Motor Starter Contact resistance

[Teslaphreak]

Looking for a little assistance / experience with determining the impact of motor starter contact resistance. Given a 150 HP 3 PH induction motor used in an air compressor application. I don't have my notes with me right now, I believe FLA ~ 163. Continued issue of tripping the breaker at start up. Motor checks good, ohms and meggar. If the unit does start, the compressor operates correctly, I'm ruling out mechanical issues at this time. Breaker is HMCP set same as an identical unit + - 2100 (approx 13x). One thing I did note is a high differential between starter contact resistance. Again, wihout my notes, approx A=150 micro ohms, B=160 micro ohms, C=2000 micro ohms.

 

[dpc]

If the HMCP tripping at startup, the instantaneous trip setting is probably still too low. The HMCP is a motor circuit protector and does not even have a thermal element - so it is not tripping due to heat buildup.

Newer high-efficiency motors can have brief asymmetrical inrush currents that are extremely high.

It's also possible the breaker magnetic trip unit is bad.



David Castor

http://www.cvoes.com

 

[Teslaphreak]

The breaker has been replaced recently. I don't want to raise the setpoint until I've investigated this other issue further. I suspect it is a contributing factor. Thanks for the quick response

 

[rbulsara]

The thing to investigate is the HMCP setting or the HMCP itself, as already advised, not the contact resistance.

What is the setting? Is HMCP tested by a testing compnay?

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

One thing is pretty certain to me - increased contact resistance cannot cause an HMCP to trip.

Any chance this breaker has a shunt trip connected to ground fault protection?

David Castor

http://www.cvoes.com

 

[davidbeach]

Have you checked the unloaders? A failed unloader would cause starting trouble but not running trouble, so it may be too early to rule out mechanical difficulties.

 

[controlsdude]

double check the part number maybe its a different trip class on the overload unit. Or maybe there is a trip class setting on this unit.

 

[Teslaphreak]

davidbeach: yes, the unloaders have been checked and even forced to unload.

dcp: "One thing is pretty certain to me - increased contact resistance cannot cause an HMCP to trip."
Not even on phase unbalance? I^2 * R through the contacts alone are on the order of 13x. Assuming approx 2100 amp inrush at startup. Pretty sure HMCP breakers with the right settings activated protect against phase unbalance. I'm just not sure how to mathematically model it.

 

[DTR2011]

Try exercising the HMCP and take resistance readings again.

Regardless of if this is the source of the problem, the resistance value is not acceptable.

 

[dpc]

The only protection in an HMCP is the magnetic trip element. It is set to trip for short circuits at a level above the locked rotor current of the motor. If the motor overload relay is tripping, the problem could be a lot of things, including the contact resistance. But if the HMCP itself is tripping, it almost has to be related to the inrush current, or a problem with the breaker - or both.

David Castor

http://www.cvoes.com

 

[hooverdale]

The article below might give few ideas..

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/wp/2100-wp005_-en-p.pdf

 

[jraef]

Micro ohms of resistance is going to create milliamps of current imbalance at best. That is not going to have any measurable effect on current that could in any way affect the HMCP tripping.

This issues has been discussed many times. It's a well established fact now that "energy efficient" motors have higher levels of instantaneous magnetizing current, for the same reasons they have higher efficiencies. Tripping of Magnetic Trips on circuit breakers is a widespread problem, so much so that it has been addressed in the NEC, starting in I think 2002 and amended in 2005. There is a well known chart, Table 430.52, that states the maximum allowable settings for circuit breakers, If all you read is that chart, it says you can set an Instantaneous (Magnetic) Trip to a maximum of 800% of Motor FLA. But if you read further, and many people fail to, it goes on to say that if that doesn't work, you can now set it for up to 1700% of motor FLA (it used to say 1100%, then 1300%) if 800% is proven not to work. EE motors can sometimes take up to 2200% instantaneous inrush, so nuisance tripping has become a real pain in the rear all over. Motor starter mfrs have had to add options of increasing the breaker sizes they use in Combination Starters because of it.

Start there, chasing micro ohms of contact resistance is tilting at windmills in my opinion.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Teslaphreak]

I didn't belabor all the specific insights I have to this problem. It may be "tilting at windmills" as you say. May question, as poorly expressed as it may have been, is, can anyone "show" me, mathematically, that is inconsequential? What are the implications? To use an extreme example, at some point increased resistance is tantamount to single phasing, thereby causing a phase imbalance. At what point does it cross over? Yes, I can replace the breaker...again, yes I can have it tested. I can have a testing service check the motor again...I can raise the trip value up to 17x, but that may not be addressing the real problem.
Thanks for your input

 

[rbulsara]

You cannot solve a problem that does not exist.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

I did not say that the contact resistance should not be investigated or corrected. I am saying that this is not causing the HMCP to trip. Did you even read my response? This circuit breaker does only one thing - it trips when current in any one of the three phases exceeds the instantaneous trip magnitude. It would be perfectly happy operating with only two phases or only one phase connected to it. It knows nothing about imbalance, it knows nothing about heat, and it knows nothing about contact resistance. It is a very stupid device and generally not a terribly precise one.

It should not trip on locked rotor current, or else the motor can never be started. It would be perfectly content to let the motor single-phase and catch on fire. After the fire starts, it might trip after a short circuit occurs.

Problems occur during starting because the total current drawn by the motor can exceed the symmetrical locked-rotor current by a significant amount, due to the dc offset current. These breakers respond to this current. This offset current is not the same for each start - it depends on the voltage phase angle.

If you have "insights" you have not told us about, then we are basically just playing a guessing game.

David Castor

http://www.cvoes.com