Page 93 - Industrial Power Engineering and Applications Handbook

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Notes
(1) C, and C, switches or contactors are interlocked among themselves, so that only one is ON at time.
(2) C, is essential to isolate the transformer after performing the starting duty, to eliminate avoidable transformer
u losses. Moreover the transformer is only short-time rated.
0.4K
Figure 4.4 Auto-transformer starting (or 40% tapping)

kVA(,,,, = continuous rating of the autotransformer.
.
Tapping = 45 100% (4.1)
Since the transformer will be in the circuit for only 15
From this equation, the desired tapping of the auto- to 20 seconds, the approximate short-time rating of the
transformer, to limit the starting current to a desired value transformer can be considered to be 10-15% of its
can be determined. continuous rating. The manufacturer of the auto
transformer would be a better judge to suggest the most
appropriate rating of the transformer, based on the tapping
Example 4.1 and starting period of the motor.
A squirrel cage motor has its lSi on DOL as six times its rated
current. Find the required tapping on an autotransformer to
limit the starting current to 1.5 times.
Rating of contactors
Star contactor C, and AIT contactor C2 must be rated for
the square of the percentage tapping. For a tapping of
or 50% 80%, for instance, the rating of the contactors Cl and C2
will be (0.Q2 or 64% of the full-load current of the motor.
Rating of autotransformer The main contactor C,, however, will be rated for the
full-load current.
Since an autotransformer is in the circuit for only a short For power and control circuit diagrams refer to Figure
period (during the start only), it can be short-time rated. 4.4.
The rating of the autotransformer can be calculated from
kVA (CMR, = 43 . kV ' I,, (4.2) Example 4.2
For a 3.3 kV 450 kW motor, with a full-load current of 100 A
where and starting current on DOL as six times the rated current,
kV = applied voltage, and the kVA rating of the transformer for 50% tapping will be