a) Describe briefly with the aid of sketches the essential constructional features of a typical dc motor. Explain how a dc motor develops and a back emf.
b) A permanent magnet dc motor is delivering zero torque when it runs at 1800 rev/min and has 50 V dc applied to its terminals. The armature resistance is 2.0 ohm. What current will the motor need in order to deliver a torque of 2.0 Nm?
Hence, calculate at this torque the corresponding values of:
(i) Emf
(ii) Speed
(iii) Output power
(iv) Efficiency
c) If the motor heated up causing the field flux to reduce to 95% of its previous value and the armature resistance to increase to 125% of its previous value, how would the current, speed and efficiency change, assuming the supply voltage and torque remained at 50V and 2.0Nm respectively?