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Chapter 3: Applications
All SR44 Soft Starters are microprocessor controlled, and this gives them a number of advantages.
Firstly, there are no adjustments to be made for the energy saving function; all calculations
necessary to find the best degree of phase-back of the thyristors for any load condition is made by
the microprocessor. Secondly, the start always synchronizes with the supply voltage, and a special
structure of turn-on pulses virtually eliminates the inrush currents normally associated with motor
start-up. This happens every time. Lastly, there is the absolutely stepless starting process,
otherwise found only with primary resistance or reactance electromechanical starters – but without
the wasted energy, and with the opportunity to control the maximum current allowed to flow
during the starting process. Other features such as soft stopping are included to give considerable
control over all modes of induction motor operation.
3.3.7 – Reliability Considerations
Reliability is an aspect which is of increasing concern regarding electronic controllers for
induction motors. There is little point in installing an expensive item of electronic equipment to
save potentially considerable amounts of money if the device is unreliable to the point that vital
processes are constantly interrupted.
There are electronic products in the market place which appear to offer soft starting more cheaply.
However, they almost always rely on less advantageous technologies such as analog control, or
half-control, where one of the two thyristors in each phase is replaced with a diode. There are
systems which only control the energy flow in one phase while the other two are directly
connected. Owing to the variable quality and performance of many so-called inverters and soft
starters available to the unsuspecting purchaser, international standards for these products have
been developed.
So far, IEC 60947-4-2 – “AC Semiconductor Motor Controllers and Starters” defines the soft starter
in every important respect, including thermal and overload performance as well as
electromagnetic compatibility. By ensuring that any motor controller equipment purchased
conforms to IEC 60947-4-2, a user should be reasonably safeguarded from shoddy or inadequate
products when specifying equipment for future installations.
A particular advantage of the use of the optimizing soft starter is its impact on the maintenance
requirements of associated electro-mechanical equipment. Optimizing lowers the surface
temperature of the motor by reducing the losses within the motor. This prolongs the motor life,
and reduces heating of the surrounding atmosphere in the process. If the atmosphere is subject to
air conditioning, reducing the heat input will reduce the air conditioning costs. Reduced starting
and running currents reduces cable losses, and contactor switching operations are carried out
under the most advantageous conditions. No current flows on switch-on since all switching is
carried out by the thyristors, which virtually eliminates the need for contact replacement.
Indeed, there are a growing number of installations where contactors are no longer employed,
being replaced by controllable circuit breakers or isolators instead.
In summary, electronic controllers for most fixed-speed applications are opening new ways of
increasing the efficient operation of induction motors, as well as offering significant benefits in
control. Prospective users should ensure themselves of the quality and performance of any
products they expect to fit, and this can be reasonably expected if compliance with the
appropriate IEC standards is demanded.
SR44 Series Soft Starter User Manual
1st Ed, Rev B 07/31/2019
3–14