40HP Blower

mrtweaver

Member
Join Date
May 2007
Location
Bloomsburg
Posts
329
I am not sure if this is the proper place to ask this question but thought why not try and see what happens.

I am not even sure I can properly pose the question so if it is confusing please ask for further details.

This is also what you get when you have a few days off and come back in on a monday morning.

We have a 40HP motor driving a blower assy. The blades on the blower assy are straight and not curved in anyway. The outfeed of the blower goes to a silo type device then into a compactor. The infeed of this blower comes off of the production floor and the vaccum that is created is used to transport the trim waste from the production floor to the compactor.

Now the big questions are:

How does the restrictions of outfeed and infeed affect the operation of the motor? IE: If you have less restriction on outfeed then you do on infeed what happens to motor current and does the motor have to do more or less work? Or if you have less infeed restriction than outfeed restriction what happens to motor current and does the motor have to do more or less work?

The problems we are having right now is we placed a VFD on the motor and sometimes the drive goes into over current and shuts down. It had been working fine until some of the dust collection bags in the silo were changed out. So this seems to tell me that there is less restriction on outfeed so motor is working harder and that is of course meaning the amps go up and hence the trip. The only way we were able to keep it running right now was to reduce the speed from 60hz to 52hz.

Looking for any feed back that would help make more sence of this in laymans terms for supervision that has the famous last words of 'I may be oversimplifing things but...'

Thanks and have a great day.
 
The simple answer to your question is that the more restricted in- or outfeed is the less work the motor does. If Infeed or outfeed is totally closed, the only job the motor does is to keep the over/underpressure at its maximum level. And that is a minor task. The hard job for the motor is to move big quantities of air. So reducing speed or restricting the flow will do the same thing, i.e. reduce the load on the motor.
 
OK. not an expert here but I have some experience with dust collection systems. I don't think it makes any difference if you restrict the inlet, outlet or both, it is just about how much air the fan is shifting and the mechanical resistance of the fan assembly. I mention the mechanical resistance bit because I have seen systems overcurrent when a bearing went bad. I think you can block the inlet or outlet and see what current is being drawn, it should be pretty close to off load, if not you have a bearing or mechanical alinement issue. Otherwise the load will come on as the cube of the speed, so dropping from 60Hz to 52Hz is a huge reduction in load.

Bryan
 
Look up affinity laws - they stipulate for centrifugal pumps and blowers that
Flow is directly proportional to speed
Discharge pressure squares as speed changes
Power cubes as speed changes.

NEVER throttle intake on cengrifugal pumps. You can with blowers however but I believe you will heat the squirrel cage due to lower cooling flow just as you get in pumps - with pumps you can burn out the shaft seals.

On blowers restricting suction or discharge reduces flow thus power. Running wide open on both suction and discharge increses flow thus power. Flow x head = power. This is exactly what you saw when they FINALLY cleaned the filters.

With this system flow is what you want to control - actually you want to control the transport velocity to ensure the junk gets conveyed clear thru the duct work to the filters. People will argue it is head / pressue ie "suction" but that is not true no flow no transport = plugged duct.

Dan Bentler
 
I would watch the motor current from the inverter display. Make sure there are no sudden increases in load which the VFD is sensitive to but that a thermal overload might not trip out on.
 
Power cubes as speed changes.

This is key.

Are you overspeeding the motor with the VFD (above base speed)? If so, current will go up exponentially.

Also, if there are major restrictions in the air flow, the same air tends to go around and around in the blower, heating up, which can lead to binding and overcurrent conditions.
 
First, power for a centrifugal fan or blower is proportional to both the flow rate (CFM = Cubic Feet per Minute) and the pressure difference across the fan from inlet to outlet. Pressure capability, power, and flow rate of the fan or blower is a function of inlet air density.

The operating point of blower is where the blower curve crosses the system restriction curve.

At constant speed, when you restrict the outlet you increase the pressure difference but you also shift the system curve upwards so that you reduce the flow rate. This results in lower power.

It is common to modulate air flow on a blower by throttling the suction - this is actually more energy efficient than throttling the discharge. Dan's right that you can create higher temperature rise, but this is not usually a problem unless you have very high pressure difference across the blower or extreme restriction. Another common problem with excessive inlet throttling or restriction with a blower is the potential to create a pulsating flow called surge. This is only a problem on higher folw and pressure units. Your blower manufacturer should be able to identify the minimum flow and maximum temperature limts for your specific unit.

At constant speed, when you restrict the inlet you increase the pressure difference across the blower and you also reduce the inlet density, resulting in less flow and lower power.

Dan's description of the fan laws identifies the way in which the blower curve is changed by changing speed. This changes the inersection of the blower curve and the system curve. When the blower is operated at a reduced speed the blower curve shifts down and intersects the system curve at a lower flow, which results in lower power.

So, if you are operating at full speed the VFD won't change the power required by the fan at all. One assumes that you put in the VFD because your conveying system doesn't always need 100% air flow.

There are a few things you can do to solve your problem. First, check the max amp settings of the VFD against the full load amps of the motor. If it didn't trip out across the line before it shouldn't do it now at full speed, as long as the old and new current limits are the same.

Second, most VFDs have a current limiting or rollback function. That means that if the motor draws more than the max amp setting the VFD will clamp the speed or reduce it until the amps are within limits. Make sure this function is enabled.

Third, adjust the acell and decell times. Sometimes you can get an overcurrent trip while the VFD is reducing speed because the blower has a high inertia and the motor turns into a generator, feeding current back into the drive's DC bus.
 
Last edited:
Tom you said
"Third, adjust the acell and decell times. Sometimes you can get an overcurrent trip while the VFD is reducing speed because the blower has a high inertia and the motor turns into a generator, feeding current back into the drive's DC bus"

Would this not be the (DC bus) overvoltage trip and not an overcurrent trip?

As far as dealing with this especially where the system is dealing with scrap (ie no control over particle size etc etc) I would
1. Install a discharge pressure read on blower
2. Install a differential pressure on the filters - mostly to know when to clean them.
3. Get system to operate with clean filters and set paramaters to allow for change in discharge pressue and or filter dP and CLEAN filters regularly.

Dan Bentler

Dan Bentler
 
When it comes to fans and pumps, Tom Jenkins is the authority on this board. I'd pay very close attention to his suggestions.

As to running this system on a VFD, I find it curious that no means other than manual is mentioned for varying the speed.

Tom may very well advise otherwise but, it seems to me that the proper control for a scrap transport system like this should regulate the fan speed so a constant velocity in the ductwork is maintained. That way, as load or filter bag plugging changes, the motor automatically adjusts to keep the velocity the same.

That would also minimize the energy consumed doing this work. For example, with clean filter bags, the motor would run slow and consume much less energy. Let the bags run in a plugged condition and, the motor will run faster consuming more energy to keep the scrap moving properly.

Kind of adds some motivation for keeping the bags clean! (Assuming someone cares, of course!)
 
Would this not be the (DC bus) overvoltage trip and not an overcurrent trip?

You're correct, Dan - I misstated the fault. Too short deceleration time will cause over voltage on the DC bus and too short acceleration time will cause over current. I generally adjust both together.

(PS Thanks, Dick)
 
One thing not mentioned is that blowers are the only fan that I know of that work against static pressure (ie: A centrifugal blower will force air through a solid wall of water flowing over a condenser where a centrifugal fan will have a hard time doing so unless there is a break in the wall).

My guess would be that the bags you are using have a smaller micron opening than what was there previously = more static pressure. Unless of course this is a centrifugal type silo (blower discharge into the side of a round tube, air discharges out the top while trash falls to the bottom). Then I would look for something choking the air discharge at the top of the silo.

Also, I find it unusual that the blower sucks trash through it and discharges it with air. Most blower transport systems I am familiar with, cotton industry, will have a blower suck clean air and discharge into a transport tube, usually at a corner. Multiple blower stations are used to maintain differential pressure. Advantage == your blowers don't eat trash and brake blades and don't clog up.

Throttling intake should work as you would not be trying to move as much mass against the same resistance.
 
Usually this kind of system should be designed to suck filtered air at the end of the conduit (after filter) like a big "shop vaccuum"
So particul mass should not affect really much the blower but filter restriction will become critical.

Do you have auto or manuel suction trap at some working station or something that can change intake restriction ?

DickDV mention a good point, you get an expensive VFD on your motor and you aren't using the energy saving potential of it. you should determine the suction pressure that you need and control the blower speed to keep it at is minimum with some "high speed jog" to remove pluggeg conduit few times in a day...
Having a diffenial pressure gauge or transmitter would also help as stated to know when to clean or change filter

For the current trip, like tom said, you should first verify the drive commissionning to make sure everything has been configured properly.
the points mentionned are the critical ones but take a look to all of thems.
If you still have problems, you may try to run it at few hertz lower (2-3) and see what's happend...Some times motor and VFD are designed very short from real power demand to save on design cost and using it at full capacity may give this result even with a well configured vfd
 

Similar Topics

I've got 6 identical regenerative vacuum blowers powered by 6 identical drives in 3 identical panels all fed from the same 1 main building feed...
Replies
27
Views
809
Good Evening , I have an " Oddball " Blower Motor , and really do not have any U.S. support . It is a manufacture called EBM Papst and the...
Replies
4
Views
1,114
Any takes on whether this is the end of GE and what will happen to their industrial automation products...
Replies
15
Views
6,444
Hello, Does anyone have an alternative idea to correlate the speed of a VFD on an aeration blower to a desired Dissolved Oxygen setpoint...
Replies
10
Views
4,354
A dust collector blower fan motor 18kW, previously run by a star-delta starter, has been replaced with a VFD( Brand: LS, Model: SV-iP5A). Our...
Replies
38
Views
11,709
Back
Top Bottom