air compressor problem

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elecmen

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Electrician
Hi, There is a problem with a a 7.5 HP 240V single phase air compressor. The compressor is fed with 8/2 MC cable and protected with a 40A breaker.The data plate states 29 A@240V The breaker periodically trips. I would like to put a 50A breaker in. Can I do this for nucense tripping? I cannot find the ampacity of the #8 MC. I do not know what insulation or temp rating is used in MC cable. Can somebody help me out?
 
If it's MC, you limiting factor is probably 100.14 and chances are you are good for 75 deg, or 50 amp.
The motor amps is a bit unusual, 7-1/2 HP 240 SPh by Code (430.248 ).would be 40 amps.
Is the motor actually marked 7.5 HP or "SPL" ?? ... often compressors carry a unusual HP rating.
In any event, there should be no problem increasing the breaker to 50 amp.
With more definite motor data (actual HP, thermal protected, etc ) it's possible to go much higher.
On a "true" 7-1/2 HP if your breaker is only for short-circuit protection, it could be a 100 amp breaker by Code, but that does not seem to be your situation based on the 29 amp nameplate.
 
I would install a 60 or even 70 amp breaker. 250% of motor full load current seldom results in tripping during starting unless there is a problem - then it is doing what it is supposed to do. As has been mentioned you could go up to 100 amp breaker for 7.5 hp without having to see that it will not hold during starting before going larger. A 40 amp breaker is only 138% of full load current, A 50 amp breaker is 172%.
 
elecmen said:
Hi, There is a problem with a a 7.5 HP 240V single phase air compressor. The compressor is fed with 8/2 MC cable and protected with a 40A breaker.The data plate states 29 A@240V The breaker periodically trips. I would like to put a 50A breaker in. Can I do this for nucense tripping? I cannot find the ampacity of the #8 MC. I do not know what insulation or temp rating is used in MC cable. Can somebody help me out?
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I know this motor well , It is a special compressor rated motor . I would start by lowering both the cut in and cut out pressure by 10% , some one might have ran it up or the compressor is getting old . The breaker could also be bad or the buss pitted . A time delay 40 would be nice but expensive . If you have to go with the 50 you will be lucky if the motor is not starting to go . 10% pressure will take a tremendous strain off the motor when trying to start or get enough pressure to cut out . Some compressers have a bleeder to empty the back pressure from the cylinder until the piston gets moving . , the bleeder line can get clogged.
 
My two thoughts would be to assure we have some overload protection before jumping too high and we can not rely on the Tables as this is most likely not a true 7-1/2 HP.
 
jetlag said:
... If you have to go with the 50 you will be lucky if the motor is not starting to go ...
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A 29 amp motor could easily draw 145 - 175 amps instantaneously at startup. Most 50 amp breakers are likely to trip at least occasionally if that is the case. The amount of instantaneous current will depend on the impedance of the supply as well as impedance of service, feeders, and branch circuit conductors. The closer the motor electrically is to the supply transformer the higher the instantaneous current will be.
 
augie47 said:
My two thoughts would be to assure we have some overload protection before jumping too high and we can not rely on the Tables as this is most likely not a true 7-1/2 HP.
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It is not a true 7.5 it is probably less than a true 5 , it uses less amps and the 40 is suppose to pull it . It has a thermal switch with reset button . I figure if he is going to fry with the 50 he might as well do it right with the 60 :grin::grin:
 
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Put a hard start cap on it. Your likely only triping because it kicks in under high pressure. Do take the time to varify the runnings amps with meter. Could be old and bearings going bad . If that fails i would up the breaker. It likely has built in overload.
 
augie47 said:
My two thoughts would be to assure we have some overload protection before jumping too high and we can not rely on the Tables as this is most likely not a true 7-1/2 HP.
Click to expand...

The 40 amp breaker does not provide proper overload protection (138% of 29 amps), it still needs separate overload no matter what the breaker size is.

Jim W in Tampa said:
Put a hard start cap on it. Your likely only triping because it kicks in under high pressure. Do take the time to varify the runnings amps with meter. Could be old and bearings going bad . If that fails i would up the breaker. It likely has built in overload.
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Hard start will actually increase the instantaneous current. Starting time will be less, but it will work harder at getting started. If there is properly working unloading valve there should be no need for hard start capacitor.
 
kwired said:
The 40 amp breaker does not provide proper overload protection (138% of 29 amps), it still needs separate overload no matter what the breaker size is.



Hard start will actually increase the instantaneous current. Starting time will be less, but it will work harder at getting started. If there is properly working unloading valve there should be no need for hard start capacitor.
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Overload should be internal in the motor. If not then we must rely on motor.
As far as the hard start i will simply tell you they often fix problems like this.
First step should be check running amps. If higher than specs you have a problem that might be cheaper to fix before it burns up.
 
Jim W in Tampa said:
Overload should be internal in the motor. If not then we must rely on motor.
As far as the hard start i will simply tell you they often fix problems like this.
First step should be check running amps. If higher than specs you have a problem that might be cheaper to fix before it burns up.
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Thats true Jim , people are sometimes to quick to bump up a breaker , Those motors have both start and run capacitors , often the cut out mechanism sticks or is sluggish to open on the start capacitor and causes the breaker to trip. this can be seen on the amp meter while starting with an analog clamp on , the needle will stay high to long or fluxuate before droping to run amps. A little WD 40 is less expensive than a motor any time , but there are other things that can prevent the motor from reaching enough speed for the start capacitor to cut out , a little to long and the breaker trips . An example is to much start pressure on compressor . :grin::grin:
 
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I forgot to mention the start capacitor it self could be bad or the contact points for the start capacitor can get pitted and burned. Many good motors have be thrown away that needed simple repair . They finally will burn up if the problem is not corrected.:grin:
 
i had a problem with nuisance tripping on a large compressor once. only seemed to happen when it was hot out. i put my clamp meter on one of the legs at the breaker and watched patiently till it tripped. i did this 3 times to verify all 3 legs. turned out it wasn't getting anywhere near the limit of the OCPD. replaced the breaker and no problems since.
 
you got some great options in the posts......
1st step = get data (who, what, where, when, how much)

2nd step = test assumptions (simplest and least expensive first)

3rd step = only introduce one change at a time and test it.

just work your way thru the options...........
 
Jim W in Tampa said:
Overload should be internal in the motor. If not then we must rely on motor.
As far as the hard start i will simply tell you they often fix problems like this.
First step should be check running amps. If higher than specs you have a problem that might be cheaper to fix before it burns up.
Click to expand...
:confused:

If motor has no internal overload protection then you must provide it. Standard inverse/time circuit breakers typically will not provide proper overload protection just short circuit and ground fault protection. If you actually try to provide overload protection with an inverse/time breaker you will often have tripping at start up when nothing is otherwise wrong, they have the wrong time/current curves.

jetlag said:
Thats true Jim , people are sometimes to quick to bump up a breaker , Those motors have both start and run capacitors , often the cut out mechanism sticks or is sluggish to open on the start capacitor and causes the breaker to trip. this can be seen on the amp meter while starting with an analog clamp on , the needle will stay high to long or fluxuate before droping to run amps. A little WD 40 is less expensive than a motor any time , but there are other things that can prevent the motor from reaching enough speed for the start capacitor to cut out , a little to long and the breaker trips . An example is to much start pressure on compressor . :grin::grin:
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I run into the opposite much more often - the centrifugal switch failed to close after the last time motor was running and therefore start capacitor is not in circuit the next time it is started. A motor with proper overload protection will be protected from this.

jetlag said:
I forgot to mention the start capacitor it self could be bad or the contact points for the start capacitor can get pitted and burned. Many good motors have be thrown away that needed simple repair . They finally will burn up if the problem is not corrected.:grin:
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An air compressor motor has enough load on it that if the start capacitor and or starting switch is open the motor will not even turn, that capacitor is needed to create enough phase shift to provide necessary starting torque.
 
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