VFD V/Hz for large propellor fan ?

[TwoBlocked]

Expect to be re-visiting an Eaton Powerxl-DG1 VFD installation. It is one of two, belt driven, propellor cooling fans for a natural gas compressor. I think 40 HP each. Was looking at the parameter for the V/Hz ratio (parameter P8.4, ID 108, page 44 of link). My understanding is that "squared" would be the best choice for this parameter. The other options being "Linear, Programmable, and Linear + Flux Optimization"

Any thoughts or comments?

https://www.eaton.com/content/dam/eaton/products/industrialcontrols-drives-automation-sensors/variable-frequency-drives/powerxl-dg1-variable-frequency-drives/powerxl-dg1-application-manual-mz040004en.pdf

 

[__dan]

The fan should be a low inertia load so it should not need anything unusual. Usually the drive has default fan setting and you might not even know what it's doing internally.

Any question like that I would go to Eaton to ask them. Only they know exactly what they would recommend and why. I do know each drive some of the params get kind of crazy for unusual applications and I've seen even high level pros doing many of them, doing it wrong. There is a point it's best to call the factory with a question like that.

 

[petersonra]

Most drive manufacturers have recommended settings for different applications, or some kind of wizard to set those kinds of parameters. Best bet is to look in the manual and see what it says. Personally I don't think a fan application requires anything special.

 

[Jraef]

V/Hz is simplest and you will not likely notice any difference in any other mode. Squared just allows some minor energy savings at lower speeds where the motor is doing less work.

 

[TwoBlocked]

Thanks, guys. Taking a step back and looking at the larger picture, what I also need to do is dig into the configuration of the other, older VFD that is controlling the other propellor cooling fan. There is only one PLC/PID controlling both, so best to try to set the VFDs up as close to each other as possible.

Seems the vast multitude of parameters in VFDs are like solutions looking for problems...

 

[Besoeker3]

V/Hz is simplest and you will not likely notice any difference in any other mode. Squared just allows some minor energy savings at lower speeds where the motor is doing less work.

Variable speed drives was my forte and I totally agree with you.

 

[TwoBlocked]

Most drive manufacturers have recommended settings for different applications, or some kind of wizard to set those kinds of parameters. Best bet is to look in the manual and see what it says. Personally I don't think a fan application requires anything special.

Yeah, that's what I am used to seeing, too. Only anymore there is a list of hundreds of parameter and their setting, but next to no information on what they actually do or when to use them.

 

[TwoBlocked]

Found this article about the effect of different v/hz ratios. My take is that using the v/hz squared ratio is preferable to a linear v/hz ratio for non-displacement pump and fans.

https://www.eaton.com/content/dam/eaton/products/industrialcontrols-drives-automation-sensors/variable-frequency-drives/active-energy-control-ia04008001e.pdf

 

[drcampbell]

... using the v/hz squared ratio is preferable to a linear v/hz ratio for non-displacement pump and fans.

Yes. The power required to drive a propeller-style fan is almost proportional to the speed cubed.
(in theory, it's exactly proportional to the speed cubed, but in practice it's usually closer to the speed^2.7)

Is Bradford County in the mountains? It may be beneficial to make an adjustment for the reduced air density at higher elevations.

 

[TwoBlocked]

They call the area "The Endless Mountains" but we are only talking about 2000 ft. But what adjustment? Max out at 65 hz or something?

Also, checked on the VFD this is paired with, an Eaton 9000X. You can only customize the V/Hz ratio at one mid point. I am pretty sure this parameter was not used, but I'll check when the time comes. To keep the two fans pulling the same, I'll probable stick with the linear V/Hz ratio.

 

[drcampbell]

They call the area "The Endless Mountains" but we are only talking about 2000 ft. But what adjustment? Max out at 65 hz or something? ...

At 2000 feet above sea level, it's probably not important.

But what am I talking about? (gee, never heard THAT before!) Usually, it means turning the fan a little faster, to move a greater volume of air, to move the same mass of air, to achieve the same heat transfer. Sometimes, it means installing a bigger fan, or one with more blades.

 

[TwoBlocked]

Or a different sized pulley. As it is, I know the design is maxed out. One time they were limping along on one fan, due to a VFD gone bad, we tried to keep hobbling along with the other one at 62hz, but it would trip on overcurrent. Fortunately the spare-spare showed up the next day.

 

[drcampbell]

Or a different sized pulley. ...

That'll also work. I have a pretty big box of different-sized pulleys, and a couple of adjustable ones.

... As it is, I know the design is maxed out. ...

Were you at a significantly higher elevation, it wouldn't have been. The power required to drive a fan is proportional to the mass of air it's moving.

 

[Jraef]

The power required to turn the fan increases by the CUBE of the speed change. If the fan motor was selected based on a max speed of 60Hz, increasing to just 65Hz (108%) will put 127% load on the motor. So unless the motor was over sized when the fan was designed, you will trip on OL (if lucky, burn out the motor if not). The same will be true of changing a sheave.

Physics is not just a suggestion, it’s the LAW!

 

[drcampbell]

Yes, but torque and current are proportional to speed².

 

[TwoBlocked]

I guess they got things running ok. Never got called. Got called on another VFD problem, though, and am having trouble understanding something. I'll start a new Topic.

 

[garbo]

Have replaced VFD'S on supply& return fans on AHU'S & exhaust fans from 5 to 150 HP and would just tell the very talented salesmen the voltage & motor amps ( he always told me that drives are rated for ampere not horsepower ).Most of the time we would pay a small fee for tech to perform start up that extended the warranty & free labor out to 3 years. Some times I would program in the motor data & ramp up & ramp down times and he would program controls. Can not ever remember him changing the settings that you mentioned. We never ran the fans below 20% and on old air handlers had up to a 10 minute ramp up time in the winter to prevent a freeze stat from opening up. Some of these drives had well over 100,000 hours controlling 40 year old plain Jane NEMA frame motors with no major problems. Read some where drive capacitors only last 75,000 hours.

 

[Joethemechanic]

Seems the vast multitude of parameters in VFDs are like solutions looking for problems...

Yeah, for most applications I see, lol.

BTW I spent lots of time growing up in your neck of the woods, Morris Run, Canton, Troy, Blossburg. Are you old enough to remember The Mohawk Airlines Flight 40 Crash? I was only like 2 miles away, I still remember the sound of the sirens echoing off the hills all night

 

[TwoBlocked]

Yeah, for most applications I see, lol.

BTW I spent lots of time growing up in your neck of the woods, Morris Run, Canton, Troy, Blossburg. Are you old enough to remember The Mohawk Airlines Flight 40 Crash? I was only like 2 miles away, I still remember the sound of the sirens echoing off the hills all night

I was living in northern NJ at the time. Now I live very close to what was the highest point of the Pennsylvania RR. If you know your local history, there's a reason the confederate flag flies there.

 

[garbo]

Thanks, guys. Taking a step back and looking at the larger picture, what I also need to do is dig into the configuration of the other, older VFD that is controlling the other propellor cooling fan. There is only one PLC/PID controlling both, so best to try to set the VFDs up as close to each other as possible.

Seems the vast multitude of parameters in VFDs are like solutions looking for problems...

They had over 500 VFD'S in the large hospital/ research center campus that I retired from. My self or a union electrical contractor would replace bad drives & drives for replacing MCC starters. We would pay a extremely talented Danfoss tech maybe $200 to perform start ups that stretched out the warranty & free labor to 3 years. He had trouble with a drive and had to go to his truck to get a 1" thick book for programming. He made changes in one of two parameters to get the 75HP VFD . Told me he attended a two day class just on programming drives.