9492600990 9-9
DECS-150 Limiters
Underfrequency Limiter
BESTCOMSPlus Navigation Path: Settings Explorer, Operating Settings, Limiters, Underfrequency
The underfrequency limiter is selectable for underfrequency limiting or volts per hertz limiting. These
limiters protect the machine from damage due to excessive magnetic flux resulting from low frequency
and/or overvoltage.
Underfrequency
If the machine frequency decreases below the corner frequency for the selected underfrequency slope
(Figure 9-11), the DECS-150 adjusts the voltage setpoint so that the machine voltage follows the
underfrequency slope. The adjustment range of the corner frequency and slope settings enables the
DECS-150 to precisely match the operating characteristics of the prime mover and the loads being
applied to the machine.
Figure 9-11. Typical Underfrequency Compensation Curve
Reset Time
The reset function introduces a time-based response to the recovery of the voltage based upon the
recovery of the speed, magnitude of the voltage dip, and the Reset Time setting in BESTCOMSPlus.
After speed begins to recover, the reset time is used to calculate the voltage recovery ramp time. The
reset time reduces the machine kW below the available engine kW during the recovery period, thus
allowing an improved speed recovery. This control is effective only during load switching when the speed
decreases below the underfrequency knee setting. If the speed stays above the knee during a load
transient, the reset function will have no effect on the recovery. This feature is typically used with a
generator coupled to a turbocharged engine with limited block load acceptance. When the reset time is
set to zero (0), the response will follow the V/Hz curve selected.
Volts per Hertz
The volts per hertz limiter prevents the regulation setpoint from exceeding the volts per hertz ratio defined
by the V/Hz High Limiter and V/Hz Low Limiter settings. The High Limiter setting
establishes the
maximum threshold for volts per hertz limiting, the Low Limiter setting establishes the minimum threshold
for volts per hertz limiting, and the Time Limiter setting
establishes the time delay for limiting. A typical
volts per hertz limiter curve is illustrated in Figure 9-12.
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