Motor Amp Draw

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Could you give a little more info? I normally think of vacuum or negative pressure with suction. A decrease in suction pressure to me would equate to an increase in vacuum. I would answer yes, the amperage would go up, but this is not an area of expertise with me.
 
I believe the less air moved the less current the motor will draw.

Keep in mind a fan is not a positive displacement device.
 
The less air that it moves the less work that it does and the less current it draws. Take a shop vac and stick your hand over the nozzle. The thing starts to make a high pitched whine because it is spinning faster. This is because it is doing less work since you have cutoff it's supply air. Less work means less strain on the motor so it runs at a higher speed.
 
The suction side maybe should have been call the inlet side.

Picture a horizontal radiator with a fan above the radiator, the fan is shrouded down to the radiator, the outlet of the fan is up, the inlet is the area through the radiator. The whole assembly is mounted on legs.

It is outdoors. There are four of them in a small area.

Picture the fan running, if another fan starts could it "starve" the air volume below fan in question? I am thinking that if the pressure in volume below the fan (the suction, or inlet side) lowers (decreased suction pressure) that the fan motor will need to work harder to maintain speed.

Picture the opposite. If the pressure under the fan increases, won't the fan motor work less hard because the air under the fan would tend to "push" the air through the fan.

What is happening is that the motor is running just over FLA, there are maximium sized overloads installed, per Code. Every once in a while the thing trips out on overload. What is making the current increase? I know that a larger motor would fix it, but that is another story.
 
The concept is simple, the more it works the more current it draws. When it's moving less air it's working less therefore it draws less current.
 
natfuelbill said:
Picture the fan running, if another fan starts could it "starve" the air volume below fan in question? I am thinking that if the pressure in volume below the fan (the suction, or inlet side) lowers (decreased suction pressure) that the fan motor will need to work harder to maintain speed.
Click to expand...

Actually along with doing less work because it is moving less air (air has mass, it takes energy to accelerate mass) the fan is now operating in a slight vacuum with less 'stuff' for the fan blades to hit and move out of the way.

Picture the opposite. If the pressure under the fan increases, won't the fan motor work less hard because the air under the fan would tend to "push" the air through the fan.
Click to expand...

In this case it is just as Trevor pointed out, less work means less energy.

Keep in mind a fan, like any centrifugal pump is not a positive displacement machine.

If you where to take a hydraulic pump (they are positive displacement pumps) and tried to throttle back the output you would overload the motor, assuming no relief valve or seal failure you could bring the motor to a complete stop.

If you tried to throttle back the inlet it would cavitate (soon destroying the pump) and the load would drop just like any fan or standard pump.
 
Okay, maybe my logic is onto to something - only it is reversed.

If the presure on the inlet side was higher, could the fan be working harder (draw more current)? Now the fan would be working harder because it is moving more stuff...

Being outdoors in a small "courtyard" area, I can envision a change in pressure during wind gusts.

I do seem to recall that one my guys mentioned that it seems to occur during rain showers...maybe the fan is moving more stuff...moisture laden air.

Again, what is happening is that the motor is running just over FLA, there are maximium sized overloads installed, per Code. Every once in a while the thing trips out on overload. What is making the current increase? It isn't voltage imbalance, loose conections. Again it may be a slightly undersized motor, but the manufacturer says that it is not.

Also, thank you guys for spending your valuable time resonding and discusing the items posted, I am grateful.
 
Starter contacts good? Measure voltage drop across starter, line to load side of overload. L1 to T1 etc, while it is in operation, or use non contact digital thermometer to scan the starter and overload block after it has run for a few minutes. A trouble spot is pretty apparent with either method if it is with the starter or overload.

Vibration has caused us grief in the past especially on high rpm fans.
 
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