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Figure 3    Motor Action
Generator Action in a Motor

Electrical Science Volume 2 of 4
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DC MOTOR THEORY DC Motors The right-hand rule for motors shows the direction in which a current-carrying conductor moves in 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  the direction of force (motion). Torque Torque  is defined as that force which tends to produce and maintain rotation.   The function of torque in a DC motor is to provide the mechanical output or drive the piece of equipment that the DC motor is attached to. When  a  voltage  is  applied  to  a Figure 5   Armature Current in a Basic DC Motor motor,  current  will  flow  through the  field  winding,  establishing  a magnetic  field.   Current  will  also flow through the armature winding,  from  the  negative  brush to  the  positive  brush  as  shown  in Figure 5. Since  the  armature  is  a  current- carrying  conductor  in  a  magnetic field,  the  conductor  has  a  force exerted on it, tending to move it at right  angles  to  that  field.    Using the   left-hand   rule   for   current- carrying  conductors,  you  will  see that the magnetic field on one side is   strengthened   at   the   bottom, while  it  is  weakened  on  the  other side.  Using the right-hand rule for motors, we can see that there is a force exerted on the armature which tends to turn the armature in the counter-clockwise direction.   The sum of the forces, in pounds, multiplied by the radius of 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 reversed and 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 action of the magnetic fields.   The force developed is directly proportional to the strength of the main field flux and the strength of the field around the armature conductor.   As we know, the field strength around each armature conductor depends on the amount of current flowing through the armature conductor.   Therefore, the torque which is developed by the motor can be determined using Equation (6-1). ES-06 Page 4 Rev. 0







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