Nice curves. I'm all for nice curves.
But starting has to take account of inertia of the fan rotor.
BFF or soft start
- Thread starter Arkansas74
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But starting has to take account of inertia of the fan rotor.
that is why the fan design engineer selects a matched motor
a fan is low interia
a blower may not be
a fan is typically an easy load to start, provided it is matched to the correct motor
usually nema design b
Not in my experience.
We did a few variable speed fan drives in cement works in the 3,000 HP range. Low inertia they were not.
But what do I know.
this is 50 hp
we know this motor starts
so lr torque is sufficient to overcome wk^2
the issue is mechanical, the bearings
they loosened the belts to take force off the bearings
now they squeal
it will require the same torque regardless of starter type to get it spinning
the issue is not the motor stalling due to inertia/mismatched load
we are losing sight of the issue
reduced torque may mask the problem but won't help the root issue
they are burning the belts via slipping
OK.
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These two posts actually address some very important points.
- Fix the REAL problem, not the symptom. The stress on the bearings should not be a problem for them, IF there was nothing else wrong. So the fact that the bearings are wearing out fast might be indicative of a bigger issue. Loosening the belts will NOT fix anything, it will move the problem somewhere else and they will continually keep throwing money at it.
- Whether or not the the blower starts with the dampers open or closed may be a really good clue as to the real source of the problem. People who are not in the HVAC business get the following concept wrong almost 100% of the time; Starting a centrifugal blower with the dampers CLOSED means the motor will start EASIER. So typically blowers are designed to ALWAYS start with the dampers CLOSED. later on, someone sees this and thinks, INCORRECTLY, that this makes the motor work harder, so they OPEN the dampers. That has the EXACT OPPOSITE EFFECT, and the motor has to strain hard from the very beginning, which can cause all sort of mechanical issues, such as unexpected bearing wear, motor base deformation, pillow block wear, fan warping, etc. etc. etc. So if you look at it when it is off and the damper is open, that's very likely to be the nexus of the problem. Fix that and you should not have to loosen the belts.
Now once that's fixed, there IS a very valid economic case to be made to replace that damper control with a VFD. As a gross general rule, the energy savings they will attain can usually pay for the cost of the VFD within 18 months, sometimes less depending on how often it runs at reduced flow. If that's of interest, you can go to this website and enter some data points, it will calculate your energy and cost savings.
If you don't know all of the questions it asks and want a "guesstimate", you click on the "Load Sample Data" link. That loads a compiled set of "metadata" points taken from all of the other users loading similar fans into the site and averaging them. It gives you a reasonable estimate.
depending on application a modulating damper or valve can reduce hp similar to a drive
factors like throttling range, pressure drop, cost of $, pump eff/operating point, hp, etc
some think starting with a damper or valve barely open increases power
power ~ mass flow air or fluid
pressure/head start to factor once moving 'stuff'
deadhead uses no appreciable power
took me years to convince this older civil pe, still don't think he believes me lol
factors like throttling range, pressure drop, cost of $, pump eff/operating point, hp, etc
some think starting with a damper or valve barely open increases power
power ~ mass flow air or fluid
pressure/head start to factor once moving 'stuff'
deadhead uses no appreciable power
took me years to convince this older civil pe, still don't think he believes me lol
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You might show the venerable PE how a vacuum cleaner motor speeds up when you block the air flow. I find it a very a very convincing demonstration.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
I've used that exact same demo, and yes, I've had to convince more than a few Civil PEs of this.
The MEs know it of course, but one of the MEs I've run into somehow "forgot" to mention it when I was called in to fix an "electrical" problem on some 600HP squirrel cage blowers. They kept burning up motors, so they added soft starters and then couldn't get them up to speed before they tripped. I got there, took one look at the dampers cranked all the way open and told them the problem. They didn't believe me because their ME had been all over it when the motors were burning up and never mentioned it, so they called him in. He came out to the building, took a look at it and said "Oh, yeah, he's right...", and left. I closed the dampers, they started like a dream. Now, why he hadn't stepped in before I was called I'll never know, but I'm grateful because I billed them for the entire 3 day trip there and back!
that's clever
Incorrect.
Lower torque will just take longer to get to the same speed.
kwired
Electron manager
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Installing a VFD and modulating blower speed while in operation - probably not a good idea. Just ramping the thing up to speed when starting vs, giving it "full york" can have advantages. The burner likely goes through a purge cycle before ignition, purge cycle probably doesn't start until a pressure switch proves there is sufficient air flow - so what if it takes 10-15 seconds to start vs 5? Purge cycle won't start until there is pressure either way.
I had pretty much exact problem you are describing maybe about 10-15 years ago on a boiler for one of my regular clients. It only driven by a 15 hp motor, but same thing, tighten belts so they don't slip when starting and then they end up replacing bearings pretty frequently. This blower runs at same speed or possibly even a little faster then the motor, so and with it's mass doesn't accelerate in an instant. Decided to try soft starter on it, had pretty noticeable results in maintenance issues right away, not going through so many belts or bearings.
My application was a steam boiler. Depending on steam load, it may only start once an hour, maybe even two hours when plant isn't in operation and not heating season, but on higher demand may start 4 or 5 times an hour.
Or lack of inertia. No motion at all I guess is still inertia - just on the opposite end of the spectrum of when running at normal speed, or the middle of the spectrum for something that can operate in reverse direction.
Boiler possibly could have had better design, but I wasn't going to redesign it. Something other then belt drive likely would have been better. Working with what I had, soft start seemed to be the best balance of how to handle this.
Exactly what happens in my application, takes longer to get to speed but works just the same once it gets there, plus we don't go through so many belts or bearings anymore. Only thing I did wrong first time was not put in a big enough soft starter. Yes it was good for 15 HP, but not really for a higher inertial load or as many starts per hour as we sometimes get. When first one failed researched a little more and figured I probably needed a larger one in the first place so replaced with a larger one, and have been going well for some time now.
The boiler I worked on I think has a damper, it is on mechanical link with gas valve. Does not fully close, but is in "low fire" position when starting the blower and stays in low fire position all through ignition cycle. Won't proceed through ignition cycle if not proven to be in low fire position.
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