generator

What ever suits the buyer/ maker.

How long is a piece of string?

I wish people could string an extra couple of words together to ask a proper detailed question.

By generator do you mean a DC set or an AC set?

The standard site mobile AC set would usually have a permanent magnet rotor as there is never any input current available to give field excitation. This is normally OK as frequency in these situations is rarely that critical and will only affect site power tools etc. (lights go dimmer and tools rotate slower) Because the excitation cannot be changed, the prime mover shaft speed will reduced as loading is increased and the alternator shaft speed has a direct impact on frequency.

On a grid supply generator frequency is critical and a constant set shaft speed must be maintained at all times, this is managed either by altering the torque of the prime mover (steam turbine) or the rotor field excitation to adapt to electrical loading.

The strength of the field excitation being an electro magnet via slip rings on the alternator shaft can be varied to change to the output conditions.l