Automation Direct Stellar SR55 Series, User Manual

Page: 184 / 202

Page B–12 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Appendix B: Soft Starter Application Considerations
m
eThod
b: W
ye
-d
elTa and oTher
r
eduCed
v
olTage
s
TarTing
s
ysTems
(
ConTinued
)
There are methods of control, for example, the Wauchope starter, which eliminate or reduce
the reconnection transients. However, such starters are expensive and have reliability
implications, and they are not widely applied for these reasons.
The wye-delta starter also has disadvantages due to the restricted starting torque available
(if you need 40% LRT to break-away, you can only increase the motor size, or revert to
direct-on-line). Combined with the severe effects of the re-switching surges, and the
additional costs of bringing six conductors from the motor to the starter instead of only three;
wye-delta only offers an imperfect solution to the problem of starting the induction motor.
m
eThod
C: p
rimary
r
esisTanCe
s
TarTer
It has long been recognized that the transition step in the wye-delta system was a source of
problems such as welded contactors, sheared drive shafts etc. For many years a method of
stepless control has been available in the form of the primary resistance starter.
This type of controller inserts a resistance in one, or more often in each, of the phase
connections to the stator at start-up, after which it is progressively reduced and shorted out at
the end of the acceleration process. Frequently, the resistances are movable blades that are
gradually inserted into an electrolyte liquid. The mechanism is usually large and expensive,
both to purchase and to maintain, and considerable heat is created by the passage of current
through the electrolyte resistor. This limits the starting frequency (because the electrolyte
has to condense back to liquid before a new start can proceed), and these restrictions prevent
this starter from being a popular option when selecting a control system. However, it has the
distinction of being the smoothest and least stressful method of accelerating an induction
motor and its load.
m
eThod
d: o
Ther
e
leCTro
-m
eChaniCal
s
ysTems
Other control methods such as auto-transformer starting (popular in North America), primary
reactance starting etc., are employed to a greater or lesser extent, to compensate for some of
the disadvantages of each type of starter discussed. Nevertheless, the fundamental problems
of electro-mechanical starters remain, and it is only in the last decade or two that their
dominance has been challenged by the introduction of power semiconductors controlled by
electronics.
b.3.5 – T
he
s
emiConduCTor
m
oTor
C
onTroller
During the 1950s, much effort was put into the development of a four-layer transistor device which
had the power to switch large currents at high voltages when triggered by a very small pulse of
current. This device became known as the silicon controlled rectifier (SCR), or in Europe, the
“Thyristor”, and it is the basis on which all soft starting systems are built. The characteristic of
most interest is the ability of the thyristor to switch rapidly (in about 5 millionths of a second) from
“OFF” to “ON” when pulsed, and to remain “ON” until the current through the device falls to zero
(which conveniently happens at the end of each half-cycle in alternating current supplies).
By controlling the switch-on point of a thyristor relative to the voltage zero crossing in each
half wave of an alternating current, it is possible to regulate the energy passing through the
device. The closer the turn-on point is to the voltage zero crossing point, the longer the energy
is allowed to flow during the half-cycle. Conversely, delaying the turn-on point reduces the time
for the energy to flow. Putting two thyristors back-to-back (or anti-parallel) in each of the phase
connections to a motor, and by precisely controlling their turn-on points, an electronic soft starter
continuously adjusts the passage of energy from the supply so that it is just sufficient for the motor
to perform satisfactorily.
So, for instance, by starting with a large delay to the turn on point in each half cycle, and
progressively reducing it over a selected time period, the voltage applied to the motor starts from
a relatively low value and increases to full voltage. Due to the motor torque being proportional
to the square of the applied voltage, the starting torque follows the same pattern giving the
characteristic smooth, stepless start of the soft starter.