PLC controlling VFD and duct detectors!

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electricalperson

electricalperson

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massachusetts
we recently installed duct smoke detectors in a building on the supply side. the fire department wants the detector to shut down the motors in the air handler when the detector goes into alarm mode. the air handlers are controlled by a VFD controlled by a PLC. basically the PLC tells the VFD to turn on when a certian time is reached. they ran a set of 2 conductors to an ice cube relay that switches the 120 volt to turn on the coils to pull in the contactors and work the vfd. im thinking we need to run a set of 2 conductors from the duct smoke to the PLC and have a tech come and land them on the terminals and program it. am i correct with that?
 
Having no certifiable knowlege of this, I probably shouldn't answer, but it seems to me :


It's usually better to operate the VFD with terminals on the VFD for starting and stopping etc. rather than killing the line voltage.

except for the smoke detectors... seems to me the smoke detectors should kill the air handler without the PLC

Now having contributed my 2 cents, you can probably go to the bank with just the opposite of my opinion.:)
 
I'd try to keep the PLC out of the duct detector shutdown, if I could. There's probably a dry contact input someplace on the VFD you can open (or close) to stop the thing. I'd look into that, and wire the duct detector into that.
 
electricalperson said:
we recently installed duct smoke detectors in a building on the supply side. the fire department wants the detector to shut down the motors in the air handler when the detector goes into alarm mode. the air handlers are controlled by a VFD controlled by a PLC. basically the PLC tells the VFD to turn on when a certian time is reached. they ran a set of 2 conductors to an ice cube relay that switches the 120 volt to turn on the coils to pull in the contactors and work the vfd. im thinking we need to run a set of 2 conductors from the duct smoke to the PLC and have a tech come and land them on the terminals and program it. am i correct with that?
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Seems to me what you want to do is to insert the smoke detector contacts into the 'ice cube' relay control circuit.

Although I disagree with the implicated control method to begin with. The 'ice cube relay that switches the 120 volt to turn on the coils to pull in the contactors' sounds to me that you are switching the supply voltage to the ASD - as VFD properly called - and that is not a smart idea. Usually there is a control ciruit on the ASD that could directly interface with the PLC outputs to START/STOP the ASD and you get your analog speed signal often from the same equipment.
 
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What kind of VFD are you using? Almost all of the Allen-Bradley vfd's I have installed have multiple dry contacts just for this purpose. It is never wise to rely on a PLC program for any emergency shutdown or control.
 
Not a big fan of input contractors for VFD's, I prefer output contractors if required, and sometimes this is a required specification for a E-stop on the drive. There is current requirement in the NEC, NFPA70E, or NFPA79 requireing the contactor when using a drive. I have normally found them to be leftovers, or bypass contractors.

Have you considered using the drive enable signal, most likely shorted with a jumper if unused. Opening this will de-permit the drive to run and allow the drive to stop safely, possibly using braking or a ramp to stop quickly. This will not happen if the input contactor is opened. You will conduct a faulted stop, due to the loss of power. Note because this is a fire system, there may be additional requirements. Safety rated PLC and monitoring systems. Other installed systems will work most likely, but could open you up to liability if something ever goes wrong.

I might use a rated monitor relay activated by the alarm state in the fire system. A reset of the relay would occur only when the alarm has been cleared at the fire panel. I would break drive enable, and verify no installed mod affects the PLC prog. The drive stays powered up but will not run.
 
im going to go look again at this. i didnt notice any contacts on the VFD but then again i just opened up the section that contained the circuit breakers, the ice cube relay and the 2 contactors along with a set of fuses
 
I will be careful in mentioning this due to AB seeming to not be a big hit around here. AB makes a safety contactor "Guard Master"(I can get numbers if you like) that should be placed on the load side of the VFD. Along with using the detectors to "disable" the VFD they should also drop this contactor on load side never line side I have seen this design before and hate it. Seen VFDs fried because of the supply being cycled. AB also has a safe off option board to install on its PF70 models to be utilized as the enable. If you can still run a signal from the detectors to the PLC this will prove to be a niffty troubleshooting tool that will save some time.
 
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Not a big fan of input contractors for VFD's, I prefer output contractors if required, and sometimes this is a required specification for a E-stop on the drive. There is NO current requirement in the NEC, NFPA70E, or NFPA79 requireing the contactor when using a drive. I have normally found them to be leftovers, or bypass contractors.

A key missing word....
 
electricalperson said:
im going to go look again at this. i didnt notice any contacts on the VFD but then again i just opened up the section that contained the circuit breakers, the ice cube relay and the 2 contactors along with a set of fuses
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should be a seperate small plastic plate to pop off where you would have a bunch of numbered terminals (where the communications wiring or LONworks terminates). Look in the manual and it will tell you which contacts are for enable, Start/Stop, and whether they are dry or not.
 
I'm not 100% clear here where your function lies in this installation. Are you just the Smoke Detector installer? If so, did you accept a contract that included investigation time of the entire system operating scheme? Without that I would not do any termination into the VFD or control system due to liability issues, unless specifically stated as a requirement in your contract (so your insurance will cover you). If something you do might have unforeseen consequences, how are you going to know if you never had anything to do with the BAS (Building Automation System) in the first place? I would insist that you will provide the circuit from your detector, someone else will need to take responsibility for interfacing it to the BAS. You could offer to do it under a separate contract if you feel qualified, but I wouldn't take it on under a simple "supply and install this detector" contract. Besides, do you really know the function of those contactors? Sometimes they are for maintenance bypass, but sometimes they are for automated emergency bypass and if you drop out the wrong one, the PLC may automatically switch to the other one. Again, without direct knowledge of the entire system, I would not volunteer for it. Attaining that knowledge may involve more billable hours than you contracted for. If you took it on as a T&M job, then I would make sure the owner understands what he is looking at.

By the way, being that this is strictly an "Emergency" shutdown situation, there is no real difference between line or load side contactors; either one will do the job. But if there is a "glitch" in the relay / control circuit and it causes the contactors cycle on-off rapidly, the VFD could be damaged regardless of which side it is on. If it is on the load side, you could also include a drop-out of the Run command so that the VFD is always "off" by the time the contactor opens. That won't help if the contactor is on the Line side though; the rapid charging / discharging of the caps will likely lead to premature failure.
 
i have yet to see a smoke detector with a dry contact.

If it is available, then the dry contact should be used in series with the logic start terminals of VSD.

Don't forget to have a manual override of this smoke detector contact,...or who will clear the smoke?

cheers.
 
chaterpilar said:
i have yet to see a smoke detector with a dry contact.

If it is available, then the dry contact should be used in series with the logic start terminals of VSD.

Don't forget to have a manual override of this smoke detector contact,...or who will clear the smoke?

cheers.
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with the simplex duct smoke detector they have a set of terminals that put out 24volts when there is an alarm. we attach a relay to those contacts from simplex and attach whatever we want to control to the C, NO or NC contacts off the relay. its not in our contract to mess with the VFD so we are just going to run the conductors to the PLC and let the tech hook it up and program it all in. also the VFD has a bypass mode on the control, itself so we can clear out the smoke if need too manually without the PLC turning it on
 
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