I have a big rolling Craftsman tank with an electric compressor on top.
The motor has a run capacitor and a start capacitor which is controlled by a centrifugal switch. When the motor spins up, the centrifugal switch interrupts the start circuit.
Initial problem was that the compressor would not spin up and would trip a breaker. Closer inspection revealed that the centrifugal switch housing was cracked and not allowing the centrifugal switch to engage the start capacitor.
I fixed the housing so now the centrifugal switch works as expected. It is closed and the starter circuit is engaged when motor is stopped. Once the motor spins up, the switch properly interrupts the start circuit. Verified.
But the motor still had a problem starting. I ordered a start capacitor because it was cheap enough to replace and the old one was made in the 1990s.
After replacing the start capacitor, the motor still has trouble starting.
I verified that:
1. without the start cap, the motor has hard time spinning up but the run cap helps it and it eventually starts, slowly though.
2. with the start cap, the motor starts great when there is no air in the tank.
3. with the start cap, the motor won't spin up if there is more than 40 psi of pressure in the tank
I tested the old start cap and the new one as well. Both provide great capacity, I can charge them with DC and they will hold the charge for days. They both will light a small bulb for a second or so. Of course I realize that's not a proper way to measure capacitance but at least it's a test that the cap is not blown.
I would love to measure the start assist windings on the motor but I do not have manufacturer's specs anyway.
What do our resident electricians think of this scenario? What do you suspect? What else would you test and how?
Thank you in advance for any useful advice.
The motor has a run capacitor and a start capacitor which is controlled by a centrifugal switch. When the motor spins up, the centrifugal switch interrupts the start circuit.
Initial problem was that the compressor would not spin up and would trip a breaker. Closer inspection revealed that the centrifugal switch housing was cracked and not allowing the centrifugal switch to engage the start capacitor.
I fixed the housing so now the centrifugal switch works as expected. It is closed and the starter circuit is engaged when motor is stopped. Once the motor spins up, the switch properly interrupts the start circuit. Verified.
But the motor still had a problem starting. I ordered a start capacitor because it was cheap enough to replace and the old one was made in the 1990s.
After replacing the start capacitor, the motor still has trouble starting.
I verified that:
1. without the start cap, the motor has hard time spinning up but the run cap helps it and it eventually starts, slowly though.
2. with the start cap, the motor starts great when there is no air in the tank.
3. with the start cap, the motor won't spin up if there is more than 40 psi of pressure in the tank
I tested the old start cap and the new one as well. Both provide great capacity, I can charge them with DC and they will hold the charge for days. They both will light a small bulb for a second or so. Of course I realize that's not a proper way to measure capacitance but at least it's a test that the cap is not blown.
I would love to measure the start assist windings on the motor but I do not have manufacturer's specs anyway.
What do our resident electricians think of this scenario? What do you suspect? What else would you test and how?
Thank you in advance for any useful advice.