Kathmandu University Department of Electrical （加德满都大学电气部门）.pdfVIP

Kathmandu University Department of Electrical and Electronics Engineering BASIC ELECTRICAL LABORATORY EXPERIMENTS (ENGG 103) EXPERIMENT 6:- Speed control of separately excited DC motor using armature voltage control Objectives: 1. Study of dc motor speed control of separately excited DC motor using armature control method. Materials and Equipments Required: i) Voltmeter (0-300 V) DC ii) Tachometer iii) Ammeter (1-5 A) DC iv) DC motor Theory: In a DC motor the armature is fed by DC current. The flux produced by field (either permanent magnet or field winding supplied by excitation dc) interacts with this armature current to produce a repulsive torque which rotates the armature of DC motor. As armature rotates the flux changes its magnitude and hence e.m.f. is induced on it. This e.m.f. opposes the supplied voltage. This e.m.f. is called back e.m.f. Back emf Eb= Ka Φω In case of dc motor back e.m.f E is zero initially i.e. during starting. (E = K Φωi.e. b b a Eb αωat starting ω= 0). Hence armature draws high current from supply during starting if initially armature voltage is set high. This high current may cause sparking and damage to motor. Due to high current the torque driving the armature is also high (T= K ΦI a a i.e.T α I ) which may cause damage to the mechanical load connected to the motor. So a armature voltage is gradually increased to the rated value. Control of speed can be understood with the motor equation = V - I R