Fairford Electronics QFE Series, Руководство пользователя

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QFE
page 3-9
Applications
FD841405_F
3.2.7 Soft-Stopping
Soft-stopping can reduce positive surge pressures in pipelines on shutdown. It is necessary
to make sure that the ramp-down time is long enough to remove the energy from the fluid
before the firing of the thyristors is stopped, otherwise the surge pressure may still be present.
Soft-stopping can also be successfully applied to loads such as conveyer belt systems where
sensitive items such as bottles are being transported.
3.2.8 Reversing Drives and Plug-braking
QFE soft-starters used in conjunction with contactor controlled reversing and plug-braked motors
show considerable benefits to the user by reducing mechanical and electrical stresses, particularly
when utilising the current limited start feature. It is required, with this type of application, to insert
a 150 to 350 millisecond delay between the opening of one contactor and the closing of the
other, to allow any residual flux in the rotor to die away. When plug-braking, there should be
some form of zero-speed detection to stop the drive after braking has been completed, otherwise
the drive may either accelerate the motor in the reverse direction or switch off before zero speed
has been reached. The QFE has a dual parameter setting feature which is particularly useful in
plug-braking where the braking settings need to be different from the normal starting ramp time
and current limit level. See section 2.4.4 and figure 2.4.6 of the Electrical Installation manual for
details.
3.2.9 Replacement of Fluid Couplings
Soft-starters can replace fluid couplings yielding benefits of higher efficiency running and
lower costs to the user. If the coupling is used to magnify the available breakaway torque, it
may be necessary to replace the fitted motor with another of a larger size or one with a high
starting torque characteristic before a soft-start can be employed.
3.2.10 Two-speed Motor Applications
Two speed motors, whether Dahlander connected or with dual windings, can be soft started
at each speed, provided that the start is initiated when the actual motor speed is less than
the synchronous speed for the winding selected. This is particularly important when changing
from high to low speeds. Of particular benefit in the QFE is the dual setting feature, which allows
different operating settings for the high and low speeds and the ability to switch rapidly between
them.
3.2.11 Multiple Motor Starting
See section 2.4.2 of the Electrical Installation manual for details.
3.2.12 Overhauling Loads
Certain applications can over-speed the motor as part of normal operation. Power flow is
then from the motor to the supply. It is important that the optimising is disabled during the
over-speed condition and reinserted during normal conditions.
3.2.13 Application Table
The table on the following page shows many common motor applications that suit the QFE soft-
starter. It lists typical breakaway torque requirements as a percentage of motor full-load torque
(FLT). For the most satisfactory soft-start in a given application, the motor should have a full-
voltage locked-rotor-torque (LRT) that is at least twice the breakaway torque. (E.g. For a
reciprocating compressor the FLT is normally in the region of 50% motor LRT.) As a general
rule, the higher the motor LRT is above the load breakaway torque, the greater the control over
the starting process.