Page 150 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

MAINTENANCE OF MOTORS

MAINTENANCE OF MOTORS 8.5

TABLE 8.2 Comparative Voltage Ratings for 60- and 50-Hz Operation*

50-Hz optional voltage ratings ±5%
Nameplate power of 80% of 60-Hz motor 85% of 60-Hz motor
60-Hz motor, V nameplate power, V nameplate power, V
230 190 200
460 380 400
575 475 500

*For 48 through 440T frames, polyphase only. The motors may operate at less than
NEMA torques. Care must be taken in using for hard-to-start and hard-to-accelerate
loads. It is advisable to consult the motor manufacturer when operating at frequencies
other than that stated on the nameplate.

INSULATION OF AC MOTORS

The quality of the maintenance and the insulation system have a significant impact on the
overall service life and performance of a motor. The main factors in increasing the life of
the insulation system are to keep it clean, dry, and cool. The insulation systems (Fig. 8.1)
in motors include the following:
1. Turn insulation. This basic wire coating is an enamel, resin, film, or film-fiber com-
bination used to electrically insulate adjacent wire turns from one another within a coil.
2. Phase insulation. Sheet material is used to insulate between phases at the coil end turns.
3. Ground insulation (slot liners). Sheet material is used to line the stator slots and insulate
the stator winding from stator iron or other structural parts.
4. Midstick (center wedge). Insulation is used in stator slots to separate and insulate coils
from one another within the slot.
5. Topstick (top wedge). This is used to compact and contain the coil wires within the
stator slots.
6. Lead insulation. Insulation materials surround lead wires.
7. Lacing and tape. These are used to tie lead wires in place on the stator end coils. Also
they are used to tie end coils together, adding mechanical strength to the end coils and
restricting their movement.
8. Varnish. Varnish treatment is employed to increase the resistance of the completely
wound stator to environmental attack of chemicals, moisture, etc. The varnish treatment
bonds the coils, connections, wedges, and stator iron into an integrated structure and
improves the components’ resistance to electrical and mechanical damage.
In modern motors rated above 500 hp, a vacuum pressure impregnation (VPI) system is
used for the insulation. The benefits are a void-free sealed insulation system with high mechan-
ical strength and thermal conductivity. The ground wall insulation of this system always
contains some form of mica tape. The varnishes used in normal atmospheric dipping are sub-
stituted by a 100 percent solids resins. The stator is placed in a large vacuum pressure tank.
Resins are impregnated into the stator and windings by alternately applying vacuum and pres-
sure while the stator is fully immersed in resin. The resins are made of polyester, or epoxy.
However, epoxy provides greater strength and higher resistance to abrasion or chemicals such
as acids and alkalies. The VPI provides a better seal in wet and corrosive atmospheres. The
absence of voids reduces the losses due to corona (partial discharge) at higher voltages.

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