Fan motor current unbalance 3

[lukin1977]

We have in operation a small centrifugal fan that is use to cool a DC motor
Fan Motor FLA is 1,5 A (picture attached)
Thermal relay is set to 0,80 A (its been set to this value for many years with same motor)

This past few days thermal relay started to trip. I checked motor current: 0,68 A / 0,70 A / 1,04 A
When the motor is off I checked bearings by spinning the fan with my hand and everything is smooth. Fan is directlly coupled to the motor`s shaft

What is the cause for unbalance current?
Does the motor need to be rewind?

Thanks in advance!

 

[jraef]

The insulation might be failing, yes. The simple check is to record the current values as you did, then disconnect and "roll" the phasing, meaning change the conductors, but not the rotation; move A to B, B to C, C to A. Then check the currents again and if the imbalance stays the same, its in the motor. If it follows the rolling, it's in the source. A small voltage imbalance will create a larger current imbalance by percentage.

Note that there are always some slight natural variations in circuit impedance of motor windings, so if the values change, that's normal. But if the issue IS a source voltage imbalance and there is nothing you can do about that, one trick is to roll the phasing anyway to take advantage of those natural differences to try to get better balance. You can do it twice of course, but the third roll is pointless (I know that should be obvious, I'm just trying to save you the embarrassment).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[lukin1977]

Thanks Jraef
Fully understood. Will do that and see what happends

 

[xnuke]

Just curious, why was the thermal relay set to a value below the rated amp draw of the motor? Typically, it's set above the nameplate amps by approximately 25%, not below it by almost 50%.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[lukin1977]

I dont know. We didn't set thermal value. I supposed it was set according to the real consumption of he motor while in operation witch is 0,70 A

Voltage at 3 phases arriving at the motor are equal: 378/379/379
We did the cables swap and the imbalance stayed the same. Problem appears to be the motor itself

 

[itsmoked]

It can also be the contactor. As it's contacts wear they start changing their contact resistance. You should measure the voltage across the contactor phases.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zlatkodo]

I checked motor current: 0,68 A / 0,70 A / 1,04 A

Hopefully is not measured with some cheap clamp-meter.

Problem appears to be the motor itself

Clean the motor and impeller. Sometimes, dirty impeller can cause an uneven air gap and unbalanced currents.
Check each phase resistance and inductance. Very often they are not equal even in production due to a variety of imperfections.
Note that the same phase resistance doesn't mean necessarily the same phase inductance.
But the question is: is it wort to deal with such a small and cheap motor? You can simply readjust the thermal protection and keep the motor working.

[URL unfurl="true"]http://winding.wixsite.com/design/single-post/2015/12/10/Amp-readings-vary-very-widely[/url]

 

[lukin1977]

itsmoked: there is no contactor. Fan is activated once main switch is ON

zlatkodo: clamp meter is of good quality. As you said, thermal has been increased to 1,5A. We could not stop the machine yet to do maintenance. When we do, we will disassembly the motor and clean everything. Also I will check wind resistance and come back with the results

 

[lukin1977]

update:

Fan motor has become noisy last few days

Today we dismantled the fan motor and found that the shaft is wear where the back ball bearing fits
Will be repairing the shaft and reinstall. We will also change both bearings and clean everything

 

[lukin1977]

Just installed the motor again and current is 0,68/0,72/0,80 A. The problem seems to be solved

 

[itsmoked]

There, you have it!

. . .

Keith Cress
kcress -

http://www.flaminsystems.com

 

[7anoter4]

I am late-as usually. However it is still a problem.
I have no experience with such a small [0.55 kW-0.75 HP] motor. In my opinion the measured current it is actually the no-load current [or close].
Since the gap is very small a small difference could change the no-load current. No-load current does not have to be equilibrated. In my opinion, +/-30% difference it could be acceptable.
Now, since we know it was a problem with bearings and it is solved I understood the bearing could displace [ eccentrically] the airgap and produce a no-load current unbalance.
However, the overload has to be set up : 1.25*1.5A and then no one could sense the unbalance.

 

[zlatkodo]

"Star" to Lukin for his photos and the feedback ( rare case in this forum).

Motor Repair and Winding Design