Minimum Field resistance.

[SJC1985]

Alright lady's and gents. I've got a DECS 125-15 (automatic voltage regulator) that is looking for a minimum field resistance of 8.3 ohms. My field resistance is at 4 ohms. Anyone know if I can add a power resistor into the mix to bring up my resistance to make the DECS happy or do I have to change DECS model to accommodate my field resistance. If I do that then my input power from my PMG to the DECS wont match anymore so that also causes me some issues.

Thanks a bunch, Steve

 

[waross]

Check the specs. You may have a failing field or you may have the wrong AVR.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

There are a lot of DESC 125-15 AVR's out there on generators with 4 ohm fields, I'm assuming this is related to your other post, is this is a CAT generator?

What you will find is that the voltage at lower loads will appear to have a bit of a step response. The most common complaint is that at low loads paralleled to a grid or network is that the power factor appears to "jump" a small amount, as the load increase and excitation output also increases this appears to settle out. If this is a worry you can add a resistor in series with the field output. I normally mount a resistor like this outside of the cabinet and away from where someone may come in contact with it, they do get hot.

The DECS 125-15 AVR is a non-current regulator, are you using it because it is what you have installed and don't want to change it?

If this is a newer CAT SR4B-HV then the bet matched AVR for it is the CAT CDVR, and a "switchgear" harness is available for mounting in your controls section.

Hope that helps, MikeL.

 

[SJC1985]

Thanks again Mike, yes this applies to the previous post. It is a CAT generator. Being this is a standby I don't think I am too worried about PF bouncing around for the short time it'll be in service, so maybe I wont bother adding resistance.

The DECS 125-15 is what we are using on all 12 generators so I'd like to stick with it. That being said if you're correct that my PMG (from previous post) is pumping out a whopping 100V then I will need to change the DECS model anyways and will look into the CAT CDVR.

Thanks again, Steve

 

[catserveng]

I have a number of customers who operate mixed fleets of engines, mostly in Landfill and Digester fueled applications that as the DECS 63-15 have failed they have been replaced with DECS 125-15 with no operational problems. With a lower input power the DECS just won't output the rated output voltage. They only we do is readjust the OEL and UEL settings to reflect the lower voltage excitation system.

I would suggest if you are going to do a load bank test and can fully check the unit out, go ahead and use the DECS 125-15 and see if it will meet your needs, I am pretty sure you will find it will work just fine. If you don't want to do a full blown retrofit you might see if you can find a DECS 63-15, as it is the same physical size and uses the same power supply module.

I am retrofitting a site next weekend that has two DECS 63-15 regulators that we are replacing with DECS 250N regulators as part of an upgrade project, I think the end user will likely put them on eBay after the conversion work is complete, and I see (and buy them) on eBay once in a while.

If you have a critical type system you may want to consider retrofitting to a newer AVR, as the current Basler replacement is larger and usually doesn't fit in the same place as the original, and wires up different. Basler will still repair the older DECS regulators IF they have the needed parts on hand, this year that has been a 50/50 proposition for me, and lead time for repair sucks.

The CAT CDVR has been a good replacement, but was originally intended to mount in a generator enclosure, but I have installed a number in switchgear with no problem. It just isn't something you want to do in a hurry. Your local CAT dealer probably has one or two CDVR's in stock, but the switchgear harness is a low volume item and usually takes some time to get.

Good Luck with your project, MikeL.