Allen-Bradley 1407-CGCM, User Manual, Page: 51 / 252

Rockwell Automation Publication 1407-UM001H-EN-P - November 2014
CGCM Unit Operation #HAPTER
6OLTS(ERTZ,IMIT
Volts/Hertz limiting acts to reduce the generator output voltage by an amount
proportional to generator frequency. This is done to protect the generator from
overheating and reduce the impact on the prime mover when adding a large load.
When the generator frequency drops, the voltage setpoint is automatically
adjusted by the CGCM unit so that generator voltage follows the
under-frequency slope.
The CGCM unit provides two configurable knee frequencies and two
configurable slopes that allow the user to define the Volts/Hz characteristic. The
slopes are expressed in PU Volts / PU Hertz. For a nominal 60 Hz, 120V system,
a slope of one corresponds to 2V per Hz. The generator output voltage is
maintained at the configured level for any frequency at or above the configured
knee frequency up to 90 Hz. Excitation is inhibited when the frequency is at or
below the 10 Hz cutoff frequency.
The Under-frequency Slope and Knee Voltages graph shows a typical Volts/Hz
characteristic as displayed in the RSLogix 5000 software CGCM unit
configuration screen.
Volts/Hertz limiting is automatically enabled in AVR mode and limits the
voltage increase in Soft Start mode.
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