Drum Motor Problem

Simple minded suggestion: mark the drum and use a watch to count RPM's.

Good idea but the motor has already been returned.

Most motors being too fast for that approach, I didn't even consider it.

I'll remember it for next time though, thanks for that.

The advantage of a fresh set of eyes and mind in action.

Another way would be to add a roller of known diameter, have it feed a length of belt material through and time the feed time, then you can calculate the rpm, but again you no longer have the motor to be able to run any tests.

I'm always suspicious of anything shipped and the greater the distance the greater the possibility of somebody dropping the crate, this can be all it takes to cause the motor or gears to bind...Doing an amp draw under full load is really the best way to test it, but I guess that ship has sailed...In my opinion if the determination is that the motor was damaged in shipment the manufacturer should be responsible for claiming damages to the carrier, unless you specified shipping choice and agreed to be responsible...of course there is the possibility that it was dropped after receiving, or damaged in some other way that has not been disclosed...wouldn't be the first time something was wired incorrectly....oops, now how did that happen?

Thanks for your reply, there are ten of these motors all exhibiting similar problems, so it's highly unlikely to be post manufacture causes.

They never got to test under any sort of real load as the conveyors would only run for a few minutes with the belts installed, and I never saw other than just a motor as I am hundreds of Kilometres from where the conveyors are built. They are around 8 metres long so freight would be substantial.

Yes that does change the picture....so there is definitely something wrong with the motors.....too bad you couldn't test them with 60hz, but then it's also possible the voltage is wrong....so there you are....

I would think if you were pulling close to Full Load Amps on the motor with no load other than the gearbox it would indicate either a mechanical problem (noisy operation) in the gearbox, a fault in the motor, or as you surmised, a 60 Hz motor labeled as a 50 Hz motor. In any of these cases, if it is a new motor, it would seem that the vendor is responsible, IMHO.

If the motors were made in Germany I don't think it's very likely that they're wound for 60Hz (they use 50Hz). If wound 60Hz for export, I think they're tight enough to put the right value on the rating plate. Just IMHO.

That was a consideration, but there is no rating plate (nowhere to put one), simply a tag attached to the lead via a cable tie which states 240V 50Hz, no other information re FLA etc.

The 240V bit could be a hint (albeit tenuous) as both Australia and Germany are 230V 50Hz, the US uses 240V 60Hz single phase in some instances although only for heavier drain appliances than this I would think. The US would likely be a big market for this manufacturer and this is the root of my incorrect frequency theory - which may be totally off target.

These doubts are why I have posted this as I have no definitive answer to any of it, it could be frequency or gear bind or something else altogether.

The drum surface temperature of 86°C is not outside the realms of reasonability for a loaded motor, although somewhat high for one that is coasting, so it may be incorrectly rated thermals.

The one thing that I am convinced of is that this is the manufacturer's problem, and so far they have denied liability.

I have submitted my report and have no other responsibility on this other than the Engineer's curse/desire to know what the cause of the fault is - which I fear will not be revealed anytime soon.

My clients advised me this afternoon that - after a somewhat rigorous debate on the subject - they have sent the motors back to the manufacturer for further investigation, that's a step forward at least.

Thanks for your responses, they do help.

"... although closer to FLA than I would have liked". With no rating plate, how do you know what the FLA is supposed to be?

John DG - I acquired the FLA rating via Email from my client who got them from the supplier so that I could test the motor, they corresponded with earlier versions.

There is a communications difficulty dealing with them as they seem to have limited English and our side have zero German.

Ramconsult - I have already suggested that they find another supplier.

67model - your points:-

1. I agree

2. The external overloads are not disconnecting, it is the internal thermals which are opening.

3. The capacitors used are actually dual 50-60Hz.

4. Agreed, or gear bind.

7. there is no cooling fan.

Re. your final point. The motor is sealed, so no chance to turn the input shaft to the gearbox without invalidating any warranty that may be available.

Thank you all for your comments, I was aware that conjecture was all that was going to be available here, but at least it seems that I haven't really missed any important points. I will await any outcome from the returned motor tests to see what eventuates but won't be holding my breath.

Thanks for the feedback, I hope you get a favorable resolution so that you can complete your project without delay/penalty.

"...My immediate thought was that the latest set of motors were more powerful than previous ones and that I had undersized the overloads..." More likely just the opposite, the motors are underrated which causes them to overheat just driving the gearbox.

Find a different supplier if possible. Any motor that runs that close to FLA when it is unloaded will probably have difficulty under full load conditions. Have the next supplier provide some certified test data that verifies that you are getting what you ordered.

Maybe they were shipped without the oil.....?

..."The motor and gears operate in a sealed oil bath, ensuring proper lubrication. Heat from the motor and gears also gets transferred through the oil to the drum where it is dissipated by the moving conveyor belt."...

1. Increased current no-load looks like overflux.
2. Your external thermal is no doubt inside curve of motor's built-in overload, which again suggests overcurrent more than your overload permits.
3. A 50Hz run cap would, with lower impedance of winding it feeds, apply more current to that winding.
4. Noisy motor suggests overflux & overcurrent.
5. If the gearing was for 50Hz, the drum would run at its correct speed, less any increased slip. However, a 60Hz motor would have less torque for same rating & its proper gearing would be higher ratio, but for a 60Hz motor the gearing would mean 20% more torque than its design level.
6. If the 50Hz voltage is equal to motor 60Hz rating, this will increase flux to about normal 60Hz level (and so torque) - but, per 5, the torque demand will be 20% up which means more rotor current and slip - I²r is 44% up & rotor overcurrent & overheat follow.
7. Cooling airflow from fan designed for 60Hz will be significantly down at 50 Hz speed - more overheating [although this is invisible, no actual temperature detector, but it may have affected an internal thermal overload which would see increased motor temperature] but would decrease time to overheat significantly.
8. You could have measured winding resistance on hot motor after your test run to trip. Comparison with cold resistance is usual method of finding winding temperature rise for comparison with motor standard requirements.
9. That the motor tripped, with no shaft load, by its internal overload seems conclusive evidence that it is seeing overcurrent & overheat for internal reasons.

To summarise, I agree with your proposal that a 60Hz motor has been supplied in error.

It is possible the gearbox is 60Hz (if supplied with motor). You [or customer] could turn the motor shaft input to gearbox, counting turns, to rotate output shaft one revolution - correct for any pulley ratio. Maybe there is a type number on gearbox which gives ratio.

It depends where the temperature is being measured; stator or rotor?