
motors. The squirrel-cage motor is nearly a constant
bearings, commutators, collector rings, brushes, etc.
speed machine. Typically its speed varies 05 per-
Motor rewinding should not be attempted by the
cent from synchronous speed from no load to full
installation support groups (Directorate of Engi-
load. The basic design of the rotor can be modified to
neering and Housing, Public Works or Civil Engi-
provide a limited degree of external speed control.
neer Shops) since it is more economical to contract
b. Wound-rotor induction unit. This AC motor has
such work to commercial shops that specialize in
connected to its collector rings the insulated phase
motor rewinding. With regard to the many thou-
windings on the rotor (fig 4-2). Through stationary
sands of fractional horsepower motors in operation
brushes in contact with the collector rings, any de-
throughout the military services, it may be more
sired value of external resistance may be added to
economical to replace a motor than to attempt to
repair it. The local electrical supervisor must make
the secondary (rotor) winding to give greater speed
this determination. Table 4-1 can be consulted to
control of the motor. Also, use of external resistance
aid in the selection of proper replacement motors.
allows the motor to deliver a high starting torque
with a relatively small inrush current.
Special consideration should also be given to high-
efficiency motors since they save both energy and
c. Synchronous unit. This type of AC motor (fig
money throughout the life of the motor. The follow-
43) has an insulated winding in both the rotor and
ing safety precautions should be observed when
the stator. A variable source of DC excitation is
working on electric motors:
supplied to the rotor winding and an AC line source
a. Make sure the machine is de-energized, tagged
is supplied to the stator winding. A synchronous
and locked out before starting work (para 122).
motor is a constant speed machine with similar


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