Fairford Electronics QFE Series, Руководство пользователя

Страница: 65 / 194

QFE
page 3-17
Applications
FD841405_F
All QFE 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 synchronises 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, found only with the primary resistance or reactance electro-mechanical 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
An aspect of electronic controllers for induction motors which is of increasing concern is that
of reliability. 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 cheaply.
They almost always rely on less advantageous technologies such as analogue 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 optimising soft starter is its impact on the maintenance
requirements of associated electro-mechanical equipment. Optimising 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 - virtually eliminating 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. Intending users need to 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.