VFD motor overspeed relay? 1

[Skogsgurra]

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?

 

[dpc]

I've never seen one used, even on large medium-voltage motors (3500 hp). You can set frequency limits on the inverter output.

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.

 

[Laplacian]

We must employ secondary overspeed trip systems for safety critical VFD's independent of the VFD logic controller.

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.

 

[jraef]

I agree with dpc. In addition, it seems rather thick headed of the "security guys" to beleive that a VFD can "run away" with output frequency. They obviuosly do not truly understand the nature of how a PWM drive works. For all intents and purposes, it must be thought of as a separate "new" power source for the motor, since there is no direct connection, magnetically or otherwise, to the incomming power that could in any way allow full voltage line power to get to the motor. What they must do is consider the failure mode potentials.

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

 

[electricpete]

I would think that if the aim is to prevent overpressurizing, the most direct and effective protection would be sensing of pressure, not speed. After all, there are many variables other than speed which affect pressure.

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[dpc]

Good point, electricpete. Pressure sensing is more direct and much simpler, in most cases. A simple pressure switch could suffice.

Frequency or speed won't really provide accurate pressure data.

 

[Skogsgurra]

Thanks for the views. But, as I said in the first posting: "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"

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.

 

[dpc]

For high-reliability trip circuits on large boilers, we generally used a two out of three voting logic (de-energize to trip) into a one out of two energize to trip logic.

Sounds like job security to me...

 

[jbartos]

Question: Have you search for it on Web? In your past postings you mentioned that it was a piece of cake for you.

 

[Skogsgurra]

Yes, jb. Of course I have.

 

[ozmosis]

It may be beneficial to spend time looking at the interface to the drive to prevent anyone tampering with it.
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.

 

[Skogsgurra]

This is a good idea, sed2. If it is good enough for hoist safety, it should be excellent for pumps. Will talk to those guys. Thanks.

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.

 

[jbartos]

Comment on skogsgurra (Electrical) Jan 27, 2004 marked ///\\\.
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.\\\

 

[jraef]

If you are able to attach a speed sensor to the motor, how about sending its output to a SIL 2 certfied PLC or other safety system that can read a square wave or encoder input? Then you can simply set up a rate counter and trip off line at a predetermined pulse rate, equating to rotation speed. Just an idea to get around the SIL 2 issue. That seems to be the killer in my mind.

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

 

[ozmosis]

Following on from the jraef tip, I was going to suggest a similar as I've done in the past with encoder and safety PLC but was put off by your security spooks aversion to putting 'more devices in that could go wrong'. This system works well but is expensive.

 

[jbartos]

Eng-tip: A good place to start would be with a frequency meter circuit or a frequency to voltage converter. Some circuits are available and have been published.

 

[Skogsgurra]

Thanks all,

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...

 

[jbartos]

Question/suggestion: Have you visited:

http://www.thomasregister.com

and typed Monitors: Frequency under Product or Service, which would returned 45 companies to approach to?

 

[Skogsgurra]

Yes - again - jb. Of course I have.

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.