Noticing a belt came off

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George Stolz

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Windsor, CO NEC: 2017
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Assuming a Danfoss VLT HVAC drive FC102 is driving a stairwell pressurization fan that has two belts: how would you determine that one of the redundant belts has come off its pulley?

The load wouldn't change all that much, right? So the VFD wouldn't really notice a decrease in the amount of energy expended turning the motor?

A CT monitoring the circuit would be useless, as the fan motor would be constantly changing speed attempting to fine tune the pressurization in the stair tower as doors are opened and closed, right?

Is the inspector asking for a bar of Unobtainium? :huh:
 

gar

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Ann Arbor, Michigan
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EE
150528-2103 EDT

I will describe the belt drive as a coupling. The stiffness of the coupling is different when comparing one belt vs two belts. Stiffer for two belts. There are two masses coupled together by the spring of the coupling. If one mass is dominate, then there is a simple torisional resonator. The torsional resonant frequency will be different for one belt vs two.

Can this be measured and detected? I don't know.

In a torque gage I have built I used a cogged belt. When I do a spectral analysis of a signal from this gage I can see a frequency component corresponding to cogs on the belt.

.
 

GoldDigger

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Install two optical sensors that watch the motion of a striped belt?
Put in two idler pulleys each connected to a rotation sensor?
Install a PLC to operate them and generate alarms?

Instead of unobtanium, this would be made if unaffordium.
 

Jraef

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There would be a change in the shaft torque seen by the drive, because with one belt gone out of two (or more) the remaining load would slip a little more. But you would not, I don't believe, be able to detect that in the drive, and even if you could, you would not be able to discriminate it from any number of other changes in air flow or static pressure having similar effects. Gar's torsional anomaly concept is interesting, and a good vector control drive might be able to detect and provide that detail with sufficient resolution for a higher level of intelligence to watch it, but that will not be someting the average HVAC drive will be capable of, so put that down as unaffordium too.

Two cheap little photo prox switches, one aimed at each belt, outputs in series to a light that says "Hunky Dory Belts". One belt breaks, no output from that prox, light goes out. Under $100.
 

__dan

Senior Member
Assuming a Danfoss VLT HVAC drive FC102 is driving a stairwell pressurization fan that has two belts: how would you determine that one of the redundant belts has come off its pulley?

The load wouldn't change all that much, right? So the VFD wouldn't really notice a decrease in the amount of energy expended turning the motor?

A CT monitoring the circuit would be useless, as the fan motor would be constantly changing speed attempting to fine tune the pressurization in the stair tower as doors are opened and closed, right?

Is the inspector asking for a bar of Unobtainium? :huh:

Start from square 1, what is the actual requirement ? Is it that the fan must run (critical uptime), if so there would be a redundant fan/motor combination. Fan B runs when Fan A starts and fails to prove.

Losing one belt I am guessing will have no practical measurable impact. The fan will run with one. Fan belts are usually a maintenance issue, scheduled PM. Failed belts give visual indication before they fail completely if you are doing scheduled PM inspections for critical equipment, burns, chunks missing, cracks, excessive wear, are detectable visually in belts that still work. They rarely break cleanly and it would be a very low probability both would break at the same time.

The norm would be finding worn and damaged belts that still work if you have scheduled PM's or monitoring for run failure when the belts break. The motor is commanded to run but the proving switch fails to detect run condition.

A photo eye on the belts will get covered in belt residue so you would have to add photo eye cleaning to the fan belt inspection/service PM.

It's not clear what the requirement is. Is it fan must run, must be monitored for run failure, or must have two belts ???
 

George Stolz

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Dan, don't try to apply logic to this, it is pure folly. In the midst of other interesting requirements, I am required to notify the FACP in the event that one of the two belts comes off.

What about this? In thinking more on this, I believe with the vibration inside the assembly housing, a roller on each belt might be a better option than a retro reflective photo eye.
 

__dan

Senior Member
Dan, don't try to apply logic to this, it is pure folly. In the midst of other interesting requirements, I am required to notify the FACP in the event that one of the two belts comes off.

What about this? In thinking more on this, I believe with the vibration inside the assembly housing, a roller on each belt might be a better option than a retro reflective photo eye.

No on the switch in direct contact with the belt. The constant vibration will beat the switch into dust pretty quickly.

The closest I would suggest is a spring loaded idler wheel on the belt that swings to the failed belt position. Magnetic type prox switch could detect the idler wheel in the missing belt position.

I would not touch it until I got an approved engineering change. I would pursue the RFI first and have the engineer, inspector, and owner approve any change before proceeding with the work. Smoke purge in the stairway could be expected to have some civilized administrative process.

Spent 8 hours today at an all day code conference. Three out of four seminars today were pure insanity.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
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Testing and Engineer
Why not try to design the problem out rather than
detection in? Direct drive fans sound like the answer
rather than a detection system. More complexity is not
always better.

JR
 

kwired

Electron manager
Location
NE Nebraska
Why not try to design the problem out rather than
detection in? Direct drive fans sound like the answer
rather than a detection system. More complexity is not
always better.

JR
but people sometimes feel like they got something for their money when it has a lot of gadgets added to it;)
 

Jraef

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San Francisco Bay Area, CA, USA
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I've done this on belt drives for large machines where it's important to know when a belt breaks because although they have redundancy, once one breaks, you have to assume they are all the same age and the rest will go soon. So the first break is an indicator, not a disaster and allows you to replace them all when convenient rather than when shut down while the boss is screaming at you and the guys cleaning out the machine wearing hazmat suits and breathers on a 100 degree day are plotting your demise...

Anything that makes contact with the belt is going to add wear to the device, and the belt. That's why I used photo prox. switches. You'll need one with automatic gain control so that it punches through the eventual buildup of belt dust on the lens though. I'm no longer tuned into the sensor world, but I'll ask around for a reasonable suggestion and post back later.
 

Jraef

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Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I've done this on belt drives for large machines where it's important to know when a belt breaks because although they have redundancy, once one breaks, you have to assume they are all the same age and the rest will go soon. So the first break is an indicator, not a disaster and allows you to replace them all when convenient rather than when shut down while the boss is screaming at you and the guys cleaning out the machine wearing hazmat suits and breathers on a 100 degree day are plotting your demise...

Anything that makes contact with the belt is going to add wear to the device, and the belt. That's why I used photo prox. switches. You'll need one with automatic gain control so that it punches through the eventual buildup of belt dust on the lens though. I'm no longer tuned into the sensor world, but I'll ask around for a reasonable suggestion and post back later.

I asked one of my sensor buddies, he suggested this one.
http://raise.rockwellautomation.com...ents.asp?CID=F00CB097AA854CD5838EC435A2DC1B9F
"Dark Operate" means the sensor status is always On, changing to Off when the receiver no longer gets the reflection back, ie the belt breaks. Wire the outputs in series to a relay and/or indicator.

phel42ef_ph.jpg

Note (from unfortunate experience):
Mount them in such as way so as when the belt breaks, it doesn't wipe out the sensor...
I ended up mounting my behind a piece of angle iron so that the leading edge of the broken belt would always hit the iron, not the sensor, then I strapped the sensor cables up under the iron.
 
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