Page 322 - Industrial Power Engineering and Applications Handbook

Protection of electric motors 12/301
instantaneous
setting 850%

TWO consecutive/
hot starts
I, 100

38 40
Time (seconds) +
Figure 12.33 Plotting of starting current and thermal curves

Use a CT of ratio 15011A. Step II: Selection of HRC fuses
I, = 123.2 A
Note If a power capacitor is connected after the relay, say
across the motor terminals, to improve the p.f. of the machine, I,, = 750% of 1,
then only the corrected value of the current must be used.
For example, if a capacitor bank of 130 kVAr is used for an Le. 924 A
individual correction of the machine then fs = 19.0 seconds
130
I, = I Consider four equally spread starts/hour. From the selection
~'3 2.64 chart in Figure 12.34, select the characteristics B-6 and
x
= 28.43 A determine the fuse rating as 350 A on the ordinate
corresponding to a starting current of 924 A on the abscissa.
where, /c = capacitor current
Since at 0.85 p.f. = 123.2 A Step 111: Selection of bimetallic overcurrent relay
Consider the available setting range as
I, (active) = 123.2 x 0 85
(a) Overcurrent: 80-125%
= 104.72 A (b) Instantaneous unit: 800-1 400%
and /r (reactive) = 123.2 x 0.527
The thermal curve of the motor does not show any significant
= 64.9 A overload capacity and therefore the relay must be set as
Net reactive current = 64.9 - 28.43
:_ close to the full-load current as possible, say at a setting of
11 0%. Then
= 36.47 A
Overcurrent relay tapping = l.' 123.2 (relay rating: 1 A)
and I, (corrected) that the relay will detect 1.05 x 150
= b!(104.72' + 36.47') = 86% Of 1 A
= 110.9 A Note The factor 1.05 (typical) is the pick-up current of the
relay.
All further calculations must be based on this current.
Say the relay is set at 85%. Then it will operate at 85/86 x 11 0,
i.e. at 108.7% of I,, which should be appropriate and the
instantaneous setting
104.72A = 1.087 x 750 = 815.25%
say, 815%
This setting is only a calculation. The exact thermal curves of
the motor and the relay should be available to closely match
their characteristics at every point. An ideal relay in this case
1
sin @ = 0 527 would be one which, without tripping, will permit two
64.9A consecutive hot starts, i.e. its characteristic should fie above