Ok electric gurus, I know this is likely to be up to me to tear into this thing to find out, but I have a 7.5hp air compressor that just recently noticed laboring on startup (we have an air valve right at the compressor). I did all the normal checks I would think, the check valve for the tank, it hisses after shutdown and there is not air pressure in the manifolds or head of the compressor. I even loosened the belts for the motor to the pump and it still labors on startup so we can rule the pump out. I don't have capacitance check on my DVOM but on Ohms, both startup capacitors seem to have the same reading as the meter charges them as much as it can. Its not a great test but having two capacitors with the same reading rules the probability of it being one of them slim, but not 100%. So after doing all these checks, I flipped the motor on a few times, out of the 5-8 times the motor started like it should 2-3 times and the others labored again. This doesn't seem like a capacitor issue to me and read there is a centrifugal switch inside that can cause symptoms like mine and seems like it would be more probable. What say ye? It also sounds like the rear bearing Is noisy? but the shaft doesn't have any real deflection at.. The basics of the motor are: Single phase 2 start/ 2 run capacitors 7.5 HP Weg
You can't check resistance with the circuit energized. That said, the start caps are probably wired in series, which would give you the same readings on both if you didn't disconnect them and discharge them before measuring. I'd also be willing to bet that you have one or more weak caps. The reason for 5/8 good starts is that the phase of the windings and the single phase input were in the best spot to give a good start on those attempts, but out of phase on the others (the labored starts).
They were disconnected, I de-energize them. I understand it wasn't a great test and not conclusive, I was just looking at the resistance change as the DVOM battery inputed something into the capacitor while reading, which they both did equally, no open or shorted readings either.
Ok, cool. I'd still suspect bad caps. Even if you do get a test of resistance, it's not under any kind of load. Any way to borrow a capacitance tester?
I got the ok from the owner to tear down the motor and found bad start switch and the centrifugal spool that disengages it along with the rear rotor bearing rough, so I guess i'll be sourcing the parts to get this thing running ASAP.
By laboring on starts do you mean it sits there and hums, or it's slow to build revs, or you have to give it a push? If it just hums or buzzes on start up intermittently it could be the start switch you alluded to. But for capacitors, in my experience and that of other electricians I've talked to about it, they're either good or they're not. I've never heard of an intermittently bad cap from someone that deals with them a lot. A connection in a cap's circuit could cause intermittent problems, but the cap either works or it doesn't. Your capacitors are fine. Tank check valves can be intermittent. If you want to test that next time it labors, just turn it off, loosen the tubing fitting (downstream of the tank check), and see if pressure escapes. Tighten and see if it starts. Won't necessesarily be conclusive since you're chasing an intermittent problem, but it could be. Age could be a consideration. I've changed a lot more compressor motors than pumps as they age. Flaky pressure switches can cause starting issues, as well as any other switches in the starting circuit. And it could be something else requiring motor replacement. 7.5hp is pretty big so hope not. That's all off the top of my head. If anything else comes to the surface I'll be back.
