Heaters on 25hp motor starter keep tripping
Heaters on 25hp motor starter keep tripping
(OP)
I have a rotary screw compressor (25hp) with 64 amps indicated on the 3 phase 240 volt motor id plate. I brought in an "expert" to service the compressor because it began frequent thermal overload tripping. He found the motor is in balance and draws 56 amps. He suggested changing the starter. I installed a Nema 3 AB 509-DOD with W-72 heaters. They are rated for 65 FLA. I also noted the motor was filthy with sawdust as it is near a dust collection system. After a full day of blowing out all the dust and reconfirming all the control wiring, the darn thing still continues to trip the heaters. Do I simply need heaters with a higher FLA rating. I don't want to smoke the motor. Any help would be great !





RE: Heaters on 25hp motor starter keep tripping
RE: Heaters on 25hp motor starter keep tripping
When you say the OL heaters are tripping, do you know what the motor current is doing at the time they trip? It's highly unlikely that if your motor is truly drawing only 56A and the heater elements are rated for 65A, that they are falsely tripping. More likely when you are reading 56A everything is A-OK, but when you are not watching, your motor is getting something like a voltage change that is increasing the motor current, or a pressure relief valve on your receiver is not opening when it should. Only a load increase or a severe change in voltage, which causes an increase in current (over and under voltage will both do this), will cause OL heaters to trip. Some OL relays will also trip on current imbalance, but not the one you are describing.
Buzzing of a breaker is NEVER something to ignore! That was an irresponsible statement IMHO. Is it the 3 pole breaker that is feeding this motor starter that is buzzing, or is it a 1 pole small breaker that feeds the control power to the motor starter that is buzzing? Either way, pressing the Start button after the motor starter has already engaged should not cause ANY change in the circuit. Something is definitely wrong there.
Melting the center pole of a 3 pole circuit breaker twice in 25 years would be unusual; twice in 18 months is a HUGE problem. Either you have loose wire or busbar connections, under sized conductors, a severely imbalanced system, a bad winding in the motor, a high impedance ground fault, or any combination of the above (or something else!). No matter what, that is something that MUST be attended to! If you replaced the breaker after it happened the first time, you should have had a detailed investigation after the second failure. That is highly unusual and suspicious.
Were it my system, I would start with getting to the bottom of the melting breaker pole, that is likely a harbinger of whatever is giving you problems elsewhere. An infra-red thermal scan under load would be a great starting point. If that isn't part of your normal maintenance routine, hire a switchgear service company to come out and do that to start with.
JRaef.com
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RE: Heaters on 25hp motor starter keep tripping
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Heaters on 25hp motor starter keep tripping
I should check for lower voltage as compared to the 240 V nameplate requirement, contactors with wore surfaces and loose connections.
RE: Heaters on 25hp motor starter keep tripping
RE: Heaters on 25hp motor starter keep tripping
RE: Heaters on 25hp motor starter keep tripping
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Heaters on 25hp motor starter keep tripping
I don't think he has a clue. But maybe that's just me.
RE: Heaters on 25hp motor starter keep tripping
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Heaters on 25hp motor starter keep tripping
I would investigate your breaker, sounds like it could be single phasing which will definatley trip an overload.
A unloaded running 3 phase motor will continue to run with 1 phase removed until it is loaded enough to stall or trip an overload. It could be the breakers connection to the panel buss is deteriorating. This will cause obvious heating. Feel the breaker case, see if it is hot, especially after it has run for awhile.
RE: Heaters on 25hp motor starter keep tripping
You might want to test the breaker resistance with a high amperage ohmmeter.
RE: Heaters on 25hp motor starter keep tripping
RE: Heaters on 25hp motor starter keep tripping
If you resort to electrical contractors, look for those that say "industrial" and have a conversation with them about your problem first, to make sure they send the right guy out. They DO have some good ones...
old field guy
RE: Heaters on 25hp motor starter keep tripping
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Heaters on 25hp motor starter keep tripping
Given that the bus bar extensions are hidden behind the breakers, I would pull one or more breakers and look for dis-colorization on the bus bars.
respectfully
RE: Heaters on 25hp motor starter keep tripping