"Some of the disadvantages of Permanent magnet DC motor are as follows:
1. Permanent magnets cannot produce as high a flux density as an externally supplied shunt field.
2. PMDC motors run the risk of demagnetization. The armature current IA in a dc machine produces a magnetic field of its own. The armature mmf subtracts from the mmf of the poles under some portions of pole faces and adds to the mmf under other portions of the pole faces, reducing the overall net flux in the machine.
3. In a PMDC machine the pole flux is just the residual flux in the permanent magnets. If armature current becomes very large, there is some risk that armature mmf may demagnetize the poles, permanently reducing and reorienting the residual flux in them.
4. Demagnetizing may also be caused by the excessive heating which can occur during prolonged periods of overload.
<a href=http://in.docsity.com/en-docs/DC_Motors-Electro_Mechanical_Systems-Lecture_Slides_>DC Motors</a>"