Permanent Magnet DC [PMDC] Motor
A permanent magnet DC motor is similar to an ordinary DC Shunt motor except that it's field is provided by permanent magnets instead of salient pole wound field structure.
This post explains about the construction, operation and features of PMDC.
The permanent magnets of the PMDC motor are supported by a cylindrical steel stator which also serves as a return path for the magnetic flux. The rotor serves as an armature. It has winding slots, commutator segments and brushes as in conventional dc machines.
There are three types of permanent magnets used for such motors.
1. Alnico magnets - used in motors having ratings in the range of 1kW to 150kW.
2. Ceramic(ferrite) magnets - They are much economical in fractional kilowatt motors
3. Rare-earth magnets - made of samarium cobalt and neodymium iron cobalt
- In PMDC motor, a fixed magnetic field generated by the permanent magnets interacts with the perpendicular field induced by the currents in the rotor windings, thus creating a mechanical torque.
- As the rotor turns in response to this torque, the angle between the stator and rotor fields is reduced, so that the torque would be nullified within a rotation of 90 electrical degrees.
- To sustain the torque acting on the rotor, permanent-magnet DC motors incorporate a commutator, fixed to the rotor shaft.
- The commutator switches the supply current to the stator so as to maintain a constant angle, =90, between two fields.
- Because the current is continually switched between windings as the rotor turns, the current in each stator winding is actually alternating, at a frequency proportional to the number of motor magnetic poles and the speed.
Note: The basic principle of operation of Permanent Magnet DC motor and the Brushless DC motor[BLDC] is one as same. In PMDC, commutator is used whereas in BLDC motor power electronic switch circuit is used for switching the current in the stator winding.
Since flux remains constant, speed of a PMDC motor cannot be controlled bt using Flux Control Method. The only way to control its speed is to vary the armature voltage with the help of an armature rheostat. So these motors are used where speed control is required only below base speed.
- In very small ratings, use of permanent magnet excitation results in lower manufacturing cost
- In many cases a PMDC motor is smaller in size than a wound field dc motor of equal power rating
- Since field excitation current is not required, the efficiency of these motors is generally higher than that of the wound field motors
- low voltage PMDC motors produce less air noise
- The major disadvantage of the PMDC motor is that the permanent magnets can be demagnetized by armature reaction mmf causing the motor to become inoperative. Demagnetization can result from (1) improper design (2) Excessive armature current caused by a fault or transient or improper connection in the armature circuit (3) improper brush shift and (4) temperature effects.
- As the PMDC motors magnetic field is active at all times even when motor is not being used, these motors are made totally enclosed to prevent their magnets from collecting magnetic junk from neighbourhood. Hence, as compared to wound-field motors, their temperature tends to be higher. But it will not be an issue where motor is used for short intervals.