VFD problems

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gilly

Member
I have a small problem with some VFD's that we are working on.
The VFD is a power flex 700 a 150 hp and a 600hp.The 600hp is a 700 H. The motors are connected to a fan, very large. If we run the motors with no load they run fine we are able to control everything we need. On the smaller motor with fan connected we are able to get to around 20 hertz and the amps are around 185A. The motor nameplate is 170 with a S.F. of 1.15. This should run up to 55-60 hertz and be around 185A. Could there be an electrical problem?? Or is this more than likely a mechincal problem?? The Problem I have is everything works great when the fan shaft is not connected. Does anyone have any experience with VFD's that might have come across a problem like this.
 

charlie tuna

Senior Member
Location
Florida
Re: VFD problems

gilly,
sounds to me you don't have the drive programmed correctly. like number of poles??? voltage? call the drive manufacturer and have them walk you through the program.

when i first started installing drives, it took me longer to get them programmed than to install!!
 

RJH

Member
Re: VFD problems

Gilly,

Fans tend to have high starting loads that are effected by temperature and humidity. Often a damper is required to block off air flow into the fan when starting. This relieves the mechanical load because you are removing the material that the fan is asked to move. The clue is the correct operation with no load attached. If the drive runs correctly it is usually a mechanical load issue.

Verify your VFD settings first of all; volts, Hz, ect. You may be able to add DC boost which increases the available starting torque. Many times the issue is an inability to get the fan to its rated speed before the drive runs out of power.

The fan Mfg. or whoever designed the mechanical air handling should be able to provide data that allows a VFD manufacturer to size the drive properly.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: VFD problems

How fast are you trying to ramp these fans up?

I was recently working on some 500 HP fans and the VFDs where set up to take about 5 minutes to ramp the fan up to 1780 RPM.

I had an amprobe on these as they started and saw no spikes, only a gradually increasing current till full speed.
 

dag456

Member
Re: VFD problems

I agree, check your VFD settings. You should be able to program it to graduallly (sp) ge up to operating speed.
Let us know what you find out.
 

tony_psuee

Senior Member
Location
PA/MD
Re: VFD problems

gilly,

What is the time period it is taking for the fans to ramp up to 20Hz? As others stated, I would check the VFD parameters, specifically acceleration time, current limit, and volts/hertz. A fan that size will be a fairly large inertial load and the VFD stopping at 185A could be current limit set in the VFD.

Tony
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: VFD problems

Can we assume the motors are correctly sized for the fans?

Posted by Iwire:

How fast are you trying to ramp these fans up?
For some reason, VFD's always seem to ship with the default programming set to "give 'er he!!"

:D
Steve
 

justdavemamm

Senior Member
Location
Rochester NY
Re: VFD problems

Many VFD's will hold at frequency when the output hit's FLA (+ something) until the amps come down.

Also, I presume the motors are Inverter Duty rated at the stated HP's ?

That's all I can add as all the other replies are quite valid.
 
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