Page 90 - Industrial Power Engineering and Applications Handbook

Starting of squirrel cage induction motors 4/71
An induction motor can be regarded as a transformer LT side thus eliminating the cause of a line disturbance
with a small air gap in the magnetic circuit. When at due to a voltage dip. On the HT side, the effect of a
rest. having no induced back e.m.f., it can be regarded as voltage dip, caused by small motors, is of little
a transformer with a short-circuited secondary. A squirrel significance. However, the method of DOL switching
cage induction motor therefore draws a very high starting for larger motors is recommended only where the supply
current, as noted in Section 1.2.1. However, it reduces source has enough capacity to feed the starting kVA of
substantially in a slip-ring motor due to the high impedance the motor, with a voltage dip of not more than 59 on the
of the rotor circuit. LT side.
The starting of an induction motor does not relate to For a large LT motor, say, 300 h.p. and above, there is
simple switching alone. It also involves its switchgears no economical alternative other than DOL starting. One
to control its starting inrush current, starting torque, or can, however, employ delayed action coupling (Section
both, and its overload and short-circuit protection. 8.3) with DOL starting to start the motor lightly and
The following are common methods to start a squirrel quickly. (See also Example 7.1, in Chapter 7 for more
cage motor, depending upon the limitation, if any, on the clarity.). Soft starting through a solid-state device (Section
magnitude of switching current, I,,. 6.16.1) or a liquid electrolyte (Section 4.2.3) are costly
propositions. Autotransformer starting is also expensive
due to the cost of its incoming and outgoing control
4.1 Direct on-line starting (DOL) gears and the cost of the autotransformer itself. This is
also the case with Y/A starting. Moreover, AIT and Y/A
This is an ideal type of starting, and is simple and startings are reduced voltage startings and influence the
economical. In this type the full voltage is applied across starting torque of the motor. It is possible that the reduced
the stator windings. The torque developed by the motor starting torque may not be adequate to drive the load
is maximum. The acceleration is fast and the heat of successfully and within the thermal withstand time of
starting is low (Figure 4. I (a)). For heavy rotating masses, the motor. Unlike in smaller ratings, where it is easy and
with large moments of inertia, this is an ideal switching economical to obtain a high ir;, characteristic, in large
method. The only limitation is the initial heavy inrush motors, achieving a high T,,, say, 200% and above, may
current, which may cause severe voltage disturbances to be difficult and uneconomical. See Section 2.2.
nearby feeders (due to a large I,, . Z drop). With this in With the availability of large contactors up to 1000 A
mind, even local electricity authorities sometimes restrict and breakers up to 6400 A, DOL starting can be used for
the use of DOL starting beyond a certain rating, say, IO LT motors of any size, say, up to 1000 h.p. The use of
h.p., for small installations. For large installations, this such large LT motors is, however, very rare, and is
condition may be of little significance as, in most cases, generally not recommended. Since large electrical
the power from the electricity authorities may be available installations are normally fed from an HT network,
on HT 3.3-33 kV. When it is so, a transformer is provided whether it is an industry, residential housing, an office
at the receiving end for the distribution of power on the or a commercial complex, DOL switching. even when

11 i_

t

c
f
3
0
I, r-- -

h2
h3
______ 3 On Off
Speed + N, trip
(a) Torque and current characteristics (b) Power and control circuit diagram
Figure 4.1 Direct on-line starting