VFD Motor Termination Failures

Is there any kind of corrosive atmosphere that could be getting to your wire terminations? Aluminum vs. copper wire in the problem area? These are wild guesses (unless they turn out not to be so wild).

Humidity and local temperatures in practice doesn't have to be uniformal even in a moderately confined space. And in a nearly vapor saturated atmosphere it takes only a few degrees lower temp for condensation to happen. So look for that difference and it's cause. And ther's always the ghost explanation. Electric fields seem to attract them. Who ya gonna call?

S.M.

Do the burnt terminals show any sign of corrosion? Is it just the incoming cables or do the motor leads get burnt as well?

One common reason for terminal failures is the use of steel nuts between the incoming terminal lugs and the motor lugs.

Length of run from distribution panel to this termination point? How much greater is it then the other pumps? Are they ran separate to each pump from the source? Or are the from a common source?

I would start by changing the wire nuts to lug and bolted 1st, then take amp readings on on legs at the pecker head connections. You say the motors at your site are all wired the same, but are the mechanicals all plumbed the same. Depending on what you find with amp readings after changing to the bolted lugs. You may find the loads are to great for wisted wire nut system.

Motors of 10 hp or more I prefer bolts lugs. Stops some the issues with wire nut connections. At any rate check amp load and piping. What type of pumps are you using in this application. There maybe a suction issue if you are seeing higher then normal readings. Just a few of my thoughts with out seeing your operations.

I agree with the lugs and bolts and add:

Wrap the terminations with 33+ pads and then triple lap with 33+ tape.

What other commonalities might these three pumps have that the other 83 don't? My first rule of troubleshooting has always been " what changed?"

Are all the parameters in these drives the same as the ones that are operating well? Something like accel time, maybe?

My top 3 picks, knowing nothing else so far:

1. Standing wave generation / conductor length issues. Do a web search on "VFD standing wave" if you are unaware of this phenomenon, it's very real and although it generally hits the first turns in the motor windings because that is where the insulation is weakest, you may have created a weaker link by using wire nuts in the pecker head.But if you fix the wire nut problem you may end up moving the symptoms down into the motor windings. You should investigate this fully and possibly take steps to correct it rather than just take the easiest way out.
2. Vibration. You may have a critical speed issue that, because of the different hydraulics in this set of pumps, is causing excess vibration in the motor and thus the connections and again (not to harp on this) wire nuts are contra-indicated in ANY VFD application. In a true VFD application with analog control instead of the simplistic 3-set speeds you have, it would be unlikely that the VFD would stay in any one critical speed long enough to do harm. But having only 3 set speeds increases those chances dramatically.
3. Corrosion, as mentioned above.
   

1A. Corrosion. Look into how the conduit enters the junction box. Is moisture buildup inside the conduit draining into the JBox?

1B. Corrosion. Are there any situations where aluminum is meeting copper?

2. Harmonics. Is there a line reactor between the motor and VFD? Adding one could help reduce any current surge.

CR4 members has given good points,among that check the foundation of motors ,compare to other motors and check the vibration of running motor(3)it will effect the termination.

all the best

I think maybe .."back to basics"... which I guess you've thought of but would like a second opinion on

Start at the motor terminals, make sure they are tight, the correct type and are checked during your planned maintenance routine. Maybe terminating them with crimped lugs, if you haven't already done so!!

Cable size as mentioned.. yes check it out... how about the size and duty of the motor? Hopefully if you have 86 pumps that have been running for however long them this could be a localised thing. How long have you been having this problem?

Power quality!.. Harmonics! as was mentioned.. test your supply AND the output from ALL the VSD for harmonics. It might be that your motors are at the end of the line from the main power supply and harmonics generated by the other VSD's (assuming that there is HD) could be affecting the power system and causing the problem. If you have power quality problems then you might what to think about changing out a few drives to ones that generate less that 5% THD AND can mitigate your harmonic problems and the same time.

Ghosts? contact the local priest for an exorcism!

Hi Jim, I did notice that you mentioned the "North System" motors were where the faults were happening, are these motors further away from the MCC than the other motors, do these motors have EMI Filters, do they have a VSD Screened cable?

If all the above is correct, and the other contributers information has not given any ideas, then i am at a bit of a loss, and cannot help you any further.

As for the Ghost, yes I have seen one, I didnt realise it at the time, but it was the vision of my brother inlaw that had been murdered the same day but a year before.

And till then I was a non beliver, there were some other things that happened the same day that I cannot explain to this day.

Jim if you want to know the other experiences, send me an email through CR4.

Best Regards

Joe

I've had a similar problem with some motors. It was +/- 5 years back, also motors 10 - 15 Hp.

Some motors were 230/400 V connected in wye and others were 400/690 V connected in delta.

As far as I remember, the 230/400V motor terminals burnt away.

Then I counted the number of fibres in the wire between motor connection board and the cupper motor winding.multiplied it with the surface area of the fibre, This gave me the cable section.

When I made the ratio nominal current over section, It was a lot higher in the problem motors.

When the motors run continuously, there was no problem, but the frequent starting gave a higher warming.

Good luck

Rudy

I ran into a similar problem with an irigation pump. It was resolved by replacing the wire from VFD to motor. It was about 125ft (38 meters). After examination of the wire removed, it was brittle and had higher resistance than replacement wire. After three repairs (in 8 months) by cutting ,re-striping and instaling new wire nuts the wire became to short to repair. This forced the replacement of the wire. After replacement of the wire it has run without failure for now going into 5 years. Verify the wire is the same in all locations.

I didn't read the entire thread but whenever I encounter a burnt wire nut on a larger voltage system I like to change out the wire nuts for lugs. Insulate with electrical tape. It's a more permanent connection and moisture and other debris wont collect as they do in an open nut.

Thermal cycling is often the cause for premature loosening and subsequent over heating of the loosened connections. Each time that the wire heats up it is compresses a little by the connector and deforms. This eventually leads to a loosening of the connection. Of course, loose connections heat even more and the cycle repeats every time you start and stop. Hot wires oxidize faster and their resistance increase accordingly. It all goes in the wrong direction. Keep the connection cool and they will last longer.

If the conductors are the same and about the same length for all your pumps, the current for this particular one might be higher or cycle more often than the others. Once you exceed a certain temperature the degradation at the connection is very rapid. You might not reach such a high temperature at the connections with the others motors.

Solutions?

Do not over tighten the connections. This can reduce the effective cross section of the cable and remove the "spring effect" of the connector that absorbs the thermal expansion.

Make sure the materials are compatible (aluminium / copper). Different coefficient of expansion = stresses.

Increase the diameter of the conductors and / or the size of the connector to dissipate more heat.

Use a large crimp lug with the correct tool setting for your cables. The gauge and the thread count are important for the setting of the crimp tool and the selection of the lug.

The reason for the burned connection is a high resistance connection.This in turn can have many causes;mechanically loose,corrosion,dissimilar metals,moisture, etc.

It is difficult to get a solid connection with wire nuts,especially with motor leads because of the fine strands,vs the large strands of the feeder wire.

If you are determined to use wire nuts, it is important the you do not PRE-TWIST the wires before inserting into the wire nuts.This will cause an unreliable connection.

One solution is to use a deforming ring-type crimp connector on each wire, using the proper crimping tool.Some connectors have a built in silicon gel inside, that is extruded out when crimped.This protects against moisture penetration.

If using split bolts,when wrapping with electrical tape, it is important to stretch the tape 50% when wrapping to achieve proper sealing from moisture.Look at the tape on the existing joints.Is the tape the same width as new tape, if so, it has not been stretched properly.It should be no wider that 1/2 of the new width(3/8").

Start wrapping in the center, go to the end,the back across center,to opposite end,and back to center.Maintain stretch (50%) while wrapping.

Do this 2 times without breaking tape for moisture protection.

Hope this helps.

I came across this piece of info from an older article on VFDs: Make sure that the motor's resistance is close in value to that of the lead cabling. If motor impedance is higher - which is almost always the case for small motors - the voltage pulse will be reflected at the motor terminals. Reflected pulses can cause an increase in voltage levels as standing waves interact with transmitted pulses...

Additional info:

Lots of very good info from all replying to my issue on these 3 pumps. Thank you all!

In this area there are (3) 20HP Pumps I have no issues with. The Control panel is located approx 30 feet from the last 20HP, the (3) 15HP are approx 25 feet. I do have 3Ph NEMA 4X disconnects next to the motors. All wire is THHN 10Ga 105'C Copper. No Aluminum anywhere in the buildings. The Main Transformer is a 3500KVA 19200-480 3Ph sits in between all 3 buildings. 4000 Amp Square D Switchgear 3 sections sitting 15 Feet from XMR, Feeders are 80 Feet to Main building 2000A Westing House switchgear, 55 feet to South Building 1200 Amp I Line Panel, 75 feet to 1200A I Line panel in North Building. Each of these N/S buildings house 20 7.5HP motors and 20 20HP motors all running on Emerson SK Drives, I forget what the THD is, We just got a nice big rebate check from NYSERDA for the energy savings using these drives and due to the fact of their impedance, I did not have to install any line reactors. The Main building has 2 systems, South has (3) 15 HP and (3) 20HP identical to the North system where I have the issues with the (3) 15HP motor leads. Both systems are Mirror images of each other. Everything is exactly the same layout, same everything. Main systems have been running for just over 2 years now 24/7/365. I have never had any issues with the South system. Only issues with the North with these 3 15 HP pumps, no issues with the 20HP Pumps. Also all pumps in the Main Buildings are Baldors direct coupled P Frame Centrifugal Pumps.

All drives are programmed exactly the same way through out all 3 buildings with exception of Pf, most are .85, 40 of them are .83

Now, as stated before all pumps run 24/7/365 as we are a salt water fish farm. Corrosion: Extreme, Moisture: Extreme, Humidity: 95-100%. Yes we have many other issues related to these issues, but only on these 3 15HP Pumps where I have this connection burning issue.

I have been researching insulated screw type lugs and have yet to find any small enough for 10Ga wire that will fit inside the pecker head. No moisture exists in the pecker head. I'd like to avoid using tape, looking more likely I may have to get small copper lugs, pad and tape them. The Ghost issues developed while we were under construction in the North system. I once lived in a house that had many ghosts living with us. After many months of thinking we were going crazy, we realised what was happening and had a nice conversation with them and we co-existed for about a year. Then asked them to leave before I brought the baby home. And they did. The house was 100% quiet till the day we moved out. Even the people helping us move were asking questions on things happening at the same time.

So many unexplained happenings and visuals. The only explanation we could come up with. We have a Ghost. The building is between 50-55 years old and has been used for commercial/industrial and manufacturing over the years including a cement pipe manufacturing. Quite possible people were killed in this part of the building, I am completely speculating on this though. I just find it so strange that this is the only area of my problem. In relation to the 3 15HP pumps, the 20 HP pumps reside just around the corner on the other side of the water tank. So my next step will be to get the smallest copper connectors and see if that solves the problem. It usually take 6-8 months for this to happen. Will remove cover for inspection on monthly basis and see what happens. In the mean time will re-read all posts and look into them.

Thank you all

Jim