Do motors get "tired"? 1

[frostrobn]

We have a 30 hp, 1800 rpm, 480V, 3ph motor that drives an inclined screw conveyor that has been causing issues. At full capacity the motor trips out on overload at times, but has not done so at this capacity in the past. It has always run fairly high load at this rate (90+ % fla), but has not had an issue with chronic trips. I am investigating all of the mechanical reasons for higher loading like screw condition, bearing issues, verification of volumetric rate, etc. I am familiar with some of the electrical testing that can be done to check the motor, but I'd like to know if a motor over time can become less efficient requiring more current to obtain the same torque. Aren't there tests of the individual windings that can be done? Can anyone offer some information on this? If we were not getting down into a service factor of unity for the screw, and if the wiring had a higher rating, I would simply go to a 40 hp motor, but this is not possible at this time.

 

[oldfieldguy]

Yes, motors get "tired" but this is usually manifest in poor insulation as thermal stress, mechanical action and dirt make inroads. These things usually show up as a motor fault and tripped circuit breaker or blown power fuses.

If your motor is tripping out on overload, this isn't likely to be the cause. Take a look at your mechanical load.

The heater elements for some overload devices DO "weaken" in time as corrosion effectively raises the resistance and with it the heat produced per unit current.

I would be looking at hooking up a temporary recorder to the motor starter to see what the current is actually doing when it trips.

If it's a crew conveyor, make sure that your process is not dumping a big load on the screw, the outfeed from the screw is not backing up and you don't have bad hanger bearings letting your screw drag.

old field guy

 

[Marke]

Hello frostrobn

In addition to the answer from oldfieldguy, there is another mechanism which could cause you problems.
If the rotor becomes damaged due to excessive starting periods, starting transients or just the long term accumulation of the above, you can have rotor bars that go high impedance. Cracks can appear in the bars or attachment points to the end rings etc. In this case, the motor will not produce full torque and will slow down under load, and this will result in higher current tripping the overload.

Rotor bar damage can often be easily detected by running the motor open shaft and observing the current. If the current swings at a low frequency (equal to the slip frequency) then that is typically due to a broken bar.

Best regards,

Mark Empson
L M Photonics Ltd

 

[itsmoked]

I agree with all of oldfieldguy's comments.

More likely: shifting overloads: Switch to electronic if you can since the next step of bad overloads is single phasing your motor with disastrous results.

Increased internal conveyor resistance: Sliding surfaces, failing bearings, rubbing points or the belt surfaces increasing their friction coefficients.

Motor bearings: You can disconnect the motor and look for excessive play; radial or thrust, or just being too hard to turn.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jasonf987]

I have had a similar problem in my plant, we found that the speed of the motor was too high which in turn affected the torque we needed. We replaced our 6 pole motor with a 4 pole one which increased the avaialable torque. we also needed to include some supports for the screw as due to the length it was bending and causing the scrolls to break. since doing the modifications we have not had a single problem.