motor failure with locked rotor 1

[conveyorman53]

i have had two failures with single phase capacitor start motors connected to a conveyor. the motor is wired 22o volt through a drum reversing switch. in both failures the rotor was locked when the motor was disconnected from the gear reducer. the reducer and drive chain spin free when the motor is not connected. when new motor is installed machine runs normally and amperage reading is below full amp rating. is the failure due to end user overloading conveyor or is there another electrical cause such as reduction of voltage by power utility ?

 

[pablo02]

All I have are questions for now... are the two failures of the same motor location or separate conveyors?

Did the motor fail electrically or mechanically? (in other words, why was the motor rotor locked up?) a bearing failure? electrical failure internally? what type of coupling is used (& how does it look after a failure)? how is the coupling aligned?..

Is the customer giving you all the information on the loading of the conveyor? who has access to the drum reversing switch and how is it used when the conveyor is running? does the customer monitor the motor temperature at all during operation?

 

[pablo02]

Also, what conditions exist in the area the motor is located? dusty, wet, gritty, etc? Does the motor nameplate indicate TEFC?

 

[conveyorman53]

pablo
thanks for responding the motor in question is a c face mount, tefc the shaft when examined shows no remarkable damage and its' removal from the reducer quill is easy ... there is no smell of burnt insulation and yes the failures occurred on the same conveyor the customer of course is not monitoring the temperatures the conveyor is used to bring product from a street level loading area to the basement the conveyor is used by a wide range of personnel. there is starting and stopping under load but this not unusual for this type of conveyor application the switch shows no sign of damage and the contacts show continuity i was wondering if it were possible that there is a flaw in the motor manufacture either with the bearing or the windings or perhaps if incomplete engagement of the switch when used frequently by store personnel were creating a low voltage condition i checked the incoming power and it is within the nameplate range

 

[electricpete]

It sounds most likely like a bearing failure.
There are of course numerous possible causes of bearing failure. Disassembly/inspection might cause some clues. Some items that come to mind:
- The gearbox is imposing excessive radial loading on the motor bearings.
- contamination of the bearings by the environment
- improper greasing of the bearings - overgrease, incompatible grease, etc
- severe operation causes high temperatures which cause all the grease to run out of the bearing.

 

[pablo02]

I, like pete, think it might be a bearing failure... it might be that the gear reducer is worn (you mention starting and stopping frequently under load) -- check the gear reducer where the motor connects for any play axially or radially in the shaft... I think your motor inboard bearing is wearing out and gauling enough to lock it up (but it's only a guess without taking the motor apart...

 

[SteveKW]

When you removed the motor was it STILL locked?

If the windings are not burned I would suspect a bearing failure.

Can the motor be banged around in any way to put it out of alignment? Could it be hit by personnel or objects?

Make sure personnel are FULLY engaging the drum switch!

Have you confirmed the voltage AT the motor leads, WHILE she is LOADED?

Does the motor come with a manual overload?

 

[electricpete]

good points Pablo and Steve.

Street level- is it outdoors? TEFC does not necessarily mean the motor is suitable for outdoor use... It needs suitable degreee of ingress protection, something like IP55 or better to make sure the bearings are suitably protected from the environment.

 

[conveyorman53]

thank you everyone for your repsonses the motor in question does retain the rotor lock when removed from the gear reducer there is no apparent play in the reducer quill after removal since it is installed in a fruit store in nyc it is difficult to monitor use and all personnel the most striking problem is that the failure has occurred twice within 2 week sof each other with brand new motors and a new reducer the reducer turns freely by hand and no apparent binding is evident i can't see how a bear could fail so quickly and without resultant noise or other evidence at present i'm inclined towards the incomplete contact within the drum switch cause can a locked rotor be caused by a voltage problem such as this? unfortuneately it is impractical to disassemble the motor for inspection

 

[SteveKW]

A reversing switch not making good contacts can make a motor sound like it is locked up.

Disassembly of the motor would be most revealing as to what happened. I suspect you will find either just the run or start windings burned and have melted the insulation so as to lock up the rotor!

Before you replace the drum switch (replace it with one that is twice the HP capable), do a conveyor load test. Load it up and stop it repeatedly and confirm that it can start the conveyor under those conditions each time!

A motor with a manual overload is what I suggest. This of course means running a line leg through the overload first and then back to the switch.

 

[electricpete]

I have never heard of insulation melting in a fashion as to lock up the rotor. But then again I work with large industrial motors, not small ones.

 

[GusD]

Hello conveyorman53

From the information of the previous posts it seems that your motor is failing due to siezed bearing/bearings.
In some cases motors can burn up and some wire strands gets caught on the rotor cage fins causing locking.If this was the case,the bearings would not necessarily be faulty.
I don't think you have mention that anywhere.
There is a possibility (albeit remote) for C flange motors to fail with drive end bearing siezed.It happens because of a lot of slop (looseness)on spigots causing motor to be badly misaligned when it is bolted down.This misalignment
can be severe enough to preload the DriveEnd bearing to destruction.
We have to rebuild spigots on large Generator Armatures for that same reason.After a few assemblies and disassemblies,spigots can get fairly sloppy.Your motor most likely has a shaft Spline that inserts in the Gearbox.They will not tolerate too much misalignment.

GusD

 

[SteveKW]

Pete..

Smaller motors I have seen have a slot liners and top stick that look like plastic... and when the motor is overloaded, they sure do MELT like plastic.

 

[Shortstub]

Have personnel been trained on proper use of reversing switch? Or, is it possible that starting circuit is improperly wired so that the start-winding and/or its associated capacitor are not adequately connected?

 

[HEMillet]

Finally someone asked about the run and start capacitors. I don't think that a fault in this area would cause bearing failure though. At this point should opt for misalignment and check for it. Be agressive. Cut the motor open with a cutoff wheel or something. Especially if it occurs again. See if motor burn or bearing. Home in on the symptom. Speculation could go on indefinitely.

 

[conveyorman53]

thanks for the responses after having changed the motor the problem has not reoccurred in the past few weeks the theory of misalignment is weakest i think the reducer is new as was the original motor and there is no slop in the fit at the quill.... it would also seem less likely that the failure was due to the bearing since the wear time on the motor was only a few weeks... of couse the porblem here is that to get credit from the motor supplier cutting open the motor or disassembling it to examine it are not possible.... this has been a problem that has occurred regularly in the conveyor industry with small floor to floor conveyors operating on single phase motors... at this point the burnt insulation wrapping around the rotor to " lock " it would seem the most plausible

 

[jbartos]

Suggestion to the original posting marked ///\\i have had two failures with single phase capacitor start motors connected to a conveyor. the motor is wired 22o volt through a drum reversing switch. in both failures the rotor was locked
///This can be of a mechanical origin or electrical origin.\\ when the motor was disconnected from the gear reducer. the reducer and drive chain spin free when the motor is not connected.
///The conveyor still may be the culprit as well as the motor mechanical internals.\\ when new motor is installed machine runs normally and amperage reading is below full amp rating. is the failure due to end user overloading conveyor or is there another electrical cause such as reduction of voltage by power utility ?
///If the motor is sensitively protected (this has not been addressed yet) and the power quality is right, then the motor electrical portion will not show major symptoms, e.g. burned wiring, smell, etc. Even, if there is some wiring melt, it should not be catastrophical to lock the motor. If the motor is not sensitively protected, then the wiring melt may cause the motor to be locked.
The mechanical failures have been addressed in the above postings. The single phase motor may have a centrifugal switch that might disintegrate and potentially lock the motor. Also, the rotor cage might experience mechanical malfunction that may lock the rotor.\\

 

[RAMConsult]

I would suggest that you observe how the switch is used to operate the conveyor. My guess is that someone is using the switch as a form of speed control, jogging, by either teasing the switch or worse by switching it on and then hitting reverse to stop the belt rapidly. In either case the 'on-time' of the motor will be short and the cooling fan won't reach full speed causing overheating. Also, the reduction ratio wasn't stated but there may be a lot of 'wind-up' in the gear box before the belt starts moving or changing direction. The net result is a lot of mechanical stress on the motor which in combination with overheating is a formula for early motor failure.

 

[jbartos]

Suggestion to the previous posting: Such operation would probably burn the motor winding. This would be easily detected.

 

[RAMConsult]

jbartos, an open in the stator winding would probably not result in a locked rotor; however, it would be an indication of very abusive, high heat conditions inside the motor housing. Induction motor rotors can have cast aluminum rotor bars and end connections which become overheated and deform and/or melt, which in combination with constant reversing or plugging, may cause an unbalanced rotor and contact with the stator. Also to be considered is the construction of the rotor shaft, how the laminations are attached, and whether the bearings were intended to take any thrust loading due to end play.

Conveyorman should cajole (hey, we're from NYC!)the motor manufacturer to disassemble the end-bells and check the bearings, if the rotor moves freely so much for my overheating theory. If not then saw the motor in half lengthwise and see where the rotor failed.

Here's a thought that just occurred to me- is any type of crud getting sucked into the external cooling fan housing and jamming the rotor?