Permanent Magnet Permanent Magnet DC MotorDC Motor

ConstructionConstruction

FieldField

-Permanent -Permanent

magnetmagnet

-Continuous -Continuous

ringring

OperationOperation

SupplySupply6V,12 V or 24 V Bt. 6V,12 V or 24 V Bt. or rectified acor rectified ac

T T αα ØØII

B-H curveB-H curve- - top left quadrant top left quadrant segment is referred segment is referred to as the to as the

““demagnetizing curvedemagnetizing curve” ”

Demagnetizing CurveDemagnetizing Curve

Permanent Magnetic MaterialsPermanent Magnetic Materials

AlnicoAlnico - good properties but too low a - good properties but too low a coercive force and too square a B-H loop coercive force and too square a B-H loop => permanent demagnetization occurs => permanent demagnetization occurs easilyeasily

Ferrites Ferrites (Barium and Strontium) - low cost, (Barium and Strontium) - low cost, moderately high service temperature moderately high service temperature (400(400C), and straight line demagnetization C), and straight line demagnetization curve. However, Bcurve. However, Brr is low => machine is low => machine volume and size needs to be large.volume and size needs to be large.

Permanent Magnet Materials…Permanent Magnet Materials…

Samarium-CobaltSamarium-Cobalt (Sm-Co) - very good (Sm-Co) - very good properties but very expensive properties but very expensive (because Samarium is rare)(because Samarium is rare)

Neodymium-Iron-BoronNeodymium-Iron-Boron (Nd-Fe-B) - (Nd-Fe-B) - very good properties except the Curie very good properties except the Curie temperature is only 150temperature is only 150C C

Performance CharacteristicsPerformance Characteristics

N-TN-T

II-T-T

ŋŋ

Performance Characteristics…

Speed (rpm)

Power output

Current

Torque

Power Input

Efficiency

Speed ControlSpeed ControlThe speed-torque characteristic of a PM motor can be The speed-torque characteristic of a PM motor can be controlled by changing the supply voltage or the controlled by changing the supply voltage or the effective resistance of the armature circuit. effective resistance of the armature circuit.

The change in the supply voltage varies the no-load The change in the supply voltage varies the no-load speed of the motor without affecting the slope of the speed of the motor without affecting the slope of the characteristics. characteristics.

Speed Control...Speed Control...

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

AdvantagesAdvantages

Lower manufacturing costLower manufacturing cost

Smaller in sizeSmaller in size

Higher efficiency (no field Cu loss)Higher efficiency (no field Cu loss)

Less noiseLess noise

Little radio and TV interferenceLittle radio and TV interference

DisadvantagesDisadvantages

Heating effectHeating effect(magnetic shielding)(magnetic shielding)Can be demagnetized by armature reaction due Can be demagnetized by armature reaction due toto

i . Improper designi . Improper designii. Excessive ii. Excessive IaIa by fault or by fault or

transient transientiii. Improper brush shiftiii. Improper brush shiftiv. Temperature effectsiv. Temperature effects

Limited in size (large magnets are expensive)Limited in size (large magnets are expensive)

ApplicationsApplications12 V PM DC motor12 V PM DC motor

AutomobilesAutomobiles Windshield WipersWindshield Wipers Door locksDoor locks Window liftsWindow lifts Seat adjustSeat adjust Mirror adjustMirror adjust Anti-lock Braking SystemAnti-lock Braking System FansFans

ToysToysElectric Electric

ToothbrushToothbrush

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