air compressor problem

[Jim W in Tampa]

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.

My typo mistake
meant to say
Overload should be internal in the motor. If not then we must rely on breaker


I do believe he has some work ahead of him to make the right correction. From what has been mentioned
1 breaker might be weak
2 caps on motor might be defective
3 bearings might be going out
4 start switch might have problems
5 settings for cut in and out might been altered
6 breaker to buss bar pitted and causing heat

will assume customer does not want 10 service calls to pay for.
I would replace breaker and perhaps size it up a little
replace caps on motor
see if there is place to oil bearings
check the setting and adjust to factory default
check start switch
check buss bar not only for this breaker but the ones near it
put amp probe on it.
Your saving him money and keeping customer happy if you take the shot gun method and fix it in 1 trip.

 

[kwired]

My typo mistake
meant to say
Overload should be internal in the motor. If not then we must rely on breaker


I do believe he has some work ahead of him to make the right correction. From what has been mentioned
1 breaker might be weak
2 caps on motor might be defective
3 bearings might be going out
4 start switch might have problems
5 settings for cut in and out might been altered
6 breaker to buss bar pitted and causing heat

will assume customer does not want 10 service calls to pay for.
I would replace breaker and perhaps size it up a little
replace caps on motor
see if there is place to oil bearings
check the setting and adjust to factory default
check start switch
check buss bar not only for this breaker but the ones near it
put amp probe on it.
Your saving him money and keeping customer happy if you take the shot gun method and fix it in 1 trip.

The OP says he has a 7.5 hp air compressor motor on a 40 amp breaker.

He may have a special duty motor that is marked 7.5 hp with a nameplate amps of 29.

NEC table 430.248 says 7.5 hp draws 40 amps

NEC table 430.52 says maximum inverse/time circuit breaker is 250% of motor full load current.

Even if he uses the 29 amps marked on the motor he can go up to 70 amp breaker. A 40 amp breaker is only 138% of full load current and likely will result in nuisance tripping during starting - especially for a hard starting load like a compressor.

I would check many things you mentioned - I would not replace capacitors unless they were bad - they are good or bad not somewhere in between. I would replace the breaker with at least a 60 amp breaker and not question much in doing so - it is undersized in the first place and the 40 is oversized for overload protection - motor needs separate overload protection whether internal to the motor or not.

One thing that is left out is when does the breaker trip? If it is always at starting then the breaker is undersized, specially if running current is in normal range. If it trips while running there is a short circuit or it is overloaded at least 138%.

 

[Jim W in Tampa]

Lets not forget 1 very key issue in that this was working . Something has changed. Putting larger breaker very likely will fix the triping but might not fix the problem.

 

[jetlag]

Lets not forget 1 very key issue in that this was working . Something has changed. Putting larger breaker very likely will fix the triping but might not fix the problem.

And it will get worse untill the other breaker starts to trip . These are special motors marker compressor duty, they have an overload reset botton and overheat protector . When you are talking motors threads can go over a hundred easily :grin::grin:

 

[weressl]

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.

If you hook up an ammeter you may find that the compressor draws a much higher amp toward the end of the run, just before the pressure switch cuts off. If you increase(d) the the recommended reservoir size or have a much higher airflow that it is rated for then it will overload.

 

[augie47]

If you hook up an ammeter you may find that the compressor draws a much higher amp toward the end of the run, just before the pressure switch cuts off. If you increase(d) the the recommended reservoir size or have a much higher airflow that it is rated for then it will overload.

Good Point