Fairford synergy, User Manual

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Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
190
8.3.4 Electro-Mechanical Methods Of Starting (continued)
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.
8.3.5 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 small pulse 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 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.
8.3.6 Running Induction Motors
Once a start has been completed the motor operating efficiency becomes of interest. When working at or
near full load, the typical 3-phase induction motor is relatively efficient, readily achieving efficiencies of 85%
to 95%. However, as shown below, motor efficiency falls dramatically when the load falls to less than 50% of
rated output.
8. Applications (continued)