DC MOTOR THEORYDC MotorsThe right-hand rule for motors shows the direction in which a current-carrying conductor movesin a magnetic field. When the forefinger is pointed in the direction of the magnetic field lines,and the center finger is pointed in the direction of current flow, the thumb will point in thedirection of force (motion).TorqueTorqueis defined as that force which tends to produce and maintain rotation. The function oftorque in a DC motor is to provide the mechanical output or drive the piece of equipment thatthe DC motor is attached to.When a voltage is applied to aFigure 5 Armature Current in a Basic DC Motormotor, current will flow throughthe field winding, establishing amagnetic field. Current will alsoflow through the armaturewinding, from the negative brushto the positive brush as shown inFigure 5.Since the armature is a current-carrying conductor in a magneticfield, the conductor has a forceexerted on it, tending to move it atright angles to that field. Usingthe left-hand rule for current-carrying conductors, you will seethat the magnetic field on one sideis strengthened at the bottom,while it is weakened on the otherside. Using the right-hand rule formotors, we can see that there is a force exerted on the armature which tends to turn the armaturein the counter-clockwise direction. The sum of the forces, in pounds, multiplied by the radiusof the armature, in feet, is equal to the torque developed by the motor in pound-feet (1b - ft).It is evident from Figure 5 that if the armature current were reversed, but the field were the same,torque would be developed in the opposite direction. Likewise, if the field polarity were reversedand the armature remained the same, torque would also be developed in the opposite direction.The force that is developed on a conductor of a motor armature is due to the combined actionof the magnetic fields. The force developed is directly proportional to the strength of the mainfield flux and the strength of the field around the armature conductor. As we know, the fieldstrength around each armature conductor depends on the amount of current flowing through thearmature conductor. Therefore, the torque which is developed by the motor can be determinedusing Equation (6-1).ES-06 Page 4 Rev. 0

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