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Typical induction motors can draw 3 to 4 times their running current at startup. Circuit breakers will tolerate momentary overload, but will often trip on a sustained motor start load. The problem gets worse if the wiring resistance is high, due to long runs, undersized wiring, or poor connections along the way. Bigger, belt-driven compressors have unloading valves so that the motor does not have to start against a load. Plug connected appliances are only allowed to draw 80% of the max current ostensibly supplied by the circuit. For a 15 amp circuit, this is 12 amps; for a 20 amp circuit it is 16 amps. Your compressor is right up against the limit. It isn't much of a surprise that it pops the breaker on startup. The National Electrical Code has a whole section devoted to motor overcurrent protection. Usually, the motor provides its own overload protection, and the circuit breaker provides only short circuit protection. Actually, fuses are commonly employed instead of circuit breakers, due to their greater tolerance for startup loads. For a dedicated circuit, you could consider upping the circuit breaker size. You get extra credit for going to 10AWG wire, and replacing the plug and outlet with ones incompatible with ordinary 120V devices. Even better would be to rewire the motor and supply it with 240 Volts.
This should end your overcurrent problems. If you want to keep it at 120 volts for occassional portable use, you face the same problem almost anywhere else. One option for remote sites is to reduce the cut-in pressure to ease the motor start load.
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