Page 91 - Industrial Power Engineering and Applications Handbook

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large LT motors are used, may not pose a problem or
cause any significant disturbance in the HT distribution
network. It is, however, advisable to employ only HT
motors, in large ratings.
HT motors have little alternative to be switched other
than a DOL or a soft starting. Y/A switching in an HT
motor is neither advisable nor possible for its windings
are normally wound in a star formation to reduce the
winding's design voltage and economize on the cost of
insulation. Autotransformer switching is possible, but
not used, to avoid a condition of open transient during a (a) Line current in case of Y connected windings
changeover from one step to another, as discussed in
Section 4.2.2(a) and for economic reasons as noted above.
Moreover, on a DOL in HT, the starting inrush current is
not very high as a result of low full-load current. For
example, a 300 h.p. LT motor having a rated full-load
current (FLC) of 415 A on a DOL will have a starting
inrush of approximately 2500 A, whereas a 3.3 kV motor
will have an FLC of only 45 A and starting inrush on a
DOL of only 275 A or so. An HT motor of 3.3, 6.6 or
1 1 kV will thus create no disturbance to the HT distribution
network on DOL starting. I-
For power and control circuit diagrams refer to Figure
4.1 (b).
-_
1
4.2 Reduced voltage starting -3
or I, = 1 I,,
When the power distribution network is available on LT, (b) Line current IE2 in case of A connected windings
and the motors are connected on such a system, it becomes Figure 4.2
dcsirable to reduce the starting inrush currents, for motors
beyond a certain rating, say, 10 h.p., to avoid a large dip
in the system voltage and an adverse influence on other the motor has run to almost its full speed. Otherwise it
loads connected on the same system. Sometimes it may may again give almost a full kick as on a DOL and
also be a statutory requirement of the local electricity defeat the purpose of employing a staddelta switching.
authorities for a consumer to limit the starting inrush of Figure 4.3(a) explains this.
motor currents beyond a certain h.p. to protect the system
from disturbances. This requirement can be fulfilled by Limitations
adopting a reduced voltage starting.
Sometimes the load itself may call for a soft start and 1 Due to the greatly reduced starting torque of the motor,
a smoother acceleration and a reduced voltage switching the accelerating torque also reduces even more sharply
may become essential. and severely (Figure 4.3(a)). Even if this reduced
A few common methods to achieve a reduced voltage accelerating torque is adequate to accelerate the load,
start are described below. it may take far too long to attain the rated speed. It
may even exceed the thermal withstand capacity of
4.2.1 Staddelta (Y/A) starting the motor and be detrimental to the life of the motor.
One considers this aspect when selecting this type of
The use of this starting aims at limiting the starting inrush switching.
current, which is now only one third that of DOL starting. To achieve the required performance, it is essential
This is explained in Figure 4.2. This type of starting is, that at every point on the motor speed-torque curve
however, suitable for only light loads, in view of a lower the minimum available accelcrating torque is 15-20%
torque, now developed by the motor, which is also one of its rated torque. In addition, the starting time must
third that of a DOL. If load conditions are severe, it is also be less than the thermal withstand time of the
likely that at certain points on the speed-torque curve of motor. For more details see Section 2.8.
the motor the torque available may fall short of the load 2 This type of switching is limited to only LT system
torque and the motor may stall. See the curves in Figure for HT motors are normally wound in star. However,
4.3(a), showing the variation in current and torque in the in special cases, HT motors can also be designed for
star and delta positions and the severe reduction in the delta at a higher cost to the insulation system which
accelerating torque. may also call for a larger frame size. Also a provision
When employing such a switching method precautions can be made in the switching device to avoid the
should be taken or provision made in the starter to ensure condition of an open transient during the changeover
that the windings are switched ON to delta only when from Y to A as discussed in Section 4.2.2(b).