DC Motors DC MOTOR THEORYT = KFI_{a}(6-1)whereT = torque, lb-ftK = a constant depending on physical size of motorF = field flux, number of lines of force per poleI_{a} = armature currentGeneratorActioninaMotorA generator action is developed in everyFigure 6 Counterelectromotive Force (CEMF)motor. When a conductor cuts lines of force,an EMF is induced in that conductor.Current to start the armature turning will flowin the direction determined by the applied DCpower source. After rotation starts, theconductor cuts lines of force. By applyingthe left-hand rule for generators, the EMF thatis induced in the armature will produce acurrent in the opposite direction. The inducedEMF, as a result of motor operation, is calledcounterelectromotive force, or CEMF, asillustrated in Figure 6.Since the CEMF is generated by the action ofthe armature cutting lines of force, the value of CEMF will depend on field strength and armaturespeed, as shown in Equation (6-2).E_{CEMF} = KFN (6-2)whereE_{CEMF}= counter EMFK = constantF= field flux strengthN = speed of the armatureThe CEMF opposes the applied voltage and functions to lower armature current. The effectivevoltage acting in the armature of a motor is the applied voltage, minus the counter EMF.Armature current can be found by using Ohm’s law, as shown in Equation (6-3).Rev. 0 Page 5 ES-06

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