240V VFD substitute?
240V VFD substitute?
(OP)
I'm having a problem with a control system which requires speed control of 2 fans for coolers on a 25 MVA transformer. The fans are 5 hp and will be run at two frequencies: 45 hz and 60 HZ. The manufacturer has suggested using a VFD to achieve this. However, I really don't have the room in my control box. I'm already trying to fit 10 gallons into a 5 gallon bucket. Unfortunately I am unable to install and additional or larger control cabinet due to space constraints.
I was thinking of an saturable core reactor control or something to that effect. Does anyone forsee any problems with that? Are there any other possibilities?
I know it's seems ridiculous to run a cooler fan at reduced speed, but the reason for the dual speed is for noise limitations.
Any help or advice would be greatly appreciated.
I was thinking of an saturable core reactor control or something to that effect. Does anyone forsee any problems with that? Are there any other possibilities?
I know it's seems ridiculous to run a cooler fan at reduced speed, but the reason for the dual speed is for noise limitations.
Any help or advice would be greatly appreciated.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.





RE: 240V VFD substitute?
OTOH, working with reduced voltage isn't a solution if your fan motors do not have a suitable rotor (like NEMA D) so they can be voltage controlled.
What kind of motor do you have?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 240V VFD substitute?
Otherwise, you may consider getting a VFD with a proper enclosure that could be mounted outside of your existing control cabinet.
RE: 240V VFD substitute?
RE: 240V VFD substitute?
If, as in most of the transformer cooling fans I have applied VFDs to, they both run all the time, you can use one 10HP VFD running both fans. Keep in mind that means you only need one CB to feed it and you get rid of the contactors. Chances are you can shoehorn that single 10HP VFD in place of 2 x 5HP combination motor starters. There are some very nice "bookshelf" style VFD packages out there, meaning narrow deep boxes. I have used that trick several times because I can often go with a deeper box when footprint is restricted.
You can also look at mounting the VFDs outside of your box, using an outdoor rated VFD package. There are also some VFD/Motor combinations that you can use on the fans themselves, where the drive is integral to the motor.
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RE: 240V VFD substitute?
As jraef pointed out, he can run both motors on a single drive mounted outside if the existing controller.
RE: 240V VFD substitute?
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If we learn from our mistakes I'm getting a great education!
RE: 240V VFD substitute?
RE: 240V VFD substitute?
Are the cooling fans fitted to any of the other manufacturer's IP-rated drives any better than those used by A-B? And by 'better', I mean a properly protected fan which will survive in the type of environment where you need an IP55 or better enclosure, like the steelworks so beloved of A-B's promo graphics people and which pops up on the cover of a lot of their literature. Rockwell did show a little interest initially but never followed it up other than to tell us that it had 'passed tests'. We ended up using venturi airmovers to cool the external heatsinks, and a product which could have been a real winner was blacklisted.
I'd have a really long hard thing before using a VFD on a critical transformer if it had to be mounted in the local control cubicle. Use a multi-stage switch to bring in cooler fans as required instead.
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If we learn from our mistakes I'm getting a great education!
RE: 240V VFD substitute?
RE: 240V VFD substitute?
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 240V VFD substitute?
Bill
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"Why not the best?"
Jimmy Carter
RE: 240V VFD substitute?
Should have been said earlier; voltage control on an AC squirrel cage motor, be via it mag-amp, SCR or even connecting in Y, will produce lower TORQUE, but speed is based on frequency and the number of poles; voltage reduction does not change that. Speed reduction as a result of voltage reduction is consequential to the lower torque and the amount of load. In other words, you are not really changing the speed of the motor, you are weakening it so that it slips more. So a basic Design B every day motor will actually over load if you do this unless the torque requirements of the load drop off at the square of the voltage reduction. Even then, any change in load will cause the speed to vary. What Gunnar was alluding to is that there are AC motors, such as Design D, that can operate at high slip speed continuously without damage. That still is not really "speed control". How that would relate to your situation is that if you did use a Design D motor and dropped the voltage to a level that resulted in a 1350 Srpm speed when turning those fans, if the air density changes, the speed will change too. You could, with an SCR controller and a feedback loop, alter the voltage to try to maintain speed, but for all that trouble, a VFD would be a LOT simpler to implement.
By the way, your mag-amp idea would not likely be any smaller than a VFD, and certainly would pose as much if not more of a ventilation problem.
I say don't make this more complicated than it needs to be. Find a way to take advantage of established technology and make the VFD work. take a look at other things you have in the control panel. Many VFDs now come with a lot of 'smarts" built-in and may be able to take on some of that functionality themselves, freeing up some real estate in your box.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 240V VFD substitute?
(*) = star
RE: 240V VFD substitute?
The fans are set to run at 45 hz all the time until the transformer gets to a certain temperature, then they go to a full 60 hz.
The problem is that this is a mobile substation. Mobile units tend to attempt to fit 10 gallons into a 5 gallon bucket. Space is very limited. So is weight. I'll see if I can mount the VFDs outside the control box. That is looking like my only solution. These mobile units are turning into a nightmare.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: 240V VFD substitute?
Keith Cress
kcress - http://www.flaminsystems.com
RE: 240V VFD substitute?
Despite the absurd claims of some manufacturers, VFD's operating in an ambient temp below 0 degrees C (32 degrees F), need a space heater. Drive life falls off dramatically when subject to colder temps than that.
Also, on outdoor units, extra care must be taken to avoid condensation inside the drive. As a general rule, if the drive is running and making heat, condensation will not be a problem but, if idle for a while and then started, a flash can easily spell the premature end for a drive with condensation in it.
RE: 240V VFD substitute?
http://ww
RE: 240V VFD substitute?
Thank you all for the help.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: 240V VFD substitute?
Sledgehammer to crack a nut sort of thing.
RE: 240V VFD substitute?
Thank you all. You've been an incredible help.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: 240V VFD substitute?
TurbineGen, note that I chose a 10 HP model assuming you can drive the two motor/fans in parallel. Also I believe that the J1000 VFD does not require any air space on the sides of the VFD which also makes them more space efficient.