Permanent Magnet D.C. Motors

Permanent magnet D.C. motors are small as that of ordinary d.c. shunt motor with the difference that there is permanent magnet instead of stationary field winding for producing the required magnetic flux. These stationary electromagnets are fixed to the outer shell of the motor.

1.1 Construction

     The constructional features of permanent magnet  d.c. motor is shown in the Fig. 1. As seen from the figure the cylindrical steel stator supporting the electromagnets also provides return path for magnetic flux as it is made up of magnetic material.

      The armature (or rotor) consists of stator windings, commutator segments and brushes same as those in conventional d.c. motors. The stator is having some structure having cylindrical shell of uniform thickness which is magnetized in radial direction. The material used for permanent magnet is having high residual flux density and high coercivity. For the motors having the rating up to 150 kW the material like Alnico may be used. The ferrite magnets are used in the fractional kilowatt motors while rate earht magnets even though costly are economical in small and large motors. The material used in rate earth magnets are newly developed materials like somarium cobalt and neodymium-iron-cobalt which gives high residual flux density, high coercivity with maximum energy product.

Fig. 1 Cross section of a typical permanent-magnet motor

       The latest trend is to use neaodymium-iron-boran material which gives larger flux density, coercivity and maximum energy product than somarium cobalt. It is also having good mechanical properties and comparatively less expensive.

1.2 Working and Performance Characteristics

       These motors normally run on 6 V, 12 V or 24 V d.c. supply. This d.c. supply can made be available from batteries or from rectification (a.c. to d.c.). Because of the interaction between flux produced by permanent magnets and current carried by the armature, the torque is produced.

The equivalent circuit of a permanent magnet d,c, motor is as shown in the Fig. 2.

Fig. 2

       R_a is representing the resistance of the armature winding. The field winding connections are absent because of permanent magnets.

       In conventional d.c. motor, the relation between speed and voltage can be written as,

       Also the relation between torque and current in conventional dc motor is given as,

       But in case of permanent magnet dc motors the resultant flux Φ is constant. Hence the above relationships can be expressed as,

1.3 Performance Characteristics

The set of typical performance characteristics of  permanent magnet DC motors are shown in the Fig. 3.

Fig. 3

       The speed-torque characteristics are almost linear which makes it suitable in servomotors. With increase in torque the current also increases. The efficiency of these motors is better then conventional motors owing to absence of field losses. As the field flux is remaining constant, the speed control is not possible with flux control method. Thus the method used for speed control is armature control using a rheostat or using electronic circuits. The speeds obtained are below the normal speeds.

1.4 Advantages 

1. These motors do not require external excitation for producing magnetic fields. Thus there is saving in energy required for creating magnetic fields.

2. As the windings on the field are absent, the size of such motor is small as compared to equal rating conventional motor.

3. The cost of these machines is low.

4. The efficiency of these motors is high compared to conventional motors as the field losses are absent.

5. The motors designed up to 12 V or less produces less TV and radio interference.

6. These motors produce less air noise.

1.5 Disadvantages

1. The excessive currents in the armature windings of these motors may demagnetize the permanent magnets because of armature reaction mmf. The other sources of demagnetization are improper design, or brush shift or temperature effects.

2. The flux density produced in the air gap by the permanent magnets is limited.

3. The speeds above normal speeds are not possible flux per pole can not be controlled.

4. As the magnets are totally enclosed to prevent them from magnetic junk, as compared to conventional motors their temperature is higher which is limitation in applications where the motor is used for short period.

1.6 Applications 

      These motors are extensively used in automobiles for windshield wipers and washers. They are also used in blowers used in heaters and air conditioners. They are also used to raise and lower windows and in slot cars. Fort disc drives in personal computers these motors are used.The rating available for these motors is up to 150 kW. These motors may also be used in applications such as fans and radio antennas, electrical fuel pumps, marine engine starters, wheel chairs and cordless power tools. In toy industry, tooth brush, food mixer, ice crusher, vacuum cleaner and in portable electric tools these motors are extensively used.