Burnt motor leads at Y connection (star points) 1

[GusD]

We have experienced this problem for several years and with different motors.The problem is usually limited to burnt motor leads at the Star connection.These motors are 5 Kv induction motors.The Star point 3 leads in some cases are bolted together and taped,on other motors ,the 3 leads are brought to a large copper busbar in the Motor JBox and bolted individually.These too burn near the STAR point from time to time.
This problem have not caused any motor breakdowns (yet)and the problem is usually found when motor is removed for OHaul or other.Do any of you have an idea or a similar problem.

Thanks a lot.

GusD

 

[electricpete]

I've inspected many motor term boxes and never seen anything like that on motors. It would seem that either the neutral must be exposed to resistive heating (due to high current) or tracking (due to high voltage)

Close inspection might tell the difference. More damage on inside of insulation next to conductor => conductor heating. More damage on outside (tracking) => high voltage.

Scenario for current heating at the neutral:
- High resistance at the bolted neutral connection.
- Lower-temperature insulation used on the neutral leads than on the line leads.
- These are the only two that come to mind. It seems like anything else would affect the entire winding and certainly the line lead similarly to the neutral leads.

Scenario's for high voltage tracking at the neutral (?):
- improper or contaminated insulating materials used on neutral connections.
- power system harmonics (?)
- power system surges (?) perhaps coupled onto all three phases and meet at the neutral causing reflection/doubling of the waves (?).


I have a tough time providing any good explanation for high voltage at the neutral (hence the question marks) but I believe it is possible. I once saw once a transformer with ungrounded neutral bushing that had definite voltage damage to the neutral bushing. Described in detail here

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&safe=off&th=e093455ff55448e2&rnum=2

The explanation is unclear to me but there is no doubt whatsoever that the neutral was zapped with high voltage (either momentary or steady state).

I should ask… is there anything unique about these motors: vfd controls, reduced voltage starting, etc?
Is the power system ungrounded?

 

[jbartos]

Suggestion: Check the motor power supply for its quality in terms of a harmonic content. It should have less than 5% voltage total harmonic distortion.

 

[nukeman]

Verify hardware (i.e. nuts, bolts and washers) are correct size for the termination lugs and that they are torqued correctly. I have seen undersized bolts cause hot joints.

 

[GusD]

Thank you for the help.
To Electricpete =I did not see this mysel,it has happened in a different area of our plant.Basically all I know is hear say.
I believe that the insulation is always good but the connections show the telltale signs of burning or overheating.You are probably right about the resistive heating.These motors are 1500 Hp induction motors,2 pole , across the LStart.Pretty standard motor.

JBartos = I believe that THD is <5%.I personally have not checked the THD,but it is something that we would be quite concerned about.I understand that the problem only happens on the motors that have a star connection in the JB.

To Nukeman-The first time I heard about this problem, I question exactly the same thing about bolts ,washers etc, etc; It wouldn't be the first time we find black metal bolts,washers,and nuts mixed with just about anythingelse. As you mentioned the lug sizes are a good possibility. We all think of a nuetral as not carrying any current so lug size could be under estimated.

I'll follow up on this problem and I will share with you people whatever we find.

Thanks a lot

GusD

 

[Shortstub]

GusD,

Experienced a similar problem on a number of motors, all of equal size, and equal number of operating hours. Only the phase lead terminal connection failed... not the 'Y'-point connection. The problem was related to the 'pigtail' conductor between the winding and the terminal.

Have you checked the pigtail size? Is it solid? Or, stranded? Sometimes pigtails to terminals are too small and when subject to vibration, the copper-wire will work-harden and snap!

 

[induction]

I rewind motors I have seen this many times it is normally becuase the lead are weak basically to small for the app. It also could be that the motor is has concentic windings and there is a little imbalance when there done this way it would show on a surge tester. Next service reconect the leads back at the winding if you send it out for G.M. I have just brought out three leads and the problem never occured again. Watch for voltage imbalance also while it is running be safe when doing so if other equipment starts up the may be causing a voltage drop.