VFD Driven Blower Motor Failure 3

[Power Geek]

Application is a dust collector. Motor ran fine for a couple years, then they added some additional duct work and motors started burning out after only a few days use. Culprit appears to be excessive current draw. At 30 Hz the 25 hp motor is pulling 40A where it's nameplate FLA is 29A. I went through the standard troubleshooting checklist looking for shaft mis-alignment, bad bearings, un-grounded shield, proper wire configuration for the voltage. I started to focus on the V/Hz setting in the VFD then stumbled upon old threads with discussion about blowers without any back pressure pulling the motor all the way down the fan curve to the point of failure. Just today the client informed me that with the discharge of the blower blocked 50% they had favorable current draw.

Isn't there a way to set up the VFD to handle an open discharge configuration? That is, solve this electrically rather than mechanically?

 

[FacEngrPE]

No there is no VFD programming that can solve this power shortage, your application requires more shaft power at 30 Hz than the motor is able to provide. You could set the drive current, I[sup]2[/sup]*t, and possible a torque limit setting to suit the motor, the result would likely reduce the motor speed further to the point where the motor would over heat on lack of ventilation. Your dust collection system's capture performance would suffer.

You might be able to make this system work by adding a 2:1 reduction between the motor and the fan.
Another option is to reduce the airflow by adding resistance to the air stream, this is what your client reported.

Discussion about Fan Laws and the equations
Fan Law Calculator

 

[dArsonval]

Having looked at this post more than a couple / three times,
I'm convinced it's the first time I've read a comment, a question or an [idea concept] that
simplistically describes what's been going on for a long time.

"Isn't there a way to... solve this electrically rather than mechanically?

John

 

[jraef]

You can “solve” it electrically by enabling the motor overload protection in the VFD. I do not know of a modern VFD that does not provide motor overload protection, but on many of them, it can be disabled. If your motor burned up, it must have been disabled. The most likely scenario is that after the changes, the drive kept tripping, because the motor is too small for the task, so rather than address the real problem, someone chose instead to just disable the OL protection and allow the motor to fry.

Bottom line, on a centrifugal machine like a blower, LOAD = FLOW, so if you want more flow, it means there will be more load and therefore need more motor power.

A VFD is not a magic portal, it cannot make a 25Hp motor into a 30 or 40 Hp motor.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

Read the manual for the VFD.

There is a setting that limits the motor power. Set that correctly and the VFD will let the motor run right up to the speed limit that is it's rated power. This removes your control of the speed because this power setting will take precedence. The fan will never spin at the motor's rated speed as it's the wrong fan in an incorrectly designed (hacked) system.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Power Geek]

Before my first visit I was told they had an Allen Bradley PowerFlex installed. I arrived and found an ABB ACS310 so I was a bit unprepared to find the overload settings. I was initially thinking they simply had the setting too high. It didn't make sense that a VFD would allow an extended overload. I scrolled through the menus with limited success, not finding the overload settings. With that motor already dead I did have time to review the ACS310 manual and exchange a few messages with tech support before returning for the new motor start up.

When I returned for the new motor start up I did find the overload trip disabled. The onsite tech did implement a current limit of 40A after the new motor was installed which is why I picked that operating point for my initial post. The VFD was doing its job and limiting the current to 40A.

One of my professors once told me about the conservation of energy, the conservation of mass, and the conservation of sorrows. We never truly solve a problem, we just transfer it to someone else (preferably the mechanicals). It appears this is one of those times.

 

[Marke]

Any centrifugal load, will increase the motor load as the speed is increased, and generally, the fan or pump is very reliant on discharge pressure to limit the motor load. This is a function of the pump or fan operational curves and this is fundamental to operation of such a mechanical device.
The only way to compensate for the reduced back pressure, is to slow the fan down, which in effect is the same as reducing the size of the fan.
In some drives, there is a load limiting function that can be used to slow the motor to keep it operating within the motor limits. In all drives, there is the ability to cause the drive to trip if the motor is overloaded.

The best solution is to correct the mechanical problem so that the fan is operating on it's design curves, but there is a very common perception that a drive can be used to correct for all design issues.

The problem with reducing the fan speed, is that as you reduce the speed of the motor, you are reducing the power capacity of the motor, so you will not get the same level of work output. At 30Hz, on a 60Hz system,a motor can only deliver 50% of its rated output and if you run it at 50% of it's rated shaft load, expect the stator windings to run at an elevated temperature due to the reduced cooling of the motor. Every 10 degrees C hotter you run the stator temperature, you are halving the life of the insulation.

Best regards,

Mark Empson
Advanced Motor Control Ltd

 

[jraef]

“Conservation of sorrows”… that’s a good one, new to me.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RRaghunath]

Typically I have seen Electrical engineers in a factory are at the receiving end.
I have experienced operations staff and the mechanical engineers ganging up together to force the electrical engineer in shift to increase the Overload relay set point or disable it altogether with the excuse that production will suffer / targets will be missed.

 

[jraef]

I once had an argument with a pump vendor that was the epitome of frustration. I was hired to install a VFD and level control for a pump to maintain the level of a collection tank so that the pump would not create a vortex and suck air, losing its prime. I was given the 100HP pump nameplate to work from, selected the VFD and had my crew install it. I set up the level control system and started it up. Everything worked perfect, but because they system was shut down, we couldn't test it at maximum flow, however I (foolishly) didn't worry about that part as I was more concerned with the system response, not the equipment capacity.

The next day I got a call from the owner saying that the VFD was not keeping up and they were getting overflows. I went down there and took a look, sure enough it was doing exactly that. But the VFD was at 100% speed, so really, it was not a VFD problem. Then the maintenance guy said "Well, the OLD pump never had a problem." "Old pump? What "old pump"" and where is it?" He took me there and it was a 400HP PUMP! I found myself explaining the laws of physics to a wrench turner, he just kept saying "Well, the PUMP salesman told us this would work!"

The next day we have a meeting with the facility owners and said pump salesman, he proceeds to insist that this is a VFD problem and that I should just increase the speed of the VFD above 60Hz. I found myself explaining the Affinity Laws... to a PUMP SALESMAN! No matter how much evidence I could show him that what he was proposing would violate the laws of physics, he kept insisting that I was mistaken. The owner refused to pay me, I had to threaten them via a lawyer and in that process, explained to them that in any ensuing lawsuit, the pump salesman would be brought in to testify as to why his version of well known laws of physics would not apply here. I got paid (minus the lawyer fee) but was never asked to do another project for that facility.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RRaghunath]

jraef,
I find you as authority on rotating machinery and drives. Thanks for sharing your professional story.
Yes, these are professional challenges. We are lucky if we get knowledgeable client or at leat someone who respects knowledge and experience of the consultant.