Why we don't use a permanent magnet in a generator?

Generators are separately excited from a different source.

preferred to be electormagnetic to have the ability to control the inducing electromotive force E by changing magnetic field by changing field current, then you can control the output of power. I think this

We have been using this technology for more than10 years.

We generate using a permag rotor, rectify and invert it to240vac, then add frequency.

It gives us ac power without having to be concerned with engine revs. Actually, it's a boon, because we throttle down for light load periods, and the frequency is always exactly correct at the load. Better quality power that from the grid, really.

It's also the basis of the modern 'inverter' generators, which have now become popular.

Added bonus is that they're quieter when throttled back.

Now a days large generators are connected to grid. once it connected with grid, generator will produce reactive power and consume reactive power according to the Grid voltage level. with this condition we need to control our excitation voltage, which is not possible with permanent magnet. so all generator are excited by separately excitation system. All house hold diesel generators are with permanent magnet..

The PMG will induce high voltage when the rotational speed is on certain level. During PMG 

installation, maintenance or setting-up, prevent touching all voltage parts without protective 

insulation.

to easy  control voltage

There are permanant magnet generators and motors .But coils are commonly used for generators to create magnetic field. wounded generators are good for maintanance , it has long life and high ratings compared to PMG.