Should motor be replaced?

[striper2]

We have a 7.5 hp 460 VAC 3ph motor on a gantry crane w/ nameplate FLA of 11.5. During operation the motor pulls approx 15.5 A. The motor is about 50 yrs old, but doesn't get used very often. Operation is critical when called upon. During operation they lift gates at a spillway, runs about 15 min per lift. Is 15.5 A too high for this application or is motor ok for short durations and infrequent use.

 

[electricpete]

I would be inclined to think that it is worth replacing a 50-year old critical motor just based upon age.

I'm not sure about the current. I would be surprised if the OEM had misapplied the motor. Is it a 11.5A continuous or momentary rating? Is there anything on the nameplate that indicates the motor is suitable for short time overload? Is it possible that mechanical changes (binding) have caused increased load?

 

[electricpete]

balanced currents and voltage?

 

[busbar]

In many cases it is unwise to run any motor at a sustained 35% overload, particularly with insulation of advanced age.

Have you determined if overcurrent is from a mechanical or electrical problem? Have your determined if the motor-overload relay is correctly sized and indeed operational?

What does loss of production cost if the motor fails at an inopportune moment?

 

[GusD]

Hi striper2

Your motor is an old timer and like most old timers ,you cannot stress it too much.You don't mention the nameplate SFactor, but even at 1.15 SF it should only run at no more that 13.2 Amps for the type of cycle you experience.
If this motor has to run for 15 minutes drawing 15.4 amps, it is possible that motor overload protection is non existent.If it has never ben overhauled,perhaps it is time to refresh the bearings,clean windings and double dip same.If you do that, it may last a good number of years
once again.As you say, your motor is critical whenever it is needed.Just for some peace of mind I think I would do an overhaul.This motor has paid its dues many times over.
I do believe in condition monitoring but there are extenuating circunstances and I think this is on of them.

Thanks

GusD

 

[striper2]

Thanks for the input...Electrically everything looks ok, I'm not sure about the overloads, the starter is oversized (size 2) but I have not looked at ol's. There's not much info on the name plate either, I was assuming the 11.5 FLA was continuous duty, but I'm not sure. We don't have any historical data for comparison for mechanical or electrical problems. Sounds like at a minimum overhaul or replace alltogether...

Thanks again.

 

[electricpete]

The cost of refurbishment (disassemble, clean, replace bearings etc) is probably comparable to the cost of a new motor. Considering also the age of the motor it probably makes more sense to replace rather than attempt to refurbish.

 

[buzzp]

I am not sure of the design of the motors but we use similar motors on spillway gates as well from the 60's. The only info I have says they have 500% torque at starting. I am sure this is because the load is always there. Given the short duration of these motors run time vs downtime, I do not imagine the overloads will kick in because the motor is not in a sustained overload. Our spillway gate motors generally run no more than 3 minutes at a time with off times of hours. That old motor could run another 50 years like that. If it fails, I imagine the gate stays put. Our spillway motors can be replaced in short order so I would at least have one on standby.

 

[RadarRay]

It is probably impossible for you to answer this, but has the load always been 15.5A? Seems to me that if you take the steps to replace it, you might consider going with a larger motor. If not that, at least verify that the new motor can perform under your duty cycle.
It is possible for some reason the load required for the motor has increased in the last 50 years, although most that come to mind are because of inadequet mechanical maintenance.

 

[PowerSim]

We have faced overload problem of a gantry crane motor in our plant as well. Problem was rectified by properly adjusting the mechanical brakes. Are you sure that there is no problem from mechanical side?

 

[SteveKW]

Radarrarray has a good point if you are thinking about replacing the motor.. GO LARGER! I used to work with an ole timer and his years of experience showed me that those old pre-NEMA motors were almost twice as strong as today's motors!

Considered rewinding at a better insulation class?... Then the motor will have no problems if she still gets warm in those 15 minutes! You may be hard pressed to find an exact replacement for that motor!

 

[jbartos]

Suggestion: Considering the age of the system, the motor as well as the mechanical load need to be checked. A straightforward replacement of the existing motor by a new motor or by a new and larger motor may or may not be the correct solution since the mechanical load may also need a fix. After all, it has served for long time too.

 

[edison123]

Given the age of the motor, I am sure it has a considerable factor of safety built in terms of temp rise (may be class E). As stevekw says, if you can rewind the motor with modern H class materials with resultant increased copper, then your motor should do fine for another 50 years!