Electric Motors with VFD 1

[gumper00]

Is it common practice to operate a motor above it rated nampeplate RPM using a VFD? The direct drive fan has a 900 RPM motor but the design fan RPM is 1,250 RPM. The vendor is saying you can use the VFD to operate the fan at design RPM.

 

[MintJulep]

Which vendor?

 

[gumper00]

Does it matter?

 

[MintJulep]

I doubt that the VFD vendor knows anything about the motor or the fan.

I doubt that the fan vendor knows anything at all.

If you want to know if you can run a motor above the nameplate ratings, you need to ask the motor manufacturer. He will need to know how much POWER the fan will be drawing.

Remember that power is proportional to torque X rpm.

 

[gumper00]

Thanks, I will check with the MFR.

 

[RossABQ]

It is possible (with MANY caveats) to "overdrive" a motor with a VFD, but to do that on a new installation is stupid IMO. You usually see it when a facility needs to squeeze a little more flow/pressure from an existing installation due to changed conditions, and hopefully as a temporary situation or to meet an infrequent peak load, etc. To plan on it from the get-go to make design point smacks of a vendor trying to meet design requirements with the wrong equipment.

 

[Artisi]

if you have an existing fan / motor unit and you want to increase the speed from 900 to 1250 rpm, the power demand will increase by a factor of approx 2.7 - this is providing the fan can operate at this speed and the motor manufacturer is consulted about the motor at this speed.

I'm with RossABQ - something is wrong with the thinking on this project.

 

[simsd]

I'm curious,

who's going to pay for the new motor when the existing one fries?

 

[quark]

You can do it provided your motor has enough power to take care of the higher speed of the fan.

However, the increase in speed seems to be high. I have operated fans at speeds upto 20% higher than the rated speed of the motor. If your fan supplier is selling you the idea to reduce the cost of the fan, better avoid it.

There is a thread somewhere in the electrical fora that discusses this issue.

 

[sterl]

UNLESS THE MOTOR IS IN FACT THE RIGHT HORSEPOWER, JUST THE WRONG SPEED, or this whole installation is pretty small in scale:

You are going to need almost 3-times the horsepower of the current motor as per Artisi. And it needs to be a motor that will tolerate the higher speed as well as be VFD ready... Chances are the 900 wasn't balanced at 1450, and a 1200 might not hold balance either....And you will need to change your coupling hardware cause that new motor is most likely a different shaft size...And if you are going to live more than momentarily at the higher speed and horsepower draw you are going to change the coupling anyways cause its not going to be intended for that HP...And the overclocked 900 RPM motor is most likely going to make a lot more torque than a 1750 of the same HP running slow, so your anchoring may have to be modified.... And then you got to consider your conductors and circuit protection cause the new set up will want to draw MORE THAN 3-times the current, cause your 60-Hz motor is going to be running at 85 Hz. And you might encounter some inductions at 85 Hz & clipping that you wouldn't have with 60-Hz modulated. That means some analog conductors and instrumentation may start acting up.

Short version: Can of Worms.

 

[gumper00]

Thanks for all your responses. I have some more info that might make a difference. This is a completely new installation. There are numerous fans where this is taking place. The new info is two plug fans, each 37500 CFM for a total supply CFM of 75000, at 41.64 BHP each with 50 HP and 900 RPM. They want to run the fans at 1050 RPM. Another one is a 6000 CFM fan, 1800 RPM - 7.5hp motor. They want to run that fan at 2275 RPM.
The vendor sais these motors can do it and they are smaller frame sizes.

 

[Artisi]

Lots of luck - increase in speed increases the power required by the cube of the speed change - and wishful thinking, wrong or stupid advice won't change it.

7.5hp at 1800 is 15hp at 2250 rpm

 

[MintJulep]

Ok, one more time.

The HP number that is written on a motor is the MAXIMUM power that motor can continuously produce without destroying itself. (Subject to modification by the service factor)

A motor will attempt to produce as much or as little power as necessary to turn the connected load.

A synchronous AC motor will valiantly attempt to rotate as fast as the field frequency - right up until it either overheats, overspeeds its bearings, or the slip angle gets so large that it dies in whatever mode an excessive slip angle causes.

You need to look at the PERFORMANCE CURVE for the FAN. At 6000 CFM against the static pressure that is imposed by your system, what is the POWER that the fan requires. If the answer is less than the 7.5 HP rating of the motor, then the windings will PROBABLY not overheat. You still need to look at the bearings.

 

[Artisi]

MintJulep -- I don't think that is entirely correct if you want to increase the speed by VFD - it should depend on the capability of the cooling fan and probably a few other factors I'm really not sure about and - the power will only increase lineally not as the cube of the change which is required in this case.

 

[RossABQ]

Gumper, why can't the vendor provide a fan that meets spec at rated motor speed? Fans the size of the larger ones are pretty much custom anyway, I mean they aren't sitting on a shelf. Or maybe they are -- cancelled orders he's trying to unload them on you? This just reeks, IMO. None of my clients would even consider it if I proposed such a set-up; their specs all call for a minimum of 20% reserve capacity without motor overload or exceeding nameplate RPM and service factor. What would you do if the required volume was found to be even 5% more than present spec? What if your system curve is a quarter inch off? These things happen all the time. Leave yourself a cushion!

 

[itsmoked]

Here's another method. ALL VFDs have current readouts. You are free to turn up the speed ignoring all else (except maximum allowed blade speed) until you reach the motor nameplate Full Load Amperage(FLA). See if the resulting speed is sufficient. If it is, you're good to go. Of course you need a VFD in the first place.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[lilliput1]

Check the fan performance table. Usually it would have a formula that gives the maximum bhp = (RPM/Constant)^3
The value of the constant varies for each fan.