Fairford Electronics QFE Series, Manual

Page: 68 / 194

Applications
QFE
page 3-20
FD841405_F
3.4.4 Optimising with the QFE
During start-up, the QFE software computes and stores, by a patented method, a reference
value for the power factor. When the motor has reached full speed and is driving the load at
the torque demanded, it enters the ‘motor running’ stage. At this stage, if required, the motor
may also operate in ‘Optimising Mode’. Entering this mode can be pre-set from the QFE keypad
and stored for automatic operation, which will suit the majority of applications where it is required.
This is the default operating mode for the QFE. It can also be toggled on and off whilst running by
using either the OPTIMISE button on the keypad or, with the QFEplus version, through external
circuitry connected to one of the programmable inputs and controlled by the driven process.
In this mode the reference power factor is continuously compared with the running power factor.
From the comparison, the software continuously computes and adjusts the firing point of the
thyristors so that the best power factor is maintained. This method of continuous control minimises
the energy wastage caused by overfluxing the motor. It also maintains the power factor at the
most appropriate value for every condition of load demand. This can effect a significant reduction
in the kVA demand.
This is an operating condition that may, at light or partial load conditions, provide the benefit of
energy saving and if selected, is continuous from the dwell period until a STOP command is
initiated or the mode is disabled. It should be noted that this function is inhibited by the software
if the current being drawn by the motor exceeds 80% of the set current of the QFE. At full volts,
when the motor enters it’s running stage, with the optimising mode selected,
The method of power factor management described does not affect motor performance, neither
does it detract from the motor’s capability to respond to changes in load demand. This feature
of the QFE soft-starter is a purely electrical function which has the effect of ensuring that, at all
times, the motor delivers the torque demanded, but allows it to draw only the precise amount of
magnetising current required to support that torque output. Without this feature, the motor would
draw the maximum magnetising current regardless of load.
The optimising function cannot improve the power factor beyond what it would ordinarily be at full
load, but it does make the optimum improvement possible at any part-load.