Wound-rotor motors are
invariably started on full voltage but with external resistance in
the secondary circuit. Ordinarily sufficient resistance is provided
to give 100% torque at standstill, which means that 100% current will
be drawn from the line. If a higher torque is required to start the
load, less external resistance must be used, and the current drawn is
proportionately higher.
As the motor accelerates, the external
secondary resistance is short-circuited in one or more steps. The
locked-rotor values in Table 20-8 are generally recognized as the
minimum needed by motor designers to obtain the required torque
characteristics for general-purpose motors.
Squirrel-cage motors with these
values are usually acceptable for full-voltage starting on power
lines and also on combined light and power secondaries of 208 or 230
V, if manually controlled (infrequently started). In the case of
automatically controlled (frequently started) equipment, with 208- or
230-V motors supplied from combined light and power secondaries,
current-reducing starters to reduce the current to about 65% of these
values may be required, unless consultation with the power company
indicates that the available system capacity will permit use of
full-voltage starting. In any case, consultations with the power
company for motor applications above 25 hp are advisable.
Autotransformer starters
(compensators) are the most popular of any reduced-voltage type.
They have the advantage that the ratio of torque developed by the
motor to the current drawn from the line remains substantially the
same as for full-voltage starting.
The motor torque and the current drawn
from the line (neglecting the magnetizing current of the
autotransformer) are both reduced in proportion to the square of the
voltage impressed on the motor. The magnetizing current of the
autotransformer generally does not exceed 25% of motor full-load
current. Normally, the motor accelerates nearly to full speed on the
reduced-voltage connection and is then transferred to full voltage.
Since the circuit to the motor is
opened and then immediately reclosed, a transient inrush of current
occurs which may be of much greater magnitude than the current
normally drawn by the motor at the speed at which the transfer is
made. This transient inrush, however, is of such extremely short
duration that it does not produce an objectionable voltage
disturbance on the average power system.
Standard autotransformer starters are
provided with 65% and 80% voltage taps in sizes up to 50 hp and with
50%, 65%, and 80% voltage taps in the larger sizes. “Part-winding”
starting is being more widely used for reducing starting current.
This involves arranging the stator winding so that, by use of
adequate control devices, one part of the stator winding is first
energized and subsequently the remainder of the winding is energized
in one or more steps.
The purpose is to reduce the initial
values of the starting current drawn and/or the starting torque
developed by the motor. The usual arrangement involves energizing
one-half the stator winding on the first step, resulting in
approximately 50% of normal locked-rotor torque and approximately 60%
of normal locked-rotor current. While this torque may be insufficient
to start the motor in some applications, it permits drawing
full-winding starting current from the system in two increments.
Another method is to connect two-thirds
of the winding on the first step, by using a 4-pole contactor, in
which case the motor should accelerate promptly to full speed. The
remaining third of the winding is then connected by closing a second
contactor with only two poles.
Resistor-type reduced-voltage starters
are sometimes used. They have the disadvantage that the current drawn
from the line is reduced in direct ratio to the impressed voltage,
while the torque developed by the motor is reduced as the square of
this voltage.
The resistor is short-circuited, either
all at once or in steps, when the motor comes up to speed. The
circuit for the motor is not broken in transferring to full voltage,
as is the case with the autotransformer starter. These features make
the resistor-type starter adapted for use where “increment-type”
starting current restrictions exist.
With the resistor-type starter, the
contactors, which short-circuit the resistors as well as the line
contactors, must carry the full current of the motor, whereas in
part-winding starting, the contactors for the two parts of the
winding each carry only half the total current.
Reactor-type reduced-voltage
starters are sometimes used on larger motors, most frequently on
high voltage motors (2300 V or above), where oil circuit breakers are
necessary to provide sufficient current-interrupting capacity. In
such cases, the reactor and starting circuit breaker are placed in
the neutral of the motor. The breaker can then be of low-interrupting
capacity, since the fault current at this point is limited by the
reactance of the motor windings.
Wye-delta starting, though quite
common abroad, is used in the United States primarily for
refrigeration compressors. This starter consists of a switching
arrangement that transfers the motor winding from Y for starting to
delta for running.
The current drawn and the torque
developed by the motor are thus reduced to only one-third their full
voltage values. This very low torque, the extra contactors required,
and the current inrush when the circuit is reclosed on delta make
this scheme less attractive than others.
Motors are frequently supplied from
power systems consisting of complex networks for which calculation of
the voltage drop would be difficult. The voltage drop may be
estimated, however, if the short-circuit kVA is known at the point of
power delivery.
