Hey guys, not sure if im asking in the right place, but has anyone had success using variable speed drives in blowing fan situations?

What are the reliability, cost and performance like in this situation?

We have thought about it, but people here are nervous about using them in critical situations.

your thoughts and expirences would be appreciated.

We have about 30 critical AHU applications. The last two were 75 hp VFD units. I’m not sure I’d do it again. VFDs are simply less reliable overall than motor starters are. We had to ignore several default fault codes and enable flying start to make them acceptably brown-out and black-out tolerant.

Out of the two, we had one catastrophic failure. The time-frame for the rooms the AHU served go out of compliance in about an hour. The on call electrician jumpered around the drive and started the motor with the local disconnect. Not a good way to go.

Here’s a link to some discussion I submitted on this site awhile back:
http://maintenanceforums.com/e...321033192#2321033192

Part of my report is attached on the first post.

Good luck.

Hi trav,

VFD sometimes can be nuisance if you did not understand the functions and also the purpose of having it. Why do you need VFD from the first place? If you want to control variable load then good to have it with by-pass mode in the system. If you use it correctly and understand the VFD, it will give you the good result.

Hi Wally,

I am totally agreed with your critics and also the concern. I have came across some VFD just laid off due to unfriendly operation during the breakdown services. If you buy from the reliable vendor with all the good function in place then should not give you any problem. Nowadays, VFD has gone a lot of improvement. VFD is electronic driven and chances of hidden failure is very high unless you have good reliability analysis study and understand the component degradation process then it will help you better. Vendor has all the data if they carry out the maintenance service contract but less analysis being done to help the customer in reporting.

Welcome comment!)

Most AHU applications don't need to vary fan speed very much. Most of our fans run near enough to 60 hz (54-56 hz) that when the VFD fails, we are able to change the motor to across-the-line starting and the damper system is able to adjust to the slight speed increase.

Trav, if you want a VFD in this application, I'd definitely order one with a wrap-around bypass. And make sure the engineers design the fan speed so that it runs above, say 52 hz normally on the VFD.

I'm a fan of VFDs. We use mostly Allen Bradley. But we have had several failures of their drives.

Lammie, for the most part, capacitors and the thousands of electronic components in a VFD cannot be anticipated for failure. Most electronic items are considered random failure items. Most VFD manuals I've seen may recommend replacing capacitors after some time, but that's about it. The biggest thing with VFDs is to make sure they are clean and cool. Check the cooling fans regularly.

I guess the other thing to say about AHU applications is that the fan usually runs 24/7 at a fairly constant rate so there is less need or benefit of putting a VFD there.

Hi Wally,

Totally agreed with you the electronic can fail prematurely due to heat stress on the component if the cooling fan did not get it replace at all. The product life cycles become obsolete after 25 years and most of the manufacturer no more product support then you have to look into the junk store to get parts. Anyway, the ROI has been achieved and by the time should look for capex to get it replace.

Have you done any RCM and degradation mechansim for VFD on the critical equipment? If yes, I would like to learn from your expertise.

Lammie,
I was thinking about that very subject on my drive into work this morning. I cannot tell you a degradation rate, but I can guesstimate our failure rates. Overall, considering all sizes and all applications, I’d say we have a 9-11% failure rate on VFDs; and a 1-3% failure rate on across-the-line motor starters. Some of our oldest drives are still in operation, so I don’t think age plays a big part in VFD failure rate.

You bring up an important subject of obsolescence. I’d say we’ve replaced another 15% of our drives simply because they became obsolete- we couldn’t get replacement parts or entire unit replacements anymore. Considering total replacements over a 12 year period, I’d put VFD replacements at around 25-30%.

Hi Wally,

Can I said that the component age of the VFD is not a significant of failure? If yes, what is the VFD longest drive in operation you have operated? Have you carried out any Life Cycle Costing (LCC) of the VFD drive? If yes, I would like to get some tips from you.

Thank you for your facts and any VFD paper you have acrossed so that I can tap some knowledge on it.

Obsolescence is the power word to get the VFD equipment replacement under the CAPEX. If the FMEA has been carried out and identify the critical component part to be stored for replacement then obsolescent is not an issue?????

Hey Lammie,
So I checked with our Electrical System Owner and he agrees with my estimates. He also said they replaced some drives that were so old they had dip switches instead of parameters. I had to laugh at that.

To answer your questions:
Our oldest drive is perhaps 17 years old, many are 11 to 13 years old, and two of our newer buildings have many drives about 5 to 7 years old.

We also had a fairly wide range of manufacturers represented in our drives. ACTech, Allen Bradley, Toshiba, Reliance, TB Woods, Magnetech, Square D, and more, I’m sure.

We have not carried out any Life Cycle Costing.

Yes, I agree that critical component parts might get you by a bit longer. However, I’ve heard that drive capacitors can go bad if left on the shelf unenergized for 5 years or so. I can’t say that I’ve experienced this myself, but I heard it from drive manufacturers. They say to keep the capacitor active you should apply power to the drive every several years. I doubt anyone ever really does it.

Back to the subject at hand, I wonder about the reliability comparison between AHUs with VFDs and AHUs mechanical damper systems. Both will do the job; the mechanical damper system probably requires more maintenance overall than the VFD as a guess.

Hi Wally,

You can carry out the LCC on the drive and see the benefit of operating. I have the guideline to help to you if you are interested.

The electrolytic capacitor will get deteriotate with time due to leakage current. Just imagine the capacitor has the stored charged from the factory and mounted on the PCB, the stored charge will leak creeply to the surrounding circuit component with time. This is called leakage current effect. Most of the VFD drive failures are due to capacitor burst.That's is true, nobody do it to power up the module card and does not proper storage at all sometimes as I have seen so many sites.

VFD and mechanical damper both also acting the same functions of controlling the air flow. VFD is better option as this can link to BMS system to monitor the damper movement, the mechanical damper can be moved either motor control or manual adjustment. It is subject the engineers design decision on the facilities.

VFD will have an advantage over the mechanical movement if properly commissioned as the energy saving can be realised. Have you done any energy saving calculation by using VFD as compare to mechanical damper movement? Try it out and you will see the benefit of saving. Is your drive with closed loop system?

Have fun!:0

Yes, fun stuff. And I would be interested in seeing your lifecycle costing analysis.

I have some reservations about your comments on energy savings in AHU applications. I’d say that the motor loading on most AHUs is fairly constant. The motor may vary by only a couple of Hz if driven by a VFD. I don’t know what the current variation would be across-the-line in the same application; I haven’t taken those readings in a long time. If the motor is properly sized and operating in its most efficient range, and running continuously, how can adding the load of a VFD reduce overall energy consumption?

For energy savings, I think VFDs are most applicable when the voltage/current parameters can be reduced to match varying load, motor speed can be reduced to match run requirements (e.g. conveyors), or high start/stop duty cycle applications. But to install a VFD on a motor running in its sweet spot 24/7, probably won’t provide energy savings needed to cover the VFD purchase/installation cost. Perhaps I’ll be shown otherwise. I’d be happy to see other viewpoints on this topic. Also, consider that your power factor is negatively affected by the VFD and you might need to pay back the cost of line filters.

Hi Wally,

I have not carried out LCC for VFD. I have the guideline to carry out perhaps you can google in the net.

I am totally agreed with your remarks for VFD on the AHU as no energy saving can be obtained if the variation is narrow. Another gimmick of VFD in AHU operation, perhaps I can said that..... Someone need to put the paper to see the benefit of using VFD on AHU to convince the industries.Unfortunately, I do not deal with AHU at the moment if not I will make a point to create the case study on this and to be shared among engineers.

True....distortion of voltages will create bundle of harmonics in the system...investment of harmonic line filters another burden cost. Nowadays, the VFD emission of harmonics are regulated by standards and should not be a big issue unless you got the VFD drive from the backyard.

Trav,
Has your team decided? Along what lines did your discussions go?

Well, the new VSd's are installed in the plant and running great. We retro-fitted two 75kw VSD's in april to two forced draft fans in our refinery. Normaly they would run at 1500rpm with the IGV's heavily gagged at times to keep O2 levels on target. With the VSD's installed the motors are responding to the varing demand from the furnace perfectly - the IGV's are run wide open and most days the motor will only be doing 300-400rpm. And with O2 levels being spot on now, the furnaces are running at maximum effiency. So far savings are looking great - fingers crossed! thanks for all your comments