Construction And Working:
The permanent mag. nets of a PMDC motor are radially magnetized and mounted on the inner periphery of the cylindrical steel stator.
The stator also serves as a return path for the magnetic flux. Field coils are usually not required.
However, some of these motors do have coils wound on the poles. If they exist, these coils are intended only for recharging the magnets in the event they lose their strength.
The rotor of this motor is similar to the rotor of a conventional de motor i.e. rotor of a PMDC motor consists of armature core, armature windings, and commutator.
Stationary carbon brushes are kept pressed on the commutator surface as in a conventional dc motor.
The material used for permanent magnets is ceramics and rare earth magnetic materials, which have high residual flux as well as high coercivity.
A 2- Pole PMDC motor is Shown in Fig.
The torque is developed by the interaction between current carrying rotor conductors and the magnetic flux set up by the permanent magnets.
The equivalent circuit of a PMDC motor is shown in fig.
Advantages:
The major advantage of PMDC motors is that they do not have any field winding, so they require no excitation current.
Thus there is no continuous loss of energy in the field. Because of the absence of field windings, there is a saving in space and PMDC motors are smaller in size and cheaper in cost in comparison to corresponding rated conventional dc motors.
Disadvantages:
The limitation of PMDC motors is that the excessive current in the armature winding may demagnetize the permanent magnets and also the flux density produced in the air gap by permanent magnets is limited.
The risk of permanent magnetism getting destroyed by armature reaction (at starting/reversing or heavy overloads) has been greatly reduced by the new permanent magnetic materials like somarium cobalt and neodymium-iron-boron.
These motors offer shunt type characteristics and can only be armature controlled i.e. in PMDC motors, speed, and torque are controlled through the adjustment of voltage applied to the motor terminals, armature rheostat control, and chopper control.
Applications:
PMDC motors are used extensively in automobiles as starter motors and for windshield wipers and washers, for blowers used in heaters and air-conditioners, to raise and lower windows, in slot cars and electric toothbrushes, in personal computer disc drives, etc.