Page 160 - Industrial Power Engineering and Applications Handbook

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R Y B torque, capable of picking up the load. It is also possible
that it may need a prolonged starting time not
commensurate with the thermal withstand time of the
motor or a larger starter. In such cases, it is essential to
have a minimum base or pedestal voltage as illustrated
in Figure 6.45(a). The voltage is adjusted to the lowest
possible level, so that the I,, is kept as low as possible. In
Figure 6.45(b) we illustrate a motor with a normal starting
current of 650% I, at the rated voltage. To limit this to a
maximum of 350% of I,, we have provided a base voltage
of about 350/650 or 54% of the rated voltage. The
minimum T,, is, however, matched with the load
requirement to attain the rated speed within its thermal
withstand time. For more details see Section 3.5 and
Example 7.1. The voltage is then raised so that during
the pick-up period the I,, is maintained constant at 350%,
0 until the motor reaches its rated speed.
Since it is not practical to custom build each drive, the
normal practice by manufacturers of soft starters is to
Rotor
provide a variety of motor parameters to which nearly
all motors will fit and the user may select the motor that
000
Variable V, constant fcontrol best suits the load requirements. The other advantages of
a solid-state soft start are that an unbalanced power supply
Figure 6.44 Stator static voltage control (soft starting) is transformed to a balanced source of supply automatically
by suitably adjusting the firing angle of each SCR or
triac through their switching logistics. Also a low starting
voltage to the required level. There is thus a controlled current can economize on the size of switchgears, cables
acceleration. The maximum I,, can also be regulated to a and generator where a captive power is feeding the load.
required level in most cases, except where the T,, require-
ment is high and also needs a higher starting voltage, 6.16.2 Soft stopping
when it may not be possible to limit the I,, to the required
level. The scheme is a simple reduced voltage start and The normal method to stop a motor is to do it instantly.
does not facilitate any speed control. Since the starting This is accepted practice in most cases and the machine
current can now be regulated to a considerable extent, the stops as shown in equation (6.8) discussed later. A higher
scheme is termed a soft start. Now T,, =V2 and I,, - V. moment of inertia (MI) of the driven masses will mean a
This may be a good alternative to Y/A or autotransformer reasonably longer duration to come to a standstill, while
starting by eliminating current overshoots during the a low MI will mean a faster stop. But in loads requiring
changeover from Y to A in a YlA or from one step to high braking torques, such as conveyor systems, escalators
another in an autotransformer start. It also prevents an and hoists, the stopping time may be too short. In some
open transient condition (Section 4.2.2(a)). Also in a cases, such as a pump duty, the stoppage may be near-
soft start the switching SCRs or the triacs are bypassed abrupt. Such a situation is not desirable and may cause
through a contactor after performing a starting duty similar shocks to the motor. In a pump, an abrupt stoppage may
to a YlA, AIT or a wound rotor start. In conventional cause severe shocks and hammering effects on pipelines,
startings also the star contactor, the autotransformer or due to backflow of the fluid, Shocks may even burst
the external resistances, respectively, are cut off from pipelines or reduce their life when such stoppages are
the circuit, after performing their starting duty. Such a frequent. They may also damage non-return valves and
provision protects the starter unit from avoidable voltage other components fitted on the lines and thus weaken the
strain, saves heat losses and cools it to prepare it for the whole hydraulic system.
next switching operation. The starter is also free to perform A gradually reducing voltage rather than an instant
switching duties on other machines of a similar size switch OFF is therefore desirable for all such applications.
connected on the same line, if so desired, to save costs. This would also gradually reduce the flow of the fluid,
The starter can be used as a switching kit for a group of leading to a strain-free and smooth stopping of the
similar-sized motors. machine. In such cases, a soft stop feature, similar to a
The voltage can be varied from 0% to 100% and hence soft start, can be introduced into the same starter, which
the ZSt can be limited to any required level. Such a linear would gradually reduce the stator voltage and facilitate
voltage variation may be suitable in most cases, parti- a smooth and shock-free stop.
cularly when the motor has to pick up a light load or is
at no-load. Motors driving heavy inertia loads or loads 6.16.3 Slip recovery system (to control wound
requiring a high starting torque or both may not be able rotor motors)
to pick up smoothly with a linear voltage rise from 0
volts. This may cause a locked rotor or stalling until the As discussed earlier, the motor speed-torque
voltage rises to a level sufficient to develop an accelerating characteristics depend largely upon the rotor current