Motor issues

[brian john]

I looked at a site yesterday that has experienced some issues with several motors. This is a new facility (less than 2 years old), the distribution is with cable tray utilizing an approved multi-conductor cable. The installing contractor did not do the prettiest job with the install. It was reported by another firm that there is a ground current current (I have not seen the report as of yet). They attempted to straighten out the cables and the measured level of ground current was reduced.

The issue that prompted the first investigation was the loose of 10 motors, the rewind shop said the motors had pitting of the bearings and the winding had pinholes in the insulation. These motors are controlled by VFD's.

I do not believe this is a grounding issue problem, though we are going to look at what may be causing some of the remaining ground current.

I think the pitting and insulation damage is a different issue and I thought I read here about pitted bearings and insulation issues due to some VFD issues?

 

[joeblurton]

VFD cable

VFD cable

Was a shielded VFD cable used? Mode currents go to ground through the bearings of the motor, causing pitting. Other stray currents from welding leads or ground fault go to ground through the bearings if the conduit raceways are not bonded to the tray.

 

[masterinbama]

Was a shielded VFD cable used? Mode currents go to ground through the bearings of the motor, causing pitting. Other stray currents from welding leads or ground fault go to ground through the bearings if the conduit raceways are not bonded to the tray.

Joe is on to something here. Do they have to do a lot of welding in this facility? I did a pet food plant project once that had a lot of shaker tables and vibrating conveyors that needed repair welds quite often. After several motor failures of this type it was found that the weekend maintenance crew was not moving their ground leads so as to be as close to practical to their work. This was discovered early one Sunday morning when the #14 ground wire in a piece of 2" nonmetallic flex burned and took the 100 control wires along with a few I/O cards with it.


After changing their welding procedures their motor bearing failure rate fell of too.

 

[brian john]

I should have added I am in the process of googling this but want input from the MH experts.

They DID NOT use shielded cable from the VFD's to the motors.

This is a fairly new industrial facility, I would assume that there was some welding going on during the final stages of construction while some of the motor were on line.

The facility is 2 years old and all the motors were lost in the first year. The reason they are revisiting this is they were told to remove all the cable tray and install conduit and the problem would be minimized, prior to spending that kind of cash they want others to look at the problem.

Welding would be DC currents, is there an issue with 60 cycle AC ground current and motors?

I would assume impulses could damage the insulation on the motors.

 

[augie47]

Back in my "industrial" days I had no experience with the "mode currents", but I definitively had similar problems with welding currents, Like masterinbama, I experienced pitted bearings and melted sealtite, all from improper grounding on welding leads.

 

[ATSman]

I looked at a site yesterday that has experienced some issues with several motors. This is a new facility (less than 2 years old), the distribution is with cable tray utilizing an approved multi-conductor cable. The installing contractor did not do the prettiest job with the install. It was reported by another firm that there is a ground current current (I have not seen the report as of yet). They attempted to straighten out the cables and the measured level of ground current was reduced.

The issue that prompted the first investigation was the loose of 10 motors, the rewind shop said the motors had pitting of the bearings and the winding had pinholes in the insulation. These motors are controlled by VFD's.

I do not believe this is a grounding issue problem, though we are going to look at what may be causing some of the remaining ground current.

I think the pitting and insulation damage is a different issue and I thought I read here about pitted bearings and insulation issues due to some VFD issues?

http://www05.abb.com/global/scot/sc...3cca8e/$file/abb_technical_guide_no5_revc.pdf

http://www.barr-thorp.com:8383/news/VendorPDF/ShaftCurrents.pdf

I am trying to find a case study I read in Maintenance Technology mag that describes your problem. Had to do with how the power cables were arranged in the cable trough and how the magnetic fields were not cancelling out causing net currents that damaged the motors. Problem was solved my removing all the cables and re-routing them to cancel out the fields. A lot of work!

 

[augie47]

Very interesting article, ATSman.
Thanks

 

[brian john]

http://www05.abb.com/global/scot/scot201.nsf/veritydisplay/8c253c2417ed0238c125788f003cca8e/$file/abb_technical_guide_no5_revc.pdf

http://www.barr-thorp.com:8383/news/VendorPDF/ShaftCurrents.pdf

I am trying to find a case study I read in Maintenance Technology mag that describes your problem. Had to do with how the power cables were arranged in the cable trough and how the magnetic fields were not cancelling out causing net currents that damaged the motors. Problem was solved my removing all the cables and re-routing them to cancel out the fields. A lot of work!

But still easier than installing conduit new copper and then scraping all the tray cable.

THANK YOU VERY MUCH

 

[weressl]

I looked at a site yesterday that has experienced some issues with several motors. This is a new facility (less than 2 years old), the distribution is with cable tray utilizing an approved multi-conductor cable. The installing contractor did not do the prettiest job with the install. It was reported by another firm that there is a ground current current (I have not seen the report as of yet). They attempted to straighten out the cables and the measured level of ground current was reduced.

The issue that prompted the first investigation was the loose of 10 motors, the rewind shop said the motors had pitting of the bearings and the winding had pinholes in the insulation. These motors are controlled by VFD's.

I do not believe this is a grounding issue problem, though we are going to look at what may be causing some of the remaining ground current.

I think the pitting and insulation damage is a different issue and I thought I read here about pitted bearings and insulation issues due to some VFD issues?

The shaft ground currents due to magnetic imbalance inherent in motor manufacturing process. Motors built fro inverter service do take steps and measures to eliminate this. Usually this was not seen with with less than 75HP motors to be a problem, but there has been sporadic reports of such in smaller motors. (I have seen one with a 10HP motor.) Large morots and MV motors for inverter service often come with one bearing end insulated with a sleeve. Cable length from the inverter, shielded cable are aggravating circumstances, especially when the cable length is longer than the manufacturers maximum recommended length. The simplest solution id to install grounding attachment that rides the shaft. You can home-make it with a simple silicone/bronze spring that does not wear the shaft down or commercially available http://www.est-aegis.com/.

 

[weressl]

I should have added I am in the process of googling this but want input from the MH experts.

They DID NOT use shielded cable from the VFD's to the motors.

This is a fairly new industrial facility, I would assume that there was some welding going on during the final stages of construction while some of the motor were on line.

The facility is 2 years old and all the motors were lost in the first year. The reason they are revisiting this is they were told to remove all the cable tray and install conduit and the problem would be minimized, prior to spending that kind of cash they want others to look at the problem.

Welding would be DC currents, is there an issue with 60 cycle AC ground current and motors?

I would assume impulses could damage the insulation on the motors.

Motors and cable as well will be damaged by the inverter output signal IF the installation uses cables longer than the maximum specified by the inverter manufacturer and the motors should be inverter duty. IEEE841 motors automatically are. Cables can be fancy inverter rated shielded cables, but I've been succesfully using CLX for half the price.Output line reactors and dv/dt filters are also remedy for long cable installation. No conduit is necessary, shielded cables produce much better results. The equalization of voltage stresses along the conductor length will not be maintained inside the conduit as well as the shielded cables that utilize triplen grounding conductors equally spaced between the three phase conductors.

 

[robbietan]

I looked at a site yesterday that has experienced some issues with several motors. This is a new facility (less than 2 years old), the distribution is with cable tray utilizing an approved multi-conductor cable. The installing contractor did not do the prettiest job with the install. It was reported by another firm that there is a ground current current (I have not seen the report as of yet). They attempted to straighten out the cables and the measured level of ground current was reduced.

The issue that prompted the first investigation was the loose of 10 motors, the rewind shop said the motors had pitting of the bearings and the winding had pinholes in the insulation. These motors are controlled by VFD's.

I do not believe this is a grounding issue problem, though we are going to look at what may be causing some of the remaining ground current.

I think the pitting and insulation damage is a different issue and I thought I read here about pitted bearings and insulation issues due to some VFD issues?

I was looking for confirmation if the motors were VFD rated. been on a case where the slower motor speed due to the VFD also slowed the fan speed that cools the motor. the motors kept failing due to insulation failure since they are not VFD rated. the company provided another cooling source for the motors and the problem disappeared.

 

[brian john]

I was looking for confirmation if the motors were VFD rated. been on a case where the slower motor speed due to the VFD also slowed the fan speed that cools the motor. the motors kept failing due to insulation failure since they are not VFD rated. the company provided another cooling source for the motors and the problem disappeared.

The motors are VFD rated or designed to be utilized with VFD's.

 

[weressl]

I was looking for confirmation if the motors were VFD rated. been on a case where the slower motor speed due to the VFD also slowed the fan speed that cools the motor. the motors kept failing due to insulation failure since they are not VFD rated. the company provided another cooling source for the motors and the problem disappeared.

You attribute the insulation overheating to the insulation failure. The majority of applications are variable torque application where this is seldom represents a problem. The ASD rating on the motor has some effect on the thermal design, but the main issue is the insulation voltage rating of the winding that is usually in the range of 1200-1600V for 460V lableled motors.

 

[mike_kilroy]

perhaps a fix may be to just buy 10pc output inductors and put them in between the vfd's and motors. the bearing pitting and insulation failure described sound like caused by vfd pwm voltage spikes. On 480v system these spikes are 1200-1500v on well behaved systems, and go up from there on poorly wired systems; insulation pitting and damage on motor windings typically start at 2500v.

So redesign all the plant wiring or just buy a choke for the output of each vfd that caused problems and be done with it. chokes are cheap.

 

[robbietan]

perhaps a fix may be to just buy 10pc output inductors and put them in between the vfd's and motors. the bearing pitting and insulation failure described sound like caused by vfd pwm voltage spikes. On 480v system these spikes are 1200-1500v on well behaved systems, and go up from there on poorly wired systems; insulation pitting and damage on motor windings typically start at 2500v.

So redesign all the plant wiring or just buy a choke for the output of each vfd that caused problems and be done with it. chokes are cheap.

I agree. many motors with VFDs already come with chokes on the line side and on the load side. to protect the motor and to keep harmonics from propagating in the system

 

[weressl]

I agree. many motors with VFDs already come with chokes on the line side and on the load side. to protect the motor and to keep harmonics from propagating in the system

This statement is just a little misleading. "Many" would imply 'a significant portion' or even 'majority' of the applications and that is just not the case. If the application is engineered, it may be decided that due to the distance between the motor and drive it may be necessary to install load side compensation and it could take a variety of forms including a reactor. The load side harmonic mitigation is even more complex as it should take the system characteristics into consideration including other harmonic contributors.

To routinely apply line or load side compensation without engineering is a sledgehammer approach, could be harmful and certainly money down the drain.

 

[robbietan]

This statement is just a little misleading. "Many" would imply 'a significant portion' or even 'majority' of the applications and that is just not the case. If the application is engineered, it may be decided that due to the distance between the motor and drive it may be necessary to install load side compensation and it could take a variety of forms including a reactor. The load side harmonic mitigation is even more complex as it should take the system characteristics into consideration including other harmonic contributors.

To routinely apply line or load side compensation without engineering is a sledgehammer approach, could be harmful and certainly money down the drain.

that would be correct. however, elevator companies (for example) have line and load reactors built in their units. seems that their experience with VFDs have made them "standard issue" with their design.

 

[weressl]

that would be correct. however, elevator companies (for example) have line and load reactors built in their units. seems that their experience with VFDs have made them "standard issue" with their design.

That makes no sense at all, at least on the load side, since the motors are located either in the same or adjacent machine room from the drives or sit atop the elevator cage if it is a crawler.

Out of curiosity, can you identify the drive manufacturer with the built-in line and load side compensators?

 

[Besoeker]

I looked at a site yesterday that has experienced some issues with several motors. This is a new facility (less than 2 years old), the distribution is with cable tray utilizing an approved multi-conductor cable. The installing contractor did not do the prettiest job with the install. It was reported by another firm that there is a ground current current (I have not seen the report as of yet). They attempted to straighten out the cables and the measured level of ground current was reduced.

The issue that prompted the first investigation was the loose of 10 motors, the rewind shop said the motors had pitting of the bearings and the winding had pinholes in the insulation. These motors are controlled by VFD's.

I do not believe this is a grounding issue problem, though we are going to look at what may be causing some of the remaining ground current.

I think the pitting and insulation damage is a different issue and I thought I read here about pitted bearings and insulation issues due to some VFD issues?

The insulation damage is usually rate of change of voltage (dv/dt) resulting from the very fast switching edges of the PWM waveform. I've measured some real nasties and, I think, posted some of those waveforms here. I had one site with some 60kW drives (not ours) driving submersible motors that were dying like flies. In some cases, I measured dv/dt in excess of 2500V/us. The IEC technical specification was maximum 1600V/us. Output reactors fixed the problem.

But it isn't always the PWM that's the problem. On another site, there were a number of 800kW machines, also suffering insulation failures. A bit worrying for me given that they were our drives....my neck on the block...
The measured dv/dt was not more than 400V/us. The problem was the way the motor was wound. There are two types of winding as I understand it - I'm not a motor man. Random wound and form wound. With random wound, the turns at the opposite ends of the winding could be in close proximity thus experience greater electrical stress. The 800kW motors were random wound. The main contractor had worded the contract with the motor manufacturer that the motors were to be suitable for operation with variable frequency inverters. They weren't and the issue was thrown back on the motor manufacturer. The motors we rewound as form wound machines and the problem went away.

There are two points from this.
Hit the motor with excessive dv/dt and expect it to fail.
If the motor is not designed to be suitable for operation on a variable frequency drive, expect it to fail.

On the matter of bearing pitting it's a problem I know of but have not had any direct experience of it. Bearing currents are usually cited as the cause and there are various mitigating measures that I've read. including what type of cable is used. Installation practice is different on this side of the pond. The motors would be cabled individual with steel wire armoured cable per motor. Maybe that's why we don't see bearing pitting, at least in my experience.

Sorry folks for the long post..........

 

[robbietan]

That makes no sense at all, at least on the load side, since the motors are located either in the same or adjacent machine room from the drives or sit atop the elevator cage if it is a crawler.

Out of curiosity, can you identify the drive manufacturer with the built-in line and load side compensators?

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a mitsubishi elevator, a very common brand in our place