Page 146 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 8
MAINTENANCE OF MOTORS

An electric motor that has been properly maintained will contribute greatly to the continuous
operation and success of a plant. Since the cost of unscheduled downtime is high, regular
maintenance of motors has become one of the highest priorities in plants. A loss of a motor
could result in reduction in reliability of a system, impairment of a system, or shutdown of
a plant.

CHARACTERISTICS OF MOTORS

The nameplate data include the rated horsepower, speed, voltage, and service factor if
different from 1.0. The service factor is a multiplier that can be applied to the horsepower
of motors that are designed to handle periodic overloading.

ENCLOSURES AND COOLING METHODS

The National Electric Manufacturers Association (NEMA) classifies motors according to
environmental protection and methods of cooling. The available categories include the
following:
Open dripproof. Motor has ventilating openings constructed so that operation will not
be affected by solid or liquid particles that strike or enter the enclosure at any angle up
to 15° downward from the vertical.
Dripproof guarded. These self-ventilating motors feature louvered covers on the
sides with grilles on bottom openings to prevent accidental exposure to live metal or
rotating parts.
Dripproof forced ventilated. Dripproof guarded motors with forced ventilation are
provided by a motor-mounted blower driven by a three-phase ac motor.
Dripproof separately ventilated. These motors provide high horsepower ratings with
separate ventilation supplied by the customer.
Totally enclosed. Motors that operate in severe environments require a totally
enclosed frame. These motors are nonventilated, fan-cooled, or dual-cooled, depending
on the horsepower.
Totally enclosed, nonventilated. Motors are not equipped for cooling by means exter-
nal to the enclosing parts, generally limited to low horsepower ratings or short-time
rated machines.

8.1
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.