I have a 150 KVA generator, around 10 days ago, it suddenly started giving an output of 600 V, the electrician who came told me that the heat sink and the photo transistor were burned out, and he replaced them. To be honest they cost quite a load of money. Still the generator is not functioning, now it's not giving a voltage at all, and according to the electrician the stator and the rotor are not damaged. I fear the electrician is not good enough, so I'm doing some research on my own. Can anyone please tell me what might be wrong? Thank you

Would you please supply more data on the 150 kVA generator. What is the make and model? what is the namplate data given on the machine, i.e. kVA rating or kW rating at some power factor (pf), 150 kW at 0.8 pf, the nominal output voltage, 480/277 volts three pahse or single pahse, etc.

Thank you for your reply, but it all worked out fine, the electrician changed the AVR and the photo-transistor, and now it's working fine.