Page 94 - Industrial Power Engineering and Applications Handbook

Starting of squirrel cage induction motors 4/75
/AT = 0.!j2 x 100 x 6 = 150 A from equation (4.1) transients, in addition to causing current transients. The
- current transients may far exceed even 14-20 times the
:. kVA(CMR) = 43 x 3.3 x 150 rated current of the motor, as illustrated in Figure 4.5,
depending upon the transient recovery voltage (TRV)
= 860 kVA (Section 17.6.2). We will describe the effect of an open
An autotransformer of nearly 100 kVA continuously-rated transient condition on an LT and HT machine separately.
should be sufficient for this application.
1 LT motors In LT motors, such a situation may not
Note The above example is only for a general reference. be a matter of concern as no switching surges would
The CDF of the transformer, for the short-time rating, should
be increased with the starting time and the number of starts generally occur. The voltage would be too low to
per hour. Refer to the transformer manufacturer for a more cause a re-strike between the interrupting contacts of
appropriate selection. the contactors (Section 17.7.6) and cause surges. The
motor's own induced e.m.f. may, however, fall phase
4.2.2(a) Open transient condition during a apart with the applied voltage and the voltage across
reduced-voltage switching sequence the motor windings may double. In all likelihood the
windings of a motor would be suitable to withstand
Whenever a changeover of a switching device (contactors effect of the same (Table 1 1.4). In locations, however,
generally) from one condition to another takes place, as that are humid or chemically contaminated, or where
discussed above, in changing over from one tapping to the motor is likely to be switched on after long gaps,
another, as in an autotransformer switching, or from star it is possible that the windings may have attained a
to delta as in a Y/A switching, there appears a small time low dielectric strength to withstand a voltage up to
gap of, say, 20-80 ms before the next contactor closes, twice the rated one. Open transient conditions must
after the first has dropped. During this period, while the be avoided in all such cases.
machine will drop its speed only very marginally, and 2 HT Motors Y/A switching in HT motors is rare,
which may not influence the load, the power from across while an A/T switching may become necessary when
the motor terminals will cease for this duration (except the capacity of the feeding transformer is not adequate
its own induced e.m.f.). This time gap, when switching to withstand the start-up inrush of DOL switching, or
HT motors particularly, may cause switching transients when the drive calls for a frequent switching, such as
and prove disastrous for the motor windings, as discussed in a large compressor or pump house, and the feeding
in Section 17.7.2. This is termed an open transient transformer is not adequate for such a duty, or when
condition. The situation is aggravated further because of a number of large drives are to be switched in quick
the motor's own induced e.m.f., which may fall phase succession and the feeding transformer may not be
apart with the applied voltage and magnify the voltage adequate to sustain such heavy inrush currents.

Note
ob = Peak up to twice (= 14/J the actual /(A) current
od (it may even exceed 14-201,)
ob, = No current transient during a closed transient
switching
c'c = 1 to 2 ms. (opening + closing time of Y and 3
ISt(A) contactors respectively)
Explanation:
A. (i) ob is the current transient during changeover
from Y to A in an open transient condition.
(ii) Voltage transient across the windings: 2.45 to
\ \ 4.1 V,
T \ B. Sequence of a closed transient changeover,
(i) When the bridging resistor is introduced in
c
parallel to Y windings at point, a, the total
E
a impedance of the windings gets reduced and
current has small overshoot to ob compared
to its normal current, oa.
(ii)There is no voltage transient now and the
voltage across the windings remains at V,.
lst(Y) (iii) The Y contactor drops at bl, the impedance of
the windings becomes high, current drops to
9.
.. (iv) The bridging resistor drops at b3 and the motor
$3 \ \ current traces back its normal A current curve
Speed - 1
~1 d.
0 0 J8
Figure 4.5 Open and closed transient conditions in YlA switching