Permanent Magnet DC Motor (PMDC):
A permanent magnet DC motor is a DC motor whose poles are built of permanent magnets. Since these motors do not attain an external field circuit, they do not have the field circuit copper losses associated with DC shunt motors. They are compact than corresponding DC shunt motors. These motors are common in smaller fractional and sub- fractional horse power sizes, where the cost and space of a separate field circuit cannot be justified. Permanent magnets cannot establish as high a flux density as an externally supplied shunt field, so these motors shall have a lower induced torque.
In a PMDC machine, the pole flux is just the residual flux in the permanent magnets. If the armature current becomes extremely large, there is some risk that the armature mmf can demagnetize the poles, permanently decreasing or re-orienting the residual flux in them.
A permanent magnet DC motor is fundamentally the same machine as a DC shunt motor except that the flux of a PMDC motor is fixed. A very general diagram of a PMDC motor is shown in Figure
Since it utilizes the permanent magnet poles, this is not possible to control the speed of a PMDC motor by varying the field current or flux.