Blower question. Is starting a blower "in the free" a heavy start ?

JesperMP

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JesperMP
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Hi.

OT but surely someone here knows:
I have a blower that is not yet connected to the pipeworks, neither on the inlet nor on the outlet side.
Still, I would like to test my motorstarter setup with a softstarter (so I dont have to wait forever for the pipe connections to be finished).
When I try to start the blower, the thermorelay trips after approx 30 seconds. I guess that the blower is 75% through the start at this time.

Is my thermorelay too sensitive for the heavy start ?
Or is this an abnormally heavy start ?

I am wondering if, without the pipeworks being connected, does this make the load extra light, extra heavy, or medium ?
I know that some blowers or fans have throttle elements that are completely closed at startup, to ease the load so the startup time is shortened.
When there is nothing connected, one should think that the air simply goes the short way from outlet to inlet - in other words a light load. Or .. ?

Maybe someone have tried this.
 
Jesper,

I know this sounds counter intutive but in your case can you block the input side of the blower? Perhaps with a sheet of plywood or something simular. Blowers actually unload when restictions block the flow. With all the ductwork installed the amp draw will be less than when the blower is operated without any ductwork.
 
Very many years ago, at the beginning of my carreer.... my boss asked me this question:

"What will happen if I block the outlet of this blower? Will the current go up or down?"

My answer was: "It will go up, because the blower has to work harder"

WRONG ANSWER...

You can easily verify this by blocking the inlet of your vacuumcleaner, the speed of the motor will go up NOT down.

A blower is made to move air.

So if there is no air to be moved there is no load, all it does is rotate the air inside the blades of the propellor.

You can guess what happens if it's the other way around, when the incoming and the outgoing air are unlimited....

 
I have had this problem with blowers and soft starts, it is the long overload time as the blower starts up, you need to use a class 20 or 30 overload. milldrone is right about blower loads but if your system does not have a damper for starting then this will be a problem when the pipework is installed. A better quality soft start with a built-in adjustable thermal overload may be required.
 
I learned the same on a large vacuum system... the hard way :oops:

By opening the inlet and letting the blower move more freely... it let the magical smoke out of the pecker head, it was a 75hp motor so it was a very expensive lesson :D
 
Yes I do know that the load goes down when you block the flow, thats what I meant with a "throttle element".
The blower is sitting 8 m high up, so I dont want to perform a test if the result is given (=lazy boy).

I couldnt get my head around how much power is needed to run the blower "freely". I kinda thought that maybe the short distance from outlet to inlet (low resistance) would offset that the flow is big.
I can see now that there is no doubt about it. The blower will start easily when the installation is complete.

Thanks for spending your weekend time on me. :)
 
JohnW said:
I have had this problem with blowers and soft starts, it is the long overload time as the blower starts up, you need to use a class 20 or 30 overload. [..] A better quality soft start with a built-in adjustable thermal overload may be required.
Click to expand...
I am investigating this as we speak.
The softstarter has adjustable class setting, but not the overload that is sitting in front of it (so it has class 10), and it is the overload that trips.
Someone goofed up it seems.
It is possible that the overload will just barely be able to allow the blower to start when the ductworks is finished.
 
Something still seems wrong here. GIT stated in the first post that the fan had only gotten to 75% of full speed when the OL let go. It seems to me that, even with no restrictions, at 75% speed and less, the load should be quite low compared to full speed.

Remember that, when starting a fan, the only real load is the fan wheel inertia. That makes the acceleration rate the primary determinant of motor load and amps up to about 3/4th speed. Above that point, the load due to moving air comes into dominance and controls most of the motor load on up to full speed.

Watch the motor current as you are ramping up to speed. If it stays below about 300-350% of FLA, you simply can't start much softer than that. If it is higher than that, lengthen out the accel ramp and see if that helps. Remember, overload classes are only meaningful when starting across the line. I would set the overload curve out to Class 30 and then let the ramp rate control the current levels.

Even a motor with no load at all will require around 275% FLA to start turning so that is your baseline for lengthening your ramp.
 
Something still seems wrong here. GIT stated in the first post that the fan had only gotten to 75% of full speed when the OL let go. It seems to me that, even with no restrictions, at 75% speed and less, the load should be quite low compared to full speed.

Remember that, when starting a fan, the only real load is the fan wheel inertia. That makes the acceleration rate the primary determinant of motor load and amps up to about 3/4th speed. Above that point, the load due to moving air comes into dominance and controls most of the motor load on up to full speed.

Watch the motor current as you are ramping up to speed. If it stays below about 300-350% of FLA, you simply can't start much softer than that. If it is higher than that, lengthen out the accel ramp and see if that helps. Remember, overload classes are only meaningful when starting across the line. I would set the overload curve out to Class 30 and then let the ramp rate control the current levels.

Even a motor with no load at all will require around 275% FLA to start turning so that is your baseline for lengthening your ramp.
 
Sorry about the double post. One other thought. Some softstarters have an output relay that can be programmed to operate when the starter reaches full conduction. If yours has that feature, you can arrange a contactor to short across the OL until you are up to full speed at which time it opens and protects the motor normally.

You may even have a feature in the software which permits the OL calculation to be blocked until up to full speed thus accomplishing the same thing as the above contactor.

I've seem both of these used to get high inertia loads going.
 
DickDV said:
Something still seems wrong here. GIT stated in the first post .
Click to expand...

JesperMP are you trying to take over my name?... there is only room for one GiT on this fourm ;) ;)
 
DickDV said:
Sorry about the double post. One other thought. Some softstarters have an output relay that can be programmed to operate when the starter reaches full conduction. If yours has that feature, you can arrange a contactor to short across the OL until you are up to full speed at which time it opens and protects the motor normally.

You may even have a feature in the software which permits the OL calculation to be blocked until up to full speed thus accomplishing the same thing as the above contactor.

I've seem both of these used to get high inertia loads going.
Click to expand...

On the one hand, I like the concept of bypassing the OL's during start-up because you could then select or set the OL's to be more SENSITIVE at normal operating speeds......on the other hand I was wondering what would happen if the overloads were bypassed and start-up was attempted when the motor or it's load were locked or jammed in some way? (corrosion, lack of use, foreign object, etc.)

Stationmaster
 

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