Page 942 - Industrial Power Engineering and Applications Handbook

28/892 Industrial Power Engineering and Applications Handbook
Material of enclosure - aluminium As in Table 28.6, by simple interpolation, for condition of
Busbar configuration - as Figure 28.33a location against serial number 2, derating = 0.77
Busbar support made of SMC or DMC (Section 13.6.1). Due to skin effect
Distance between two busbar supports: 400 mm (Figure Nil, as it is already considered in Tables 30.2, 30.4 and
28.33(b)) 30.5, while establishing the basic ratings of the busbars.
Due to proximity effect
(ii) Data available Approximately 20%, since S < 2b. It is recommended to
Aluminium busbars, from Tables 30.4 and 30.6 have the centre spacing S at least 2 x 152.4, Le. 305 mm.
Electrical conductivity = 1 If the width of the enclosure poses a limitation, a more
Minimum tensile strength = 2050 kgf/cm2 (Table 30.1) appropriate configuration such as in Figure 28.34 or the
Minimum cross-breaking or yield strength = 1650 kgfkm’ technique of interleaving as in Figure 28.35 may be adopted
(Table 30.1) to achieve better utilization of the active metal. In our
Busbar supports: from Table 13.1 4 calculations we have considered all these alternatives.
Due to voltage variation
We have already discussed the impact of voltage variation
on an industrial drive in Section 1.6.2(A-iii). The impact of
this on a bus system may not be the same. A bus may
Mechanical properties DMC SMC have to supply lighting, heating and other resistive or
in kgf/cn? in kgf/cn?
inductive loads. All such loads except electric motors will
perform low at lower voltages and hence draw a lower
Minimum tensile or shearing 250-500 500-900 current. Generally, we can assume that the loading on a
strength bus, as enhanced by the industrial drives, will be almost
Minimum compressive 1200-1 800 1600-2000 offset by the decrease of loading by the other loads if we
strength assume industrial drives to be up to 50% of the total
connected load. Usually therefore no derating will be
Minimum cross-breaking or 700-1200 1400-1 800
flexural strength (bending necessary for a lower voltage, except for large installations
strength) where the drives may constitute the bulk of the load.
Consideration of voltage variation will therefore depend
upon the type of installation. In our calculations, however,
Hardware we are ignoring the impact of this.
Frequency variation
At higher frequencies up to 3% skin and proximity effects
Mechanical properties High tensile (HT) Ordinary MS would be slightly higher, but can be ignored as their impact
fasteners as in (mild steel) will be only marginal.
IS0 4014 fasteners as Considering all these deratings the total derating for the
and IS0 898, in IS0 4016, configuration of Figure 28.33(a) or (b) will be
grade 8.8 grade 4.6
= 0.815 x 1 x 1 x 0.77 x 0.8
Minimum tensile = 0.50
strength 8000 kgf/cm2 4000 kgf/cm2
Minimum cross-breaking Basic current rating of 4 x 152.4 mm x 6.35 mm aluminium
or yield strength 4400 kgflcm’ 2200 kgf/cm2 busbars per phase as in Table 30.4
= 4240 A
(Iii) Deriving the actual current rating :. Effective rating after considering all possible deratings
Applicable deratings: = 4240 x 0.5
= 2120 A
Due to higher ambient temperature
For 50°C as in Table 28.3 and Figure 28.10 = 0.815 These sections of busbars are not adequate for the required
Due to altitude current rating of 2500 A. The rating of the bus system can,
Nil, since the installation of the equipment is assumed to however, be improved by almost 20% and make it suitable
be within 2000 m above the mean sea level for the required rating by providing the busbars and the
Due to grade of busbars inside of the enclosure with a nonmetallic, matt finish
For E-91 E or its equivalent, as in Table 30.6 = 1 .O black paint. If voltage variation is also to be considered,
Due to size of enclosure and environmental conditions of then this bus may not be suitable for the required duty
location even after painting.
The Enclosure is of nonmagnetic material, therefore it will
be devoid of hysteresis and eddy current losses. (iv) Voltage drop
Heat dissipation factor Rdc for E-91 E grade of conductor, from Table 30.7
- Cross-sectional area of active aluminium = 32.38 pWm per conductor at 20°C
Area of enclosure
:. Rdc at an operating temperature of 85°C
(3 x 4 x 152.4 x 6.35) + (2 x 152.4 x 6.35)
i.e. = Rdc20 [l + a20 (02 - el)]
380 x 870 = 32.38 [l + 0.00363 (85 - 20)]
- 14 x 152.4 x 6.35
380 x 870 = 32.38 (1 + 0.236)
= 0.041 or Rdc = 40.02 pR/m per conductor and
or 4% for the phase = ~ 40.02 or 10.005 x Q/1000 m
4