compressor wont start under load

by Joe

7.5 hp 80 gal speedaire
Dayton motor kept blowing breaker - took motor off and was told it was bad and replaced with Baldor motor of same specs
Now when any load on motor breaker blows
What we have done so far:
removed belts - motor starts fine and pulls correct load
Installed belts, drained tank, motor starts fine
removed and cleaned check valve - reinstalled - filled tank and removed line to see if there was back pressure - everything was fine
removed safety valve from top of head - compressor starts fine
removed unloader valve line - compressor starts fine
-- in both of these previous situations there was air in tank but obviously not full load with air leaking at startup
reattached unloader valve line while compressor running and unloader valve worked fine when tank was full
removed and cleaned unloader valve - reinstalled - same problem; no start under load
Motor spins several revolutions and then when pressure builds slightly the breaker blows
Installed new breaker - same problem
Nothing else on that line - dedicated to compressor
Any suggestions?
Sorry I don't have a picture - vertical style compressor

__________________
Bill says...

Joe, two things that come to mind.

Empty the tank and the compressor starts fine? Is that correct? Do that, let the compressor run up to normal cut out, and monitor the unloader valve to be sure that it is opening and dumping the air over the piston when the compressor shuts off. If the unloader doesn't work, the trapped air is adding too much load to the motor and it pops the breaker.

Make sure there is no change in the power supply, no extension cord, and that nothing else is pulling power from the same circuit at the same time the compressor is trying to run.

Still no joy? See the electrical troubleshooting section on the site map to find how to test your capacitors.

Cheers,

Bill