Page 198 - Industrial Power Engineering and Applications Handbook

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Table 7.3 Maximum vibration levels, as practised by Indian
manufacturers
~ ~ ~
Vibration level, peak to peak
Speed Ns (double amplitude)
rp.m. at 50 HZ LT HT motors
(microns) (microns)
1. 3000 1s 15
2. 1500 40 25
3. Up to 1000 40 25
Note In higher speed ranges and for HT motors, these levels of Figure 7.14 Motor with muffling cone and lining of round felt
shaft vibration are generally of the same order or slightly better wool to absorb the exhaust noise of air
than prescribed in IEC 60034-14, corresponding to Table 11.3.

the fan and fan cover. The basic purpose of all these driving end fan cover to absorb the friction and
is to reduce to a minimum the windage (air friction) intensity of exhaust air.
noise at the suction and exhaust points:
7.1 0 Service factors
By providing a unidirectional axial flow fan
By providing a sound-absorbing fan cover at the
non-driving end, as shown in Figure 7.12 When a motor is expected to operate in unfavourable
By transforming the intake axial air flow to a radial conditions such as:
air flow, as illustrated in Figure 7.13, thus
significantly reducing frictional and hence suction Intermittent overloading
noise Higher ambient temperatures
By changing the geometry of the fan cover, i.e. by A restricted temperature rise as for a spinning mill, a
providing muffling cones (noise-hod) at the driving refinery or a hazardous area
end (Figure 7.14) and providing felt wool on the Frequent starts, stops and reverses
or any such conditions during operation
Non-driving end
and when it is not possible to accurately define their
likely occurrences or magnitudes, it becomes desirable
for the motor to have some in-built reserve capacity. To
account for this, a factor, known as the ‘service factor’,
c is considered when selecting the size of the motor. A
Air in
c ‘service factor’ in the range of 10-15% is considered
adequate by practising engineers. With this service factor,
c no more derating would normally be necessary. See also
Example 7.1 at the end of the chapter.
Figure 7.12 Sound-absorbing fan cover at the non-driving
end
7.1 1 Motors for hazardous locations
Areas prone to or contaminated with explosive gases,
vapours or volatile liquids are at risk from fire or
explosions. A hazardous area is a location where there is
a risk of fire or explosion due to the formation of an
explosive mixture of air and gas or inflammable vapour.
Normal motors may emit sparks or some of their parts
accessible to such environments may reach a temperature
high enough to ignite inflammable surroundings during
normal running. Special motors have thus been developed
for such locations and may be one of the following
types:

1 Flameproof (FLP) or explosionproof type (Ex. ‘d’)
2 Increased safety type (Ex. ‘e’)
3 Pressurized type (Ex. ‘p’)
Figure 7.13 Radial flow of air to reduce suction noise 4 Non-sparking type (Ex. ‘n’)