Well that engine running at 600 RPM is making less than 6HP. That means you can only support around 3KW of electrical load. If I read correctly you are running 250V single phase out of your alternator? If that A/C pulls 9-12 amps when running, that is 2.25-3KW of load, or all that engine is capable of powering. Unfortunatly, the compressor needs several times that curremt at startup. That is probably what is causing you problems. As the voltage falls off under load, the A/C actually trys to draw more current to make up for the lost potemtial that the voltage drop causes...