VFD motor overspeed relay?
VFD motor overspeed relay?
(OP)
Hi all drive experts,
Does anyone of you have any experience with overspeed sensing on variable speed motors? The application is from pressurized vessels where the pump/compressor is fed from a VFD and there is a risk that someone sets the speed reference so high that the resulting pressure exceeds what the vessel can take.
The security guys have this thinking: "Nothing is secure. And if it is - it will still fail. Nothing that the drives people say are worth listening to. Safety devices shall be totally independent of the inverter"
So anything you tell them will be ridiculed and no good. Pressure sensors and pressure switches are being used to supervise the systems today, but they need supervision and calibration several times each year. A frequency sensitive relay that cannot be manipulated, has SIL 2 and can be connected to the inverter output is needed. Does anyone have any experience with such relays? Pitfalls? Comments?
Does anyone of you have any experience with overspeed sensing on variable speed motors? The application is from pressurized vessels where the pump/compressor is fed from a VFD and there is a risk that someone sets the speed reference so high that the resulting pressure exceeds what the vessel can take.
The security guys have this thinking: "Nothing is secure. And if it is - it will still fail. Nothing that the drives people say are worth listening to. Safety devices shall be totally independent of the inverter"
So anything you tell them will be ridiculed and no good. Pressure sensors and pressure switches are being used to supervise the systems today, but they need supervision and calibration several times each year. A frequency sensitive relay that cannot be manipulated, has SIL 2 and can be connected to the inverter output is needed. Does anyone have any experience with such relays? Pitfalls? Comments?





RE: VFD motor overspeed relay?
But if you are sensing motor shaft speed/position anyway, adding a contact output to shut down on high speed would be fairly simple to do. Just one more thing to fail.
Seems like that pressure vessels should also have other means of protection, such as relief valves.
RE: VFD motor overspeed relay?
We accomplish this by using a frequency converter fed into a Moore SPA to trip a breaker feeding the VFD.
Frequency is taken off the VFD output by a separate CT. This CT secondary loops through a small instrument CT which is the input on the frequency converter.
I don't know what non-programmable devices will provide the same functionality of the Moore SPA in our application, but there must be something.
We have used TachPax speed switches to detect shaft rpm on new installations, but for a retrofit applications frequency is more easily accessible.
RE: VFD motor overspeed relay?
If the front-end converter fails (and it would need to be catastrophic), it would become a dead short phase-to-phase and would clear the upstream protective device. If one, some, or all of the output transistors fail they usually just stop conductin. If the short out, (again, catastrophically), the worst case is that DC is put onto the motor, which would damage it but not make it run faster. The output frequency is totally controlled / created by the microprocessor, determined by the application of the proper PWM pattern. It cannot "accidentally" decide on its own to apply a higher frequency than commanded (assuming of course that Artificial Intellegence has not yet been achieved). If anything fails, again it just quits functioning, which means NO output. With proper lockouts, the frequency limits in programming are demonstrably more relaible than external relays / sensors etc. If any component failed, the result would be non-operation, never uncontrolled operation. For the VFD frequency limits to allow unsafe operation, someone would need to actively change them. Even SIL 2 certified PLCs can be sabotaged if someone gets apst the password protection so I don't see a major difference here. For limit relays to allow an unsafe condition, all that would need happen is interference, a broken wire or a single failed component. Far moe unreliable in my opinion.
Also, I seriuosly doubt there is such a thing as a relay that "cannot be manipulated". You could probably get someone to make a frequency trip relay that is pre-set to a particular frequency and potted into epoxy, but again if someone were to cut or short an output wire, what good is it? Then you would need to find one designed to work on a PWM output. Not impossible, but it limits the field a bit more. The deal-killer would be to find someone to get SIL 2 certification on it. I hope the "security guys" will allow you an unlimited budget for this!
Quando Omni Flunkus Moritati
RE: VFD motor overspeed relay?
imho the pump should trip/block if pressure gets too high. At even higher pressure of course there should be some kind of pressure relief device to provide backup.
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RE: VFD motor overspeed relay?
Frequency or speed won't really provide accurate pressure data.
RE: VFD motor overspeed relay?
So I welcome comments from someone that has tried that "road to sucess" - or has found a relay that does the job when connected to PWM and has some degree of safety integrity level.
I start to think that two cheap frequency relays working together could be accepted by the risk analysis people. But probably not by the production manager - two relays = double trouble.
RE: VFD motor overspeed relay?
Sounds like job security to me...
RE: VFD motor overspeed relay?
RE: VFD motor overspeed relay?
RE: VFD motor overspeed relay?
If the VFD is sized correctly and set-up right it should protect the system (with current monitoring etc) before any real damage is done if somebody was really out to sabotage your plant and manage to overide the upper frequency limit.
Hoist motors have centrifugal switches that bring in a brake in the event the motor runs away with itself. Might be worth looking into this for ideas.
RE: VFD motor overspeed relay?
dpc, They have something against pressure sensors and pressure switches. It is about calibration and checking. And if I ask them to put in THREE...
The voting idea is good, though. Have been using it for web break detection in paper machines. There, nuisance trips from the web break detector is a real er.. well.. nuisance.
RE: VFD motor overspeed relay?
Yes, jb. Of course I have.
///I am surprised that you do not have good results. There appear to be about 45 companies involved in frequency sensing/monitoring. This Forum can hardly match that expertise since it is eng-tips Forum only. However, as you can see, volunteers try hard for you.\\\
RE: VFD motor overspeed relay?
Rotational speed is an issue in turbine and compressor systems for the oil and gas industry. When searching, try looking for ESD (Emergency Shutdown Systems) equipment, a term more commonly used in the US for systems similar to those having SIL 2 compliance.
Quando Omni Flunkus Moritati
RE: VFD motor overspeed relay?
RE: VFD motor overspeed relay?
RE: VFD motor overspeed relay?
This kind of comments and thinking do not "appear" when you do a search on the web. That´s why (after doing a thorough search that gave me lots of overspeed and cranking automation and other dubious frequency measuring devices - including our own did appear) I thought that I should put the question here.
It now looks like the good old shaft mounted overspeed switch will be used. There is some concern about motor shaft coupling. It has to be SIL 2 as well...
RE: VFD motor overspeed relay?
http://www.thomasregister.com
and typed Monitors: Frequency under Product or Service, which would returned 45 companies to approach to?
RE: VFD motor overspeed relay?
I even have their (Thomas register) blue books (three thick books) in my book shelf. And among the devices presented there are murphy, hobut, dynalco, nupart, vacon etc frequency monitors, frequency detectors, speed switches and so on. None of these stand up to the SIL 2 criterion.
Why is this so important to you? You seem to be obsessed by this subject. You can relax now. Problem solved.