Inrush trip issues

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Ok let me try and get all the proper information in my first post.

I have a 225a square D breaker tripping the second I press the start button on a 125hp rotary screw compressor. I had this issue with this same machine at installation. Customer replaced the breaker with a new one, and it worked properly for 1000 hours give or take a few. Here is the kicker. Now it started tripping at startup again, but for three consecutive start attempts the breaker will trip instantly as the starter closes. Motor never even starts to turn, but on the fourth attempt it will start 95% of the time this is what will get it started. The breaker reset and restart attempts have to be right away though because any delay in trying it again will start he three count over again. The motor never actually starts to turn. Now for the details.

I can easily rotate the motor by hand.
Inrush on the breaker is currently maxed at 2250.
125hp motor.
I get 480 volts from L1 to L2, 479 volts from L2 to L3, and 478 from L1 to L3.
When unit is running I read 159, 159, 154 amps, and voltage on all phases drops only about 1 volt. The amp readings are almost ten above nameplate of the motor.
The equipment is only 40 feet from breaker, and has a disconnect on the wall outside of the unit.
The motor has no shorts to ground from any of the windings, and reading between 1.8 and 2.0 ohms on each winding when isolated from each other . (Didn't have a megohmeter with me)
With the motor removed front the starter it opens and closes perfectly with no tripping of the breaker. All connections are right, and no signs of heat damage of any sort.

The he only issue I can find (and it's a big one) is the wire size. The wire is printed as 3awg, but very hard to read, and my calculations show I should be using 4/0. Is that my issue? I would imagine the wire would handle the inrush, but burn up over time from excessive heat. I didn't install the electrical and the contractor is going to replace it with the proper size, but would that wire size difference cause an inrush trip of that breaker? And is a 225a breaker big enough? I was originally thinking it was another bad breaker but that just doesn't seem right to have two in a row
 

ron

Senior Member
I would think the physical difference between the #3 and 4/0 would be easily detectable.

4/0 is the size to match the 225A breaker.

If it is the smaller #3, maybe so much voltage drop that the motor is having a hard time starting.
 
I would think the physical difference between the #3 and 4/0 would be easily detectable.

4/0 is the size to match the 225A breaker.

If it is the smaller #3, maybe so much voltage drop that the motor is having a hard time starting.

The odd thing is they look very similar in size. I should say the wire installed looks very similar in size to 4/0 I looked again and I can make out the dot matrix style printing of "3 awg", but would that small of a wire cause an inrush trip or a burnout of the wiring before a short circuit trip? That's what I'm wondering. Wire size won't increase or decrease inrush current right?
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Ok let me try and get all the proper information in my first post.

I have a 225a square D breaker tripping the second I press the start button on a 125hp rotary screw compressor. I had this issue with this same machine at installation. Customer replaced the breaker with a new one, and it worked properly for 1000 hours give or take a few. Here is the kicker. Now it started tripping at startup again, but for three consecutive start attempts the breaker will trip instantly as the starter closes. Motor never even starts to turn, but on the fourth attempt it will start 95% of the time this is what will get it started. The breaker reset and restart attempts have to be right away though because any delay in trying it again will start he three count over again. The motor never actually starts to turn. Now for the details.

I can easily rotate the motor by hand.
Inrush on the breaker is currently maxed at 2250.
125hp motor.
I get 480 volts from L1 to L2, 479 volts from L2 to L3, and 478 from L1 to L3.
When unit is running I read 159, 159, 154 amps, and voltage on all phases drops only about 1 volt. The amp readings are almost ten above nameplate of the motor.
The equipment is only 40 feet from breaker, and has a disconnect on the wall outside of the unit.
The motor has no shorts to ground from any of the windings, and reading between 1.8 and 2.0 ohms on each winding when isolated from each other . (Didn't have a megohmeter with me)
With the motor removed front the starter it opens and closes perfectly with no tripping of the breaker. All connections are right, and no signs of heat damage of any sort.

The he only issue I can find (and it's a big one) is the wire size. The wire is printed as 3awg, but very hard to read, and my calculations show I should be using 4/0. Is that my issue? I would imagine the wire would handle the inrush, but burn up over time from excessive heat. I didn't install the electrical and the contractor is going to replace it with the proper size, but would that wire size difference cause an inrush trip of that breaker? And is a 225a breaker big enough? I was originally thinking it was another bad breaker but that just doesn't seem right to have two in a row

125 HP @ 480 volt = 156 amps
Min. CU conductor @ 75 degree is 3/0 (156 X 125%=195)
Inverse time breaker 156 X 250% = 390 amps Next size up allowed = 400 amp.

It would seem that your 225 amp breaker is woefully undersized.
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
If it is the smaller #3, maybe so much voltage drop that the motor is having a hard time starting.
I would agree with that. But why would changing the breaker early in this sequence of events make the problem go away? And why would the problem come back after 1000 hours or so of run time? What had changed in each of those two instances?

My best guess is that the process of replacing the breaker at the beginning of this story also allowed the terminations to be properly tightened. Then, with the extra heat that resulted from the higher than appropriate starting current (i.e., due to the excessive voltage drop along the small conductors), the contact between the conductors and the breaker terminals has degraded over time. That would cause even more voltage drop at that part of the circuit, resulting in even more voltage drop up to the motor terminals, higher starting current, and eventually a complete failure to start. So I suggest getting a close look at the breaker, including an inspection of the wires at the point of termination, and including a test of the tightness of the connections.

 

Pizza

Senior Member
Location
Minnesota
125 HP @ 480 volt = 156 amps
Min. CU conductor @ 75 degree is 3/0 (156 X 125%=195)
Inverse time breaker 156 X 250% = 390 amps Next size up allowed = 400 amp.

It would seem that your 225 amp breaker is woefully undersized.

Agreed


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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160108-1700 EST

Wire size may be of some significance, but an under size breaker for the inrush is probably the real cause.

40 ft of #3 copper is about 0.16 * 0.04 = 0.007 ohms. At 2000 A the voltage drop is about 14 V, not huge compared to a line to neutral of 277 V.

The present breaker may have become more sensitive with time or ambient temperature.

What is strange is the breaker holding after several quick tries.

The instantaneous trip characteristic my differ compared to the thermal trip of the breaker. Is it possible that instantaneous trip increases with some increase in internal component differential temperature?

Also motor winding resistance may increase after the several tries to start, thus reducing inrush current.

.
 

Jraef

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Location
San Francisco Bay Area, CA, USA
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I would agree with that. But why would changing the breaker early in this sequence of events make the problem go away? And why would the problem come back after 1000 hours or so of run time? What had changed in each of those two instances?

My best guess is that the process of replacing the breaker at the beginning of this story also allowed the terminations to be properly tightened. Then, with the extra heat that resulted from the higher than appropriate starting current (i.e., due to the excessive voltage drop along the small conductors), the contact between the conductors and the breaker terminals has degraded over time. That would cause even more voltage drop at that part of the circuit, resulting in even more voltage drop up to the motor terminals, higher starting current, and eventually a complete failure to start. So I suggest getting a close look at the breaker, including an inspection of the wires at the point of termination, and including a test of the tightness of the connections.

I agree with everything in the above being a possibility, but the part I highlighted in red is the only thing that doesn't work. If you drop the voltage, you drop the current, that's what a reduced voltage starter does. And if that was the case, the motor would still ATTEMPT to start and rotate, the tripping would not be instantaneous and the OP stated that it never even twitches. That's indicative of a magnetic trip.

156A FLC on the motor, the magnetic inrush can be as high as 2600-3000A under the right circumstances, especially if it is a new "energy efficient" motor design. It's a known issue with new motors now. The same things they do to improve efficiency also make the magnetic inrush worse. 2600A inrush, 2250A max trip setting, it's a game of Russian Roulette. Sometimes if the closing point hits on the right part of the sine wave, it doesn't trip, sometimes it does. Basically you were lucky for a long time, but the luck ran out.

If not an EE motor, it might also be that because of the undersized conductors, the insulation is breaking down and leaking to ground, which makes it worse. A tiny bit of moisture in the conduit facilitates that even more so, but after heating the conductors up 3 times, the moisture is being evaporated. If you wait too long, the moisture creeps back in or condenses and you have to start over. Bottom line, the first problem is the undersized breaker and possibly undersized conductors. Change it to 3/0 and put in at LEAST a 300A breaker, I'd go with a 400A as suggested. There is nothing more expensive than having to do the same thing twice, except doing it a third time.
 

charlie b

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I agree with everything in the above being a possibility, but the part I highlighted in red is the only thing that doesn't work. If you drop the voltage, you drop the current, that's what a reduced voltage starter does.
I am a bit pressed for time presently. But let me quickly ask, what part of the OP's description leads you to conclude that there is a reduced voltage starter?

 

Jraef

Moderator, OTD
Staff member
Location
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Electrical Engineer
I am a bit pressed for time presently. But let me quickly ask, what part of the OP's description leads you to conclude that there is a reduced voltage starter?

I didn't mean he had an RV starter, I was just using that as an example as to why reducing the voltage reduces the current, not increases it.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Another thought is to install a correctly sized MCP (Motor Circuit Protector) rated breaker, rather than a standard thermal/magnetic. They're designed to tolerate the inrush. The overloads provide thermal protection.
 

Aleman

Senior Member
Location
Southern Ca, USA
Change it to 3/0 and put in at LEAST a 300A breaker, I'd go with a 400A as suggested. There is nothing more expensive than having to do the same thing twice, except doing it a third time.


I can see the reasoning behind this but then you have a motor that draws less than half of the protection level at running speed. Isn't this also a problem? If he lost a phase or developed a high
load due to misc circumstances, the motor would be able to self destruct easily without tripping the breaker unless there is also an overload. I didn't see the OP mention any OL protection. I don't
often work with motors this large but on smaller motors I have replaced breakers with slower trip curves to prevent inrush trips.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Another thought is to install a correctly sized MCP (Motor Circuit Protector) rated breaker, rather than a standard thermal/magnetic. They're designed to tolerate the inrush. The overloads provide thermal protection.
Not really any more. The only difference now is just that the MCP doesn't have thermal trips.

Years ago, thermal mag breakers had fixed mag trips, not adjustable. So MCPs were released that had no thermal trips, but an adjustable mag trip so it could be turned up to meet motor requirements without nuisance tripping. But modern thermal mag breakers above 15A now almost all have adjustable mag trips, and the trip adjustment range is exactly the same as an equal sized MCP. The only reason mfrs use MCPs on starters now is because for them, they are cheaper since the OL relay has to be there anyway.

And, you and I can't add a mag-only (MCP) breaker in the field if it is not a direct replacement of one that was originally there. Mag-only breakers are not UL listed and restricted by the NEC to only be used in factory built and listed assemblies.
 

Jraef

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I can see the reasoning behind this but then you have a motor that draws less than half of the protection level at running speed. Isn't this also a problem? If he lost a phase or developed a high
load due to misc circumstances, the motor would be able to self destruct easily without tripping the breaker unless there is also an overload. I didn't see the OP mention any OL protection. I don't
often work with motors this large but on smaller motors I have replaced breakers with slower trip curves to prevent inrush trips.
There has to be an overload relay. In theory you MIGHT be able to use a breaker if the thermal trip rating HAPPENED to be within that maximum 125% of FLA, but that's not only rare, it's unlikely that the mag trips on that breaker would hold in.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Not really any more. The only difference now is just that the MCP doesn't have thermal trips.

Years ago, thermal mag breakers had fixed mag trips, not adjustable. So MCPs were released that had no thermal trips, but an adjustable mag trip so it could be turned up to meet motor requirements without nuisance tripping. But modern thermal mag breakers above 15A now almost all have adjustable mag trips, and the trip adjustment range is exactly the same as an equal sized MCP. The only reason mfrs use MCPs on starters now is because for them, they are cheaper since the OL relay has to be there anyway.

And, you and I can't add a mag-only (MCP) breaker in the field if it is not a direct replacement of one that was originally there. Mag-only breakers are not UL listed and restricted by the NEC to only be used in factory built and listed assemblies.

Don't want to hijack this thread, but I didn't know that. We've had several older MCC's with thermal/mag breakers correctly sized that trip on startup regardless of the magnetic setting. We replaced them with same sized MCP's and they quit doing it. Maybe it was just because of breaker age or a tired thermal element. Worked, though.
 

don_resqcapt19

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Location
Illinois
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retired electrician
... But modern thermal mag breakers above 15A now almost all have adjustable mag trips, and the trip adjustment range is exactly the same as an equal sized MCP. ...
I don't think I have ever seen one. Are they breakers that are only used in starters?
 

don_resqcapt19

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Location
Illinois
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Don't want to hijack this thread, but I didn't know that. We've had several older MCC's with thermal/mag breakers correctly sized that trip on startup regardless of the magnetic setting. We replaced them with same sized MCP's and they quit doing it. Maybe it was just because of breaker age or a tired thermal element. Worked, though.
It will work just fine, but a violation of 430.52(C)(3).
 

kwired

Electron manager
Location
NE Nebraska
I agree the 3AWG conductors mentioned are too small for the application, but would like to point out that by increasing them to the proper size it very possibly reduces overall circuit resistance enough that the starting current raises enough to trip this breaker even more often then it trips now.

I have a Square D sliding motor calculator that recommends a 225 amp (type LAL) breaker for this motor, but keep in mind that is just a recommendation by Square D that works in many cases. Actual NEC maximum permitted is the previously mentioned 400 amp, and even then if it still won't hold during starting it can be increased some more.
 
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