Permanent Magnet DC Motor

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Permanent Magnet DC Motor

ConstructionField -Permanent magnet -Continuous ring

OperationSupply 6V,12 V or 24 V Bt. or rectified ac T IB-H curve - top left quadrant segment is referred to as the demagnetizing curve

Demagnetizing Curve

Permanent Magnetic MaterialsAlnico - good properties but too low a coercive force and too square a B-H loop => permanent demagnetization occurs easily Ferrites (Barium and Strontium) - low cost, moderately high service temperature (400 C), and straight line demagnetization curve. However, Br is low => machine volume and size needs to be large.

Permanent Magnet MaterialsSamarium-Cobalt (Sm-Co) - very good properties but very expensive (because Samarium is rare) Neodymium-Iron-Boron (Nd-Fe-B) very good properties except the Curie temperature is only 150C

Performance CharacteristicsN-T

I-T

Performance CharacteristicsEfficiency

Torque Power Input Power output

Current

Speed (rpm)

Speed ControlThe speed-torque characteristic of a PM motor can be controlled by changing the supply voltage or the effective resistance of the armature circuit. The change in the supply voltage varies the no-load speed of the motor without affecting the slope of the characteristics.

Speed Control...with the change in the effective resistance of the armature circuit, the slope of the curve is controlled and the no-load speed of the motor remains the same

AdvantagesLower manufacturing cost Smaller in size Higher efficiency (no field Cu loss) Less noise Little radio and TV interference

DisadvantagesHeating effect (magnetic shielding) Can be demagnetized by armature reaction due to i . Improper design ii. Excessive Ia by fault or transient iii. Improper brush shift iv. Temperature effects Limited in size (large magnets are expensive)

Applications12 V PM DC motor Automobiles

Windshield Wipers Door locks Window lifts Seat adjust Mirror adjust Anti-lock Braking System Fans

Toys Electric Toothbrush

http://www.walter-fendt.de/ph11e/electricmotor.htm

http://www.freescale.com/files/microcontrollers/doc/train_ref_material/MOTORDC