high current issue

[jumacao]

We have a fan motor in a chamber. We observer a current above high limit in one fan. WHat happen?

 

[tygerdawg]

Could be MANY things. My first thoughts would be:
(1) blocked or obstructed fan
(2) failing bearings
(3) corroded electrical connections
(4) failing motor windings
(5) blocked or obstructed ducts or filters


TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[MintJulep]

Blocked or obstructed fans, ducts or filters cause motor current to go DOWN, not up.

 

[DRWeig]

MintJulep is correct -- broken ductwork joints or missing seams cause current to go up (excess airflow).

Good on ya -- let us know what you find!

Goober Dave

 

[zekeman]

Few other possibilities:
1) Voltage at fan too low
2) Wrong run capacitor if PSC motor
3) Running fan "wide open" meaning insufficient ducting.
I assume you are running multiple fans from your description.
In that case, stop all other motors and just test the one in question. since voltage drops from insufficient wiring could be possible. Or just swap motors to see if location is the problem.

 

[electricpete]

Blocked or obstructed fans, ducts or filters cause motor current to go DOWN, not up.

I believe it depends on the type of fan. Would be the case for radial flow fans. Would not be the case for axial flow fans.

First suspect is usually the fan/pump and fluid system, not the motor. Compare air flows and pressures if possible with other similar loops or with fan curve (which includdes also BHP can be converted to current). I agree with Zeke about swapping motors for a check.

Is it a 3-phase induction motor? Check current balance (are three currents reaonable close). Also check voltage is possible (low voltage is a possible source of high current).



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[racookpe1978]

stopped/slow motor causing "low current"? Usually, the opposite occurs.

High current sensed into controller <= excessive draw (in the motor) <= reduced EMF (in the motor) <= stopped or reduced speed motor <= blocked fan.

A too hot motor gives excessive resistance gives extra current flow to get same rotation speed. That could come from plugged up cooling vents or a very, very, very dirty motor and bad fan blades.

Bad bearing giving you too much mechanical resistance to rotation? Direct drive fan or belt and pulley?

I'd get an optical speed sensor and check performance.

 

[electricpete]

stopped/slow motor causing "low current"? Usually, the opposite occurs.

It's not clear who you are talking to because no-one before you in this thread has said anything about stopped.slow motor.

However your comment
"High current....<= blocked fan"
would be correct for an axial flow fan, incorrect for radial flow fan. My point was it depends on the type of fan.

"A too hot motor gives excessive resistance gives extra current flow to get same rotation speed."
Sounds like gibberish to me. For an induction motor the biggest effect of temperature is increase on rotor resistance. Higher temperature -> higher rotor resistance -> higher slip for a given load level (derivable from the equivalent circuit). To get to the same speed (slip) at higher rotor temperature, you'd need lower load which would equate to lower current.

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[racookpe1978]

I was assuming (without a sketch granted) that the motor shaft and drive shaft of the pump are linked.

Thus, fan speed - regardless of type of fan - will be proportional to motor speed. And that the motor current will behaive all other common AC motors: EMF, current, speed, etc.

 

[mauricestoker]

Is the fan direct drive or belted? Was this start-up? Any previous history? You've given about enough information to tell what color your shirt is by mentioning your age. Maybe a squirrel got out of the cage and got its tail stuck in the sheave (if belted).

 

[electricpete]

I was assuming (without a sketch granted) that the motor shaft and drive shaft of the pump are linked.

Thus, fan speed - regardless of type of fan - will be proportional to motor speed. And that the motor current will behaive all other common AC motors: EMF, current, speed, etc.

All correct. Does not change the fact that your previous comments were incorrect.

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[electricpete]

To hopefully bring this subdiscussion to a close:

High current ... <= blocked fan.

Is not correct for all types of fans in all operating ranges. Look at the BHP vs flow curves, you will find many that have lowest BHP at lowest flow.

"A too hot motor gives excessive resistance gives extra current flow to get same rotation speed."

Let's motor is at 100% load and is 90% efficient. Half of the losses are I^2*R losses (total of 5%). Now increase the temperature by 100C. Resistance increases 40%. This causes two effects: a small one and a big one:
Small effect: I^2*R losses go up by 40% from 5% to 7% and current increases by approximately 2%
Big effect: Rotor resistance goes up by 40% which means the slope slip vs load up by 40%. Load would need to go down 40% to get to the same slip which would cause current to decrease by 40%.

Sounds more complicated than it needs to be? Yes because we brought in the idea of maintaining the "same rotation speed" (It was not me that brought it in). The relevant aspect for this thread... in this extreme case 100C increase we might possibly see 2% increase in current from losses (or less if the load decreases due to that increased slip).


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