PMDC or Permanent Magnet Direct Current Motor

Permanent Magnet DC motors are useful in a range of applications, from battery powered devices like wheelchairs and power tools, to conveyors and door openers, welding equipment, X-ray and tomographic systems, and pumping equipment, to name a few. They are frequently the best solution to motion control and power transmission applications where compact size, wide operating speed range, ability to adapt to a range of power sources or the safety considerations of low voltage are important.
The synchronous permanent magnet motors, brushless or sinusoidal, are employed more and more widely also in the industrial field, especially in small and medium power servo drives. They are principally used in high performance situations, in which the particular specifications justify their cost, which is usually very high, due to the presence of permanent magnets in the element of merit the mobile (rotor).

Construction of PMDC

As it is indicated in name of permanent magnet DC motor, the field poles of this motor are essentially made of permanent magnet. A PMDC motor mainly consists of two parts. A stator and an armature. Here the stator which is a steel cylinder. The magnets are mounted in the inner periphery of this cylinder. The permanent magnets are mounted in such a way that the N – pole and S – pole of each magnet are alternatively faced towards armature as shown in the figure below. That means, if N – pole of one magnet is faced towards armature then S – pole of very next magnet is faced towards armature.

Permanent magnet

A number of magnetic materials are available for permanent magnets. These include ceramic oriented ferrites, rare earth permanent magnets, and Alnico, although Alnico’s use is waning. Ceramic oriented ferrites, typically made with barium or strontium, develop into products with lower energy than Alnico. Therefore, they have become the material of choice in most PM motors, replacing Alnico, because of their greater resistance to demagnetization, ease of forming, and low cost.
Other magnets, used in particular situation, are made from rare-earth material, like samarium or neodymium. Rare-earth magnets are typically two to three times stronger than ferrite or ceramic permanent magnets. In Electric Motors, the use of rare-earth magnets enables greater performance to be obtained from a smaller, lighter motor. Clearly this has its attractions for electric vehicles, where a lighter, more efficient motor reduces the amount of stored energy that has to be transported in the form of petrol, hydrogen or batteries.

Advantages of the Permanent Magnet DC Motor

– They are smaller in size.
– For smaller rating Permanent Magnet reduces the manufacturing cost and thus PMDC motor are cheaper.
– As these motors do not require field windings, they do not have field circuit copper losses. This increases their efficiency.

Disadvantages of the Permanent Magnet DC Motor

– Permanent magnets cannot produce a high flux density as that as an externally supplied shunt field does. Therefore, a PMDC motor has a lower induced torque per ampere turns of armature current then a shunt motor of the same rating.
– There is a risk of demagnetization of the poles which may be caused by large armature currents. Demagnetization can also occur due to excessive heating and also when the motor is overloaded for a long period of time.
– The magnetic field of PMDC motor is present at all time, even when the motor is not being used.
– Extra ampere turns cannot be added to reduce the armature reaction.