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14 l Automatic Voltage Regulator – AVR-A-OPT-06
4.2 PROTECTIONS – U/F OPERATION
Figure 4.1 shows the curves for voltage variation as a function of frequency variation. For nominal frequency operation,
U/F is disabled. When rotation decreases (for ex. when shutting down), excitation diminishes, reducing the output voltage
of the alternator. The drop follows a straight line in order to arrive at 0V in 0Hertz. For the case shown on Figure 4.1, the
adjustment of U/F was done on the limit of the nominal frequency.
Figure 4.1: U/F curves
This mode of operation is determined by trimpot U/F, jumper JHz and associated components. The JHz jumper determines
the operation frequency, following the following logic:
JHz, position 1 and 2 = 50Hz
position 2 and 3 = 60Hz
The U/F trimpot determines the trigger point of the U/F mode, which can be the nominal frequency (Fn) down to 1/3 of Fn,
and leaves the factory adjusted to 10% below Fn. For operation in 60Hz it is adjusted to 54Hz and for operation in 50 Hz
it is adjusted to 45Hz (see Figure 4.2). These values can be altered according to the needs of each application.
Figure 4.2: Actuation point for U/F protection
ATTENTION
1. Do not set the U/F protection below 20 % of the rated alternator frequency. The configuration should be done
according to Figure 4.2 to avoid problems when shutting down.
2. The frequency limited by U/F is the frequency of the waveform that is at the power supply input of the
regulator and not at the sensing input (output voltage of the alternator).
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