Heaters on 25hp motor starter keep tripping 3

[klsieber]

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 !

 

[klsieber]

One more point. When the NO momentary start button is depressed, the breaker in the panel board makes a slight buzzing sound that my hired "expert" says not to worry about. I has already melted the middle leg on the breaker twice in the last 18 months... seems like something I should be concerned about.

 

[jraef]

I would say you have more than one immediate issue, but they may be able to be traced to an overlying main problem.

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
"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

 

[itsmoked]

I would get out a clamp-on ammeter and sit and watch the current for a loooooong time. Certainly thru all aspects of the compressor's cycle. I suspect something mechanical that is overloading the motor. If you have a ton of dust in the motor why wouldn't you have a ton of dust in the actual compressor? What protection does the air inlet have?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[aolalde]

Check the voltage and current. Cleanning the external motor surface is good for the motor thermal performance but has no relation with the thermal current protection.
I should check for lower voltage as compared to the 240 V nameplate requirement, contactors with wore surfaces and loose connections.

 

[dpc]

Good suggestions. I'd just add that the ambient temperature at the motor starter could also be an issue. If it is extremely hot at the starter, this can cause tripping at lower values of current than expected. It could also possibly cause some of the breaker problems you've mentioned.

 

[klsieber]

Thanks for all the great advice. My compressor "expert" was back today and changed oil and filters. He rechecked all line and control wiring connections. He says it all checked out and feels the problem is downstream from the starter. He also indicated the compressor itself is not creating a load beyond the capacity of the motor. He is 80% sure the problem has to be in the squirrel cage motor. Does it make sense at this point to replace the 25 hp motor with a new one? Thanks for your attention.

 

[itsmoked]

I sure wouldn't. I'd find someone who would actually find the problem rather than suggest where it isn't.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

If he can explain how the bad motor made the breaker melt down I'd be interested in that explanation.

I don't think he has a clue. But maybe that's just me.

 

[jraef]

As a general rule, motor winding problems are not intermittent, they tend to be either there or not there. That doesn't rule out the odd occasion of a broken rotor bar that only opens up under stress or a winding that is shorting to ground only when there is a lot of internal condensation, but these kinds of things are very rare. Remember the Occam's Razor rule: work from a theory that introduces the fewest assumptions and postulates the fewest hypothetical entities. In this case, since you have "ruled out" mechanical loading (in quotes because I wouldn't totally buy into that yet), then the next simplest explanation is a periodic voltage drop. A Chart recoding power meter can be rented fairly inexpensively to see if there is anything going on in your incoming supply.

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

 

[badasselectric]

Screw compressors only approach fla when they approach max psi.What pressure is it operating at and what is it rated at? Slight pulley changes allow for different cfm/psi configurations at a given hp.Reducing pressure will reduce current. Once you determine the pressure switch or regulator set point has not changed I would continue with electrical trouble shooting.

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.

 

[dpc]

A bad breaker pole could explain the meltdown and the overload tripping on single-phasing.

You might want to test the breaker resistance with a high amperage ohmmeter.

 

[klsieber]

I've done some research and feel it's time to hire an electrical specialist rather than an air compressor specialist. Can any of you suggest how I might find the right guy in the Gainesville, Florida area. I don't feel comfortable going to the yellow pages as most electicians in this area wire single phase residential and commercial and may be no help with 3 phase issues like this. Thanks for all your help.

 

[oldfieldguy]

Try

http://www.netaworld.org

for a listing of companies who actually work on industrial power equipment. They're likely to be a bit more pricey than a regular electrical contractor. These folks will likely know how to look for problems in all parts of your electrical system.

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

 

[itsmoked]

Another thing is ask around your business area and talk to other proprietors with similar equipment or hopefully a more complex facility. Ask them who they use and if they are pleased with the result.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I would suspect a bad connection between the bus bar extension and the bus bar. This would explain the failed breakers and could cause intermittent unbalances that would trip the O/L relay.
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

 

[Chazney]

klsieber, I have a similar problem. Did you ever solve it. What was the answer? Thanks.