Motor Starter Contact resistance
Motor Starter Contact resistance
(OP)
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.





RE: Motor Starter Contact resistance
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
www.cvoes.com
RE: Motor Starter Contact resistance
RE: Motor Starter Contact resistance
What is the setting? Is HMCP tested by a testing compnay?
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor Starter Contact resistance
Any chance this breaker has a shunt trip connected to ground fault protection?
David Castor
www.cvoes.com
RE: Motor Starter Contact resistance
RE: Motor Starter Contact resistance
RE: Motor Starter Contact resistance
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.
RE: Motor Starter Contact resistance
Regardless of if this is the source of the problem, the resistance value is not acceptable.
RE: Motor Starter Contact resistance
David Castor
www.cvoes.com
RE: Motor Starter Contact resistance
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RE: Motor Starter Contact resistance
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.
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RE: Motor Starter Contact resistance
Thanks for your input
RE: Motor Starter Contact resistance
Rafiq Bulsara
http://www.srengineersct.com
RE: Motor Starter Contact resistance
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
www.cvoes.com