What is the major advantage provided by a separately excited

What is the major advantage provided by a separately excited DC motor versus a permanent magnetic DC motor?

Solution

As the name suggests, in case of a separately excited DC motor the supply is given separately to the field and armature windings. The main distinguishing fact is that, the armature current does not flow through the field windings, as the field winding is energized from a separate external source of DC current.

The permanent magnet DC motor consists of an armature winding as in case of an usual motor, but does not necessarily contain the field windings. The construction of these types of DC motor are such that, radially magnetized permanent magnets are mounted on the inner periphery of the stator core to produce the field flux. The rotor on the other hand has a conventional DC armature with commutator segments and brushes

Torque Equations:

for separately excited dc motor,

T_g = K_a I_a the torque in this case can be varied by varying field flux , independent of the armature current I_a.

for permanent magnetic dc motor,

The torque equation of DC motor suggests T_g = K_a I_a. Here is always constant, as permanent magnets of required flux density are chosen at the time of construction and can’t be changed there after. For a permanent magnet DC motor T_g = K_a1I_a

Where, K_a1 = K_a. which is another constant. In this case the torque of DC Motor can only be changed by controlling armature supply.