Field generators and hybrid systems are a special case of engine driven alternators.
Permanent magnet alternators are often ideal for generators. They are relatively small, light weight and efficient for their power capability, and do not require excitation to start producing power.
There are many applications that require a custom generator. There may be noise limitations, or unusual environmental operating conditions, or special fuels, or systems that must sit for months but be ready to start immediately. These can all require a dedicated design.
Field generators start with a set of requirements, leading to an engine selection. From there, we match an alternator and power regulation electronics, and any other components required by the customer.