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15 hp RPC struggles to start 7.5 hp 3 phase air compressor - add cap?

I have had my RPC up and running for ten years or so without a hitch. Confident, I decided to rebuild a 3 phase air compressor despite persistent rumors they can be hard to start with an RPC. I spun it up today for the first time. It started fine with the tank empty. (There is no pressure gauge installed on the tank at present.) However, with the tank partly full the motor pulley spun quite slowly as if overloaded, and never came up to speed before the output breaker on my RPC blew. I tried again with 2 other 3 phase machines running and the motor started fine.

Let me digress into unloading systems for a moment, please bear with me. The air pump is from the Champion R series. Those have robust unloaders. The discharge tubes vent to outside air as long as the motor isn't running. As soon as it starts running, a centrifugal gizmo closes a valve and this allows air to begin building. It is difficult for me to imagine how if the centrifugal unloader seems to be working that it could somehow not really be working. It's pretty robust.

My current use model for this machine is for occasional use, so it's not enormously burdensome to have to turn on a couple of extra machines. But it's wasteful of power, and noisy. I'm wondering if I could simply add some run capacitors to boost the RPC's heavy starting capability. Does that seem possible to you?

metalmagpie
have you tested the capacitors? about 10 yr is a typical life span for some, then when they die it causes a host of problems with motors drawing more current and popping a motor or a breaker if it dies short or OC
 
have you tested the capacitors? about 10 yr is a typical life span for some, then when they die it causes a host of problems with motors drawing more current and popping a motor or a breaker if it dies short or OC
Once the RPC is started, the capacitors are a bit irrelevant. The "start" capacitors are obviously out of the circuit.

Yes there are "balance" capacitors, but the "balance" they produced is thrown off as soon as you connect a large motor to the RPC. (Just due to the way the capacitors work).

Unless you have the basic capacity to produce larger currents without excess voltage drop, no practical amount of capacitors is going to do much good. That capacity comes from the "size" of the idler motor.

(Obviously, a normal 3 phase motor, supplied with 3 phase power, does not have any start capacitors, so there are none on the load motor)
 
With enough start capacitors you may be able to start the 3 phase air compressor without an RPC.

That's why i suggested a current operated contactor switching additional caps into the circuit
 
Once the RPC is started, the capacitors are a bit irrelevant. The "start" capacitors are obviously out of the circuit.

Yes there are "balance" capacitors, but the "balance" they produced is thrown off as soon as you connect a large motor to the RPC. (Just due to the way the capacitors work).

Unless you have the basic capacity to produce larger currents without excess voltage drop, no practical amount of capacitors is going to do much good. That capacity comes from the "size" of the idler motor.

(Obviously, a normal 3 phase motor, supplied with 3 phase power, does not have any start capacitors, so there are none on the load motor)
I've just had experience with a dead cap 1/5 run caps were dead and almost shorted causing the voltage on that leg to basically pull the voltage down and run worse when loaded, was fine when unloaded.
only way I caught it was when it ran for a minute or so, it was getting slightly warm (82C) compared to the others that were all still room temp.
 
Yes, parts if defective are a problem.

The "caps are irrelevant" was in regards to changing a value to improve performance. It won't work unless the basic current capacity is OK.

Once that is known OK, then a capacitor change, or, better, a boost transformer, can correct the balance to what is needed.
 








 
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