Permanent magnet vs field winding in machines
Machines are mainly classified into two types those are motor and generator which may be of AC or DC type. Permanent magnet and field windings are used to produce a magnetic field around the armature or conductor. A permanent magnet is nothing but the magnet which is used for the small rating of the machines mostly which is used in dynamos. Dynamo is nothing but a machine which converts electrical energy to mechanical energy as well as mechanical to electrical energies i.e., motor and generator respectively .The advantage of the dc machine over the AC machine is that dc motor will work as a motor and as well as a generator.
The field winding is nothing but, a winding is wounded on the conductor and which winding is energizes, it produces the magnetic field as the same way of permanent magnets. Usually, field windings are used for greater magnetic field strength as compared to permanent magnet and we can’t able to design the permanent magnet for the large rated machines. By using this field winding we can control the speed of the machines in some cases. But, it consumes energy while working. In permanent magnets we cannot control the speed of the machines but the risk of operation is very less and losses are negligible.
From the above figure (a) , where the north pole (N), south pole(S) are the two permanent magnets. so the no of poles in the machine is 2.those are permanent magnets .
From the above figure (b) , the north pole (N), south pole(S) are the permanent magnets. so the no of poles in the machine is 4.this poles are made by the field windings. 