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agility
TM
Series Soft Start User Manual
MAN-AGY-001. Version 04. 21/09/2018
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, star-delta only offers
an imperfect solution to the problem of starting the induction motor.
Method C: Primary Resistance Starter
It has long been recognised that the transition step in the star-delta system was a source of
problems such as welded contactors, sheared drive shafts etc., and 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.
Method D: Other Electro-Mechanical Systems
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.
The Semiconductor Motor Controller
During the 1950’s, 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 smallpulse
of current. This device became known as the silicon controlled rectifier (SCR), or in Europe, the
‘Thyristor’; 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
Applications (continued)
