I recommend you talk to the manufacturer of the 300HP motor. They should have the background, knowledge and test equipment to identify the reason for these motors only lasting five years. If they can't help you or won't help you then you really should be concentrating on finding a new motor...
There are several different types of motors and control approaches that could be used to meet your requirements depending on your cost target for 200 systems (system = motor + control). If you tell me a short summary of your system design requirements and system cost target maybe I can help.