D. C. Motors & Shunt Motor

Chapter 7.8 D. C. Motors

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • 244Fundamental Electrical and Electronic Principles (a) PVPVVRoLLoLfIIIIwatt; so amptherefore A80045077 8311.ffaLfaaREVampAampAvolt45020022580 05475450..IIII12252580 050 139Vtherefore ohmRa1.. Ans (b) When W, Vthus APVVRoLff50046050046008 7460233111II.0002308 72 34600 13....Ahence, Avolt(IIaaaEVR111111199475 4)therefore V E.Ans Note : Although the load had changed by about 60%, the fi eld current has changed by only about 2.2%. This justifi es the statement that a shunt generator is considered to be a constant-fl ux machine. 7.8 D.C. Motors All of the d.c. generators so far described could be operated as motors, provided that they were connected to an appropriate d.c. supply. When the machine is used as a motor, the armature generated emf is referred to as the back-emf, Eb , which is directly proportional to the speed of rotation. However, the speed is inversely proportional to the fi eld fl ux . In addition, the torque produced by the machine is proportional to both the fl ux and the armature current. Bearing these points in mind, we can say that: Speed, Eb (7.4)and torque, TIa(7.5) 7.9 Shunt Motor When the machine reaches its normal operating temperature, Rf will remain constant. Since the fi eld winding is connected directly to a