Permanent Magnet DC Motor

Permanent magnet dc motors are same as that of ordinary dc shunt motor. Permanent magnet DC motor uses permanent magnet unlike DC shunt motor.

Construction of Permanent Magnet DC Motor

Cylinderical steel stator supporting the electromagnets also provides return path for magnetic flux as it is made up of magneitc material like iron. The rotor consist of slots for windings, commutator and brushes same as those in conventional dc motors. The stator having some structure having cylinderical shell of uniform thickness which is magnetized in radial direction. Here the material used for permanent magnet is having high residual flux density and high coercivity. For motor having the rating upto 150 KW the material like Alnico may be used. The ferrite magnets are used in the fractional kilowatt motors while rare earth magnets eventhough costly are economical in small and large motors. The material used in rare earth magnets are newly developed material like somarium cobalt and neodymium-iron-cobalt which gives high residual flux density, high coercivity with maximum energy product. Using neodymium-iron-boran is the latest trend which gives larger flux density, coercivity and maximum energy product than somarium cobalt. It is also having good mechanical properties and comparatively less expensive.

Working of Permanent Magnet DC Motor

Permanent magnet dc motor normally run on 6V, 12V or 24V dc supply. This dc supply can be made available from batteries or from rectifier (ac to dc). When dc supply is given to the stator of the permanent maget dc motor, then rotor conductor experiences a mechanical rotational force, this direction of rotation of a conduction is given by Fleming's left hand rule. A Force experienced by the rotor conductor is given by F = B.I.L Newton.

Advantages of Permanent Magnet DC Motor

• Low cost as it do not require any external excitation for producing magnetic fields.
• High efficieny as compared to conventional dc motor as field losses are absent.
• Very silent in operation.
• The size of this motor is very small as compared to conventional dc motor, due to the absence of field winding.

Disadvantages of Permanent Magnet DC Motor

• The flux density produced in the air gap by the permanent magnet in limited.
• Speed above the rated speed is impossible.
• As the poles of the stator is cylinderical type to prevent them from magnetic junk, The temperature is higher which is limitation in applications where the motor is used for short period of time.

Application of Permanent Magnet DC Motor

• Permanent maget dc motor are used in blowers which is extensively used in heaters and air conditioners.
• These motors are extensively used in automobiles for windshield wipers and washers.
• These motors are used in industries which manufacture tooth brushes, food mixer, vacuum clenear etc.

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