I am using 3 pole breaker although I am only using 2 poles. 220V single phase

Two scenarios. First of all a 50 amp breaker is designed to run a continuous load at 80% or its trip rating. 80% of 50 is 40 amps. If you are drawing 42 amps for long periods I could see it tripping the breaker(heat build up in breaker). Scenario 2 is that the starting current of a motor is much higher than the running current or amps. The code allows to go to the next size breaker 60 amps, but your wire should be rated to handle the 60 amps. You must have a fairly large air conditioner to draw that kind of amperage, but it is possible if your air conditioner is that large . I assume there are no other issues. The unit works fine outside of the nuisance trip?