Dear Kwabena,

Welcome,

I am not a machine engineer but i will try to answer your question from a conceptual point of view. You may need to verify my answer by searching the web.

In order to connect a generator to the electric grid it must produce voltage with specific waveform that could not be produced by the permanent magnet. Strictly speaking one can not produce sinewave with high purity and acceptable harmonic levels. The second main cause is that the perm ant magnet is not controlable which is required to control the machine operation. With the proper distribution of the magnetizing winding one can produce the required waveform which must be as pure as possible sinewave.

Best wishes

Permanent magnet motors are practical for small power systems, but may lose their magnetism during a power surge. Also the efficiency is less.