DC MOTOR THEORYDC Motors(6-3)IaEtECEMFRawhereIa= armature currentEt= terminal voltageECEMF= counter EMFRa= armature resistanceDCMotorSpeedThe field of a DC motor is varied using external devices, usually field resistors. For a constantapplied voltage to the field (E), as the resistance of the field (Rf) is lowered, the amount ofcurrent flow through the field (If) increases as shown by Ohm’s law in Equation (6-4).(6-4)IfERfAn increase in field current will cause field flux (Ff) to increase. Conversely, if the resistanceof the field is increased, field flux will decrease. If the field flux of a DC motor is decreased,the motor speed will increase. The reduction of field strength reduces the CEMF of the motor,since fewer lines of flux are being cut by the armature conductors, as shown in Equation (6-5).(6-5)ECEMFKFFNA reduction of counter EMF allows an increase in armature current as shown in Equation (6-6).(6-6)IaEtECEMFRaThis increase in armature current causes a larger torque to be developed; the increase in armaturecurrent more than offsets the decrease in field flux as shown in Equation (6-7).(6-7)TKFFIaThis increased torque causes the motor to increase in speed.T NES-06 Page 6 Rev. 0
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