3 Phase problems

[millwall ken]

Hi all,
can someone please provide me with a list of the most common faults that you will encounter with 3 phase equipment such as ac units, motors, etc.
Cheers Boys.

 

[malcolmsanford]

Droping a phase
Wrong direction
wiring up star/delta wrong
AC Units though not 3 phase could be compressor overlaods

Off the top of head but I'll try and think of more

 

[Marvo]

You mean equipment / manufacturers faults, installation faults or user faults?

 

[millwall ken]

You mean equipment / manufacturers faults, installation faults or user faults?

Faults associated with equipment on BMS systems mate.

 

[Marvo]

The most common problems we encounter are failed contactors. About half are just old and the other half are burned due to poor connections. Half of the burned connections or more are caused by poor connections of the wiring.

Supply phase failure often results in thermal overloads tripping which need to be manually reset usually.

Out of all faults on motor systems we encounter, more than half are caused by poor installation.
Out of all motor failures most are inadequately protected against overload current or against supply phase failure. We've had a couple of motor failures where VFD's have cause induced rotor voltages which destroyed the bearings prematurely.

 

[pmac10]

The most common problems we encounter are failed contactors. About half are just old and the other half are burned due to poor connections. Half of the burned connections or more are caused by poor connections of the wiring.

Supply phase failure often results in thermal overloads tripping which need to be manually reset usually.

Out of all faults on motor systems we encounter, more than half are caused by poor installation.
Out of all motor failures most are inadequately protected against overload current or against supply phase failure. We've had a couple of motor failures where VFD's have cause induced rotor voltages which destroyed the bearings prematurely.

Marvo, i work with VFD,s and motors everyday but have never heard of that as a cause of bearing failure, could you explain that one a bit further as it sounds like one to look out for in the future, thanks.

 

[Canoeboy]

Marvo, i work with VFD,s and motors everyday but have never heard of that as a cause of bearing failure, could you explain that one a bit further as it sounds like one to look out for in the future, thanks.

Its to do with current flowing in the bearings causing them to fail, I have done invertors for 30 years and only seen it twice now (motors of 55Kw and above), it depends on a number of factors the main one being motor design

You can buy special insulated bearings to stop this happening or a much cheaper solution is grounding rings on the rotor shaft See AEGIS™ Shaft Grounding Brush: Bearing Protection for Life (they are just one supplier)

You would need a scope to see the problem and it doesn't normally happen for quite a few months down the line (one of my cases was 3 years)

And the only way you would know is when the motors bearings are changed if the engineer doing the job knows what hes looking at with the track marks in the bearings

A really good article on it is here

 

[Marvo]

Marvo, i work with VFD,s and motors everyday but have never heard of that as a cause of bearing failure, could you explain that one a bit further as it sounds like one to look out for in the future, thanks.

We've had problems with Emmerson and Danfoss earlier vector drive VFD's on Weg motors. Motor bearings failed catastrophically under warranty. Post mortem showed it was due to induced rotor voltage discharging through the bearing. The inner and outer race was microscopically pitted, lubricant contamination and high running temperatures resulted from excessive friction. It turned into a legal issue, our insurance covered it but eventually the drive manufacturers covered the cost of the damage in both cases, more because of the volume of business we do with them than anything else. They refused to accept liability in writing but they have made design changes to prevent this in the case of at least one of the manufacturers.

We fit a rotor earthing kit as standard with VFD installations. With newer generation drives it might no longer be necessary but old habits die hard especially after you've been bitten a couple of times.
http://www.culvermotor.com/images/stories/marathon-sgr.pdf
The link probably explains it better

 

[Canoeboy]

We've had problems with Emmerson and Danfoss earlier vector drive VFD's on Weg motors. Motor bearings failed catastrophically under warranty. Post mortem showed it was due to induced rotor voltage discharging through the bearing. The inner and outer race was microscopically pitted, lubricant contamination and high running temperatures resulted from excessive friction. It turned into a legal issue, our insurance covered it but eventually the drive manufacturers covered the cost of the damage in both cases, more because of the volume of business we do with them than anything else. They refused to accept liability in writing but they have made design changes to prevent this in the case of at least one of the manufacturers.

We fit a rotor earthing kit as standard with VFD installations. With newer generation drives it might no longer be necessary but old habits die hard especially after you've been bitten a couple of times.
http://www.culvermotor.com/images/stories/marathon-sgr.pdf

The link probably explains it better

Strangely enough the 2 motors i spoke of were both WEG's

 

[Marvo]

Possibly a coincidence, 80% of the motors around here nowadays are WEG.

Both the rotor earthing kit and the insulated bearing system are workarounds. Neither approach is addressing the root problem which is the developed potential in the rotor. If the supply is balanced then rotor voltage should be 0 volts. It can only be inbalanced VFD output from the IGBT output that would cause it. I've never heard of similar problems on motors without VFD control.

 

[Canoeboy]

We fit filters now (as discussed by your attachment) to the invertor outputs if a customer requests them. If you scope the output then its a near perfect balanced sine wave.

 

[millwall ken]

We fit filters now (as discussed by your attachment) to the invertor outputs if a customer requests them. If you scope the output then its a near perfect balanced sine wave.

Excuse me for being the village idiot here fellas, but can we please get back to the orginal post. Only, there is only so much that you can read in a text book, but there is nothing like real life experience.

 

[pmac10]

sorry ken, my hijack is now over, phase to phase short dont think has been mentioned, easiest method of location is to stand back, set the mcb, and look for flash/bang/smoke (you will only need to do this once).

 

[millwall ken]

sorry ken, my hijack is now over, phase to phase short dont think has been mentioned, easiest method of location is to stand back, set the mcb, and look for flash/bang/smoke (you will only need to do this once).

Cheers fella, as you can obviously tell I don't really have much knowledge of this subject over than testing motors in exams!!! I've applied for a job in a hospital, and they want you to be knowledgeable on the subject. My own experience is of commercial, some domestic, and property maintenance - what I'm doing now. So any help is greatly appreciated.

 

[pmac10]

no control circuit voltage, look for a 'phase sequence' or 'phase rotation/detection' relay. as the name suggests this protects against incorrect direction of rotation of motors and pumps or anything really where this is a critical factor or may detect the loss of a phase. three phases connected correctly will energise and onboard relay to enable your control circuit.