12-25-2016 01:00 PM
12-23-2016 04:33 PM
Excitation will not shut down at 25 Hz because the Minimum frequency is set to zero. The min voltage % rated sets the maximum voltage drop when in the volts/hz mode.
Take a look in your manual, there should be a graphic in the systems operation section that shows the Loading and Stopping Profile (pg 11 in the manual I have in front of me).
It does a good job showing you the effects of the various settings. From my version of the manual
The digital voltage regulator will begin to build
voltage following the “Soft Start” settings before the
configurable underfrequency point has been reached.
After the configurable underfrequency point has
been reached, the digital voltage regulator will begin
to build voltage following a volts per hertz profile.
When the speed reaches the knee frequency point,
the loading/stopping profile takes effect. The startup
profile function will not be initiated again unless
the frequency drops below the underfrequency
point. The underfrequency point is defaulted to 25
Hz, with a range of 20 to 40 Hz. This is the same
underfrequency set point that is used by the loading
and stopping set point. The knee frequency point is
the point at which the digital voltage regulator will
regulate to the voltage specified by the generator
output voltage parameter.
Soft start rate is two parts, Soft Start Bias, this is the voltage the CDVR tries to build to at initial start as fast as possible, then ramps to the nominal voltage setting per the Soft Start Time setting. Take a good look at the manual describing this parameter, it can follow the volts/Hz slope if the engine accelerates slower than the soft start time.
Hope that helps, MikeL.
12-23-2016 12:50 AM
12-22-2016 02:28 PM
Actually you don't have to "shut off" the regulator, you just need to make sure the Volts/Hz settings are properly set. Unless you are trying to prevent ANY voltage generation while at idle speed, the CDVR will limit excitation based on the frequency, the Minimum Frequency setting can is used to remove exciation below a setpoint frequency between 20 and 40 Hz.
If this was an older unit with an analog regulator like a Basler SR4 then disabling the AVR was common when idling, however no longer needed if the CDVR is properly setup.
Also, how exactly are disabling excitation? By opening the F1(+) and F2(-) leads, or using the Excitation Disable inpit to the CDVR. Hint, if you are breaking the field output wires, REALLY a bad idea, can lead to a failure of the field output driver. If you need to disable excitation for ANY reason, use the Excitation Disable input on P9-9.
Look in your manual (about page 27 in the one I have) discussing the "Startup Tab), and take a good lok at parameters associated with the Under Frequency Mode.
I think you are over complicating this, thousands of units are out in the field that use the CDVR and can operate at idle with no problems.
Hope that helps, MikeL
12-22-2016 03:58 AM
I currently have a fleet of Caterpillar Gensets, which have had their voltage regulators replaced from AVR6 to CDVR. These gensets have an idle speed that differs from the rated speed, as a result the field excitation circuit needs to be disabled whenever a genset is running in idle. Each of these gensets have a station, consisting of control plc and panel. Switching a genset to idle mode is controlled by a mode switch on the panel. This switch is an input to the PLC. When this switch is placed in the "idle" mode position, it causes the PLC to open a relay contact, resulting in the field excitation circuit to be opened. This disables voltage in the SR4 alternator's stator winding, causing a loss of sensing fault in the CDVR, resulting in it shutting down. When the mode swich is moved from "idle" to another position, the field excitation circuit is closed, however as the CDVR has now shut down there is no sensing voltage present, causing the PLC to recognise an undervoltage event, resulting in it shutting the genset down.
I am currently considering a resolution to this issue, my proposed solutions is to use a one shot monostable relay to send a reset fault signal to the CDVR whenever the field excitation relay is closed by the plc (this is triggered when the mode switch state exits "idle"). There potential risks associated with this solution, as the CDVR has a soft starter, and I am unsure of the CDVR start up latency after it receives a fault reset signal. The voltage ramp rate of this soft starter is proportional to engine speed. Since the CDVR will be reset when the engine is operating at rated speed with no stator winding voltage, the soft starter ramp rate would be quite high. Would resetting the CDVR at rated engine speed when there is no stator winding voltage cause a dangerous voltage overshoot? After receiving a fault reset signal, what is the latency for the CDVR to bring the stator voltage back to operating level (415V L-L in Australia).
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