Permanent Magnet DC Motors – Operation & Applications

Permanent magnet dc motors employ a permanent magnet instead of stationary field winding (as with the case of some other types of dc motors) for producing the required magnetic flux. These stationary magnets are normally fixed to the outer shell of the motor as illustrated in Figure 1.0.

Features of a Permanent Magnet DC Motor

The constructional features of permanent magnet dc motor are illustrated in Figure 1.0 below.

As shown in the figure above, the cylindrical steel stator supporting the stationary magnets also provides return path for magnetic flux as it is made up of magnetic material.

The armature (or rotor) consists of slots of windings, commutator segments and brushes same as those in conventional dc motors. The stator is having some structure having cylindrical shell of uniform thickness which is magnetized in radial direction.

Operation of a Permanent Magnet DC Motor

The equivalent circuit of a permanent magnet dc motor is shown below:

This motor typically run on 6 V, 12 V, or 24 V dc supply. Because of the interaction between flux produced by permanent magnets and the current produced by the armature, torque is produced.

Related: DC Shunt Motor Speed Control

Advantages of Permanent Magnet DC Motors

• Since these motors do not require external excitation for producing magnetic fields, there is saving of energy needed for creating magnetic fields.
• They have a high efficiency compared to conventional motors as there are no field losses.
• Since these motors don’t have field windings, the size is small compared to equal rating conventional motors.
• These motors produce less air noise.
• The motors that are designed up to 12 V or less produce reduced amount of TV and radio interference.
• The cost of these motors is low.

Disadvantages of Permanent Magnet DC Motors

• The flux density produced in the air gap by the permanent magnets is limited.
• The excessive currents in the armature windings of these motors may demagnetize the permanent magnets because of armature reaction mmf. Demagnetization may also be caused by temperature effects, improper design or brush shift.

Also read: AC vs DC Power & Motors –  Differences and Benefits

Applications

• Permanent magnet dc motors are used in heaters and air conditioners.
• They are widely used in automobiles for windshield wipers and washers.
• They are used in personal computers for disc drives.
• Other applications of these motors include: toys, tooth brush, food mixer, cordless power tools, vacuum cleaner, fans, etc.

Related: Electrical Motors