Page 332 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/31 1
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*MPR: Motor protection relay

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Figure 12.45 Typical schematic and control wiring diagram for power circuit of Figure 12.44

Starting time of the motor for motors having a starting time of not more than 15
Number of consecutive starts seconds. Similar charts from other manufacturers can
Number of equally spaced starts per hour also be obtained for different ratings of fuses and different
Consider the rating of the fuse at about twice the rated starting times of motors.
current for a DOL starting and about 130% for an
A/T starting. The rating will depend upon the I* - t Vacuum contactors
characteristics of the HRC fuses and must ensure that
they remain intact while handling short-duration Normally a vacuum contactor (Figure 12.52) is preferred
excessive current during normal operation, such as to an ordinary air break contactor for the following reasons:
during a start, and clear a fault condition quickly.
Since the contacts make and break in vacuum there is
Current-Time (I2 - t) characteristics no oxidation of the contacts. The contacts thus do not
deteriorate and provide a longer working life.
The typical current-time characteristics for 50-630 A, Dielectric strength of high vacuum is approximately
6.6 kV fuses are shown in Figure 12.49. The cut-off eight times that of air (Figure 12.53).
current characteristics are given in Figure 12.50. The arc energy dissipated in vacuum for a given
interrupting current is one tenth of that in oil (Figure
Method of selection 12.54).
It calls for less maintenance.
The manufacturers of HRC fuses provide a selection Very high contact life, say, over 1.5 million operations,
chart to help a user to select the proper type and size of which is more than enough for lifetime operation.
fuses. For the fuses covered in Figures 12.49 and 12.50 High operating rate suitability, say 1000 or more
we have also provided a selection chart (Figure 12.5 I), operations per hour. This makes them suitable even