There is yet to be a reliable current switch for VFD's.
If you are using CVD's I would recommend a current switch though. They are reliable (assuming you do your research) and inexpensive and provide instant feedback to the BMS when problems arise.
I have tons RIBXGTAs on the line side of VFDs without issue. Have done so for years.
Originally Posted by brookscs
I should have been more clear, I am referring to a current switch to be placed on the load side of the VFD drive, not the line side.
Originally Posted by orion242
Just replaced two more. Both were on the line side of two different ABB VFD's. Check out the date. Replaced with the same type and with the setpoint turned down.
"It's not that I'm smart, it's that I stay with the problem longer”
Hmmm, okay Thanks. Always good to know this sort of thing, even if it hasn't been my own experience.
Originally Posted by xarralu
Now, while we've used the precise devices you show, a great many of them, none are installed on the line side of VFD's.
So I'd have no real clue if such utilization is problematical.
Typically if using any sort of decent VFDs (ABB, Danfoss, etc) we utilize the VFD's built-in facilities to set up monitoring.
Where we use the items you show is plain old fixed speed type applications with standard motor starters. Where customer wants some sort of fan belt loss or whatever monitoring as versus simple Go/No Go monitoring.
Now, I've found bad 608's, but not often. And such failure, in my experience, is mostly upon initial start up or shortly there after. The usual deal, probable factory defect.
Mostly what I find when one is reported as not working is installation error, it isn't adjusted correctly, or the "jaws" aren't properly closed or the contact of the "jaw" is dirty.
The "jaw" sometimes comes loose. Had this issue time to time where there is significant vibration. For instance, one installation with several of split core type CT's in the electrical panel of a large condensing unit that was both very noisy and vibrated a good bit. Finally ran some electrical tape around the things to correct the problem.
On one job, when I went around doing my testing and commissioning thing, found that the installers had a bunch with the trip points set incorrectly. Had to do the 20 turns counter-clockwise bit to reset them, then adjusted them.
And here and there found some where someone has decided to use them to pull in some auxiliary relays (using the CT as a switch). You can do this, and coil voltage for the aux relays was right (24V), the problem was that they were pretty good size relays, and when switched "off", back voltage was generated of sufficient magnitude to exceed the rating of the CT internal solid state switch. I replaced the failed CTs, and installed transorbs on all and problem went away.
But, otherwise, no real issues with them. However, as stated, I haven't used em in the way you describe above.
How effective were you in detecting issues with the VFD through the BMS? From what I understand, because of the fluctuation one of the ways to monitor the VFD is to "filter" out the high end fluctuations. Is this the direction that you went?
Originally Posted by osiyo
I'm not quite sure what you mean by "detecting issues with the VFD through the BMS".
Originally Posted by brookscs
This thread was/is generally about what method folks thought best to use as proof of air flow, a mechanical switch or a CT.
I, myself, had not commented on my own personal views about that in question in particular. I was just asking Xarralu for more information about his experience with a high failure rate with some CTs. Just so I know more, and maybe find out something I did not know.
Now, as concerns trying to detect VFD issues with a CT.
Hmmm, haven't really tried to do that.
We have put simple Go-No Go CT's on the load side of VFD's. Or to look at it another way, on the motor leads. Just to verify current flow to the motor, when we have to work with really dumb and simple VFDs installed by someone else, or to take into account a bypass being used.
However, generally speaking, mostly what we do with the kinds of VFDs we typically work with, is to configure one of its relays to give us a "Running the motor and its up to commanded speed" signals (a "Running" input to a controller); and then configure another output relay on the VFD to give us a "Fault" input. With fault configuration set up to trip fault with Over and Under Amps, volts, etc. With most decent VFDs you have, among other options, the ability to include an Under Amps set point of some sort that'll trip a fault relay.
The above as a minimum. Depending on customer needs, and pocketbook, we go on to added a number of monitoring points such as analog feedback from VFD indicating motor speed, or current draw, etc. Or we connect to it via Modbus or BacNet and pick up all the points customer may want.
You COULD use an adjustable CT on the load side of a VFD to detect under current. But that's problematical. For a Go-No Go CT, they often have a problem that starts at some lower speed (i.e. 6 hz) where the Go-No Go CT's start to chatter. Not a big deal if you follow most manufacturer's suggestion of setting up a minimum of somewhere around 15-20 hz.
For adjustable CT's on the load side, first yah really need to use a CT designed for VFD service. Output of a VFD IS NOT a true sinusoidal wave. In fact, its choppy as heck, with sharp rises and falls, and from time to time with some quite high spikes. There are CT's made for such service. They actually apply some mathematics to smooth out the input and average out that smoothed out signal over time.
Generally speaking, in my experience, better off to just pay a few extra bucks for a bit better VFD that has the abilities to feed you the info you want and need, and monitor it. If a critical installation, install a power monitor or aux switch on the disconnect/breaker so you can tell if someone shut off the power to the VFD.
Any single method of verifying air flow has its issues.
I happen to like mechanical air flow switches. But they're certainly not perfect and have their issues. Not to mention I couldn't count the number of times I've found them installed improperly (I've seen both pipe fitters and electrician installers to that). Or found one indicating flow ... and there was indeed flow ... but not nearly enough (i.e. loose belt). Often enough neither the mechanical switches nor the adjustable CT's are adjusted correctly.
Thanks osiyo, very helpful information!
We have lots of problems with DP or air flow switches on our return fans that usually run at minimum. But at the same time the CT can show it running with belts off. The ABB VFD's have a built in proofing address that I like to use if you are running it or monitoring it with COMM.
Originally Posted by air1
DP is best if already there and wired. But like everything the switches are mechanical and need adjustment / maintenance IMO. We use the Veris product and adjust them correctly. Almost too correctly, we sometimes get a fan failure issue on a single speed fan / motor after air balance and have to re-adjust. As far as drives some have the ability to detect belt failure using running HTz and compare the load on the drive. Very good way to determine actual correct conditions on the fan motor. Just an aux relay saying the VFD is at set-point for a good status is not the best IMO.
DP sensors will always tell if the fan is actually moving air. They will require more calibration and maintainance than a current switch. Make sure to install it in a location that has easy access.