Compressor issues

[FirtstKnight]

I am a maintenance man but I do hold a journeyman's license I'll do my best to explain what is going on. I have 2 ea 50 Hp air compressor. one is approx. 2 yrs old (Com-05) the other is 20+ (Com-01). I seem to be having similar problems with both.
We are running 240 VAC 3 phase into the building to a 400 amp I-line from there a 150 amp breaker carries power to a 200 amp disconnect. with 150 amp fuses. from there it goes to a switch box that allows me to select which compressor I want to use. Com-01 the older one will run for 5-6 hrs. then it will trip the motor overload in the cabinet. by the time I get there (5 Min) I am able to reset the overload and it will run for a couple of more hrs. then it will do it again. During the cooler months we were able to run it for weeks then it started this as the temps started to rise. oil temps on the gauges seem well with in range. I have tested and replaced the temp switches they seem fine. I have replaced the motor starter with the overload and the thermal units. More recently we sent the motor out to a motor shop for evaluation . They did $1300 dollars of work to it. We reinstalled it on 6/30/21
and had the same result.. Com-05 runs most of the time but back in April while I was on vacation it tripped the breaker and blew one fuse, then the week after I got back it blew one fuse (not the same fuse) it has ran fine until 6/30/21 then it tripped the breaker and blew one of the fuses. I have done an amp draw on both compressors . Readings are A Phase= 100 amps B Phase = 87 Amps and C Phase= 88 Amps. All connections seem fine. I am at a loss

 

[Lucien Nunes]

You might save a lot of time by connecting a logger to monitor the line currents and voltages. When a trip occurs you will be able to look back over the immediate history to see how these parameters moved from normal operation to abnormal.

One possibility is that there is something wrong with the supply, perhaps one phase sags or browns out, and the motor protection gear is not acting promptly to disconnect the compressor before the overcurrent kills the fuse. That would explain the similar problem on two different compressors, and possibly a seasonal variation due to changing demand on the power grid. The log should enable you to distinguish between this kind of situation and a regular motor overload from within the compressor.

 

[Megawatt]

I am a maintenance man but I do hold a journeyman's license I'll do my best to explain what is going on. I have 2 ea 50 Hp air compressor. one is approx. 2 yrs old (Com-05) the other is 20+ (Com-01). I seem to be having similar problems with both.
We are running 240 VAC 3 phase into the building to a 400 amp I-line from there a 150 amp breaker carries power to a 200 amp disconnect. with 150 amp fuses. from there it goes to a switch box that allows me to select which compressor I want to use. Com-01 the older one will run for 5-6 hrs. then it will trip the motor overload in the cabinet. by the time I get there (5 Min) I am able to reset the overload and it will run for a couple of more hrs. then it will do it again. During the cooler months we were able to run it for weeks then it started this as the temps started to rise. oil temps on the gauges seem well with in range. I have tested and replaced the temp switches they seem fine. I have replaced the motor starter with the overload and the thermal units. More recently we sent the motor out to a motor shop for evaluation . They did $1300 dollars of work to it. We reinstalled it on 6/30/21
and had the same result.. Com-05 runs most of the time but back in April while I was on vacation it tripped the breaker and blew one fuse, then the week after I got back it blew one fuse (not the same fuse) it has ran fine until 6/30/21 then it tripped the breaker and blew one of the fuses. I have done an amp draw on both compressors . Readings are A Phase= 100 amps B Phase = 87 Amps and C Phase= 88 Amps. All connections seem fine. I am at a loss

Are you saying that it’s blowing fuses in the control panel or your 200 amp disconnect ?

 

[Marvo]

Is it a thermal type motor overload that's tripping? Does the cabinet have a ventilation fan and if so is it working?

I just had a fault on a large air handler unit where the motor thermal overload was intermittantly tripping do to high ambient temperatures inside the control panel because the ventilation fans were seized.

 

[FirtstKnight]

Are you saying that it’s blowing fuses in the control panel or your 200 amp disconnect ?

200 amp disconnect

 

[FirtstKnight]

I have contacted our supplier on this issue and have had little success . I was looking at a monitor on Tuesday. I was out yesterday . I will look some more today. I want too thank you for all the responses so far

 

[FirtstKnight]

You might save a lot of time by connecting a logger to monitor the line currents and voltages. When a trip occurs you will be able to look back over the immediate history to see how these parameters moved from normal operation to abnormal.

One possibility is that there is something wrong with the supply, perhaps one phase sags or browns out, and the motor protection gear is not acting promptly to disconnect the compressor before the overcurrent kills the fuse. That would explain the similar problem on two different compressors, and possibly a seasonal variation due to changing demand on the power grid. The log should enable you to distinguish between this kind of situation and a regular motor overload from within the compressor.

Do you have a logger that you would recommend,

 

[marconi]

You have some useful current measurements. If, using your nomenclature for the supply lines A, B and C, you label the motor leads as say 1, 2 and 3, you have the following information on connections and currents:

(A B C) - (1 2 3) - (100A 87A 88A) ie A to 1, B to 2 and C to 3.

If now you rotate the supply lines 120 degrees:

(B C A) and connect these to (1 2 3) ie B to 1, C to 2 and A to 3 and now measure the three currents, - see attachment -

then:

(1) if the currents are now (87A 88A 100A) ie they correspond still with the supply lines A, B and C - the unbalance problem is with the supply;

(2) if the current are now (100A, 87A 88A) ie: they correspond with the motor leads 1, 2 and 3 the unbalanced problem is with the motor.

 

[FirtstKnight]

You have some useful current measurements. If, using your nomenclature for the supply lines A, B and C, you label the motor leads as say 1, 2 and 3, you have the following information on connections and currents:

(A B C) - (1 2 3) - (100A 87A 88A) ie A to 1, B to 2 and C to 3.

If now you rotate the supply lines 120 degrees:

(B C A) and connect these to (1 2 3) ie B to 1, C to 2 and A to 3 and now measure the three currents, - see attachment -

then:

(1) if the currents are now (87A 88A 100A) ie they correspond still with the supply lines A, B and C - the unbalance problem is with the supply;

(2) if the current are now (100A, 87A 88A) ie: they correspond with the motor leads 1, 2 and 3 the unbalanced problem is with the motor.

I'm not sure sure what this would do because I get the exact same reading on 2 different motors. I'm pretty sure that it is a supply issue but I am not sure what is causing it.

 

[marconi]

I would do the easy test I mentioned earlier but it is up to you what you do. It does help quickly point to a supply or compressor problem.

What are the 3 line to line voltages (or line to neutral voltages) at the motor terminals, at the output of the motor contactor, at the input to the motor contactor, at the input to the panel isolator, at the supply end of the panel feeder and finally at the source of supply to installation?

Quick sum. 50hp compressor on 3 phase 240V supply at say 0.8pf.

Balanced line current = (50 x 750)/(1.7 x 240 x 0.8) = 114A. So your figures appear 'low' too.

What I would do is measure the incoming supply voltages (line or phase) and currents with compressor off and then on to calculate the external impedance of the incoming supply. I would do the same at the output of the panel isolator and output of the circuit breakers, fuses and overload trips you mentioned to do some sums for the installation and panel wiring. For balanced line currents the line impedances need to be balanced too.

What type of motor starter? DOL or Star Delta or something else?