Page B–14 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Appendix B: Soft Starter Application Considerations
b.3.6 – running induCTion moTors (ConTinued)
All SR55 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.
b.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.
To Next Page
Loading...
Need help?
General
