Welcome toLectures on D.C.Machine : 2/11/2009 Welcome toLectures on D.C.Machine Slide 2: 2/11/2009 D. C. Generator Slide 3: 2/11/2009 Portable D. C. Generator DIRECT CURRENT MACHINES : 2/11/2009 DIRECT CURRENT MACHINES DC machine Construction
The major advantages of DC machines are: easy speed and torque regulation.
The stator of the DC motor has poles, which are excited by DC current to produce magnetic fields.
The rotor has a ring-shaped laminated iron-core with slots.
Coils with several turns are placed in the slots. The distance between the two legs of the coil is about 180 electric degrees. N S Stator with
with poles Brush Rotor Field Slide 5: 2/11/2009 D. C. Generator
Principle of Operation When a conductor rotates in a magnetic field, dynamically induced e.m.f is produced in it according to Faraday’s law of electromagnetic induction Slide 6: 2/11/2009 Generated EMF Let Slide 7: 2/11/2009 Hence, according to Faraday’s Laws of electromagnetic induction, Slide 8: 2/11/2009 For Wave wound Generator No. of Parallel Paths = 2
No. of Conductors (in series) in 1 path = Z/2 Slide 9: 2/11/2009 For Lap wound Generator No. of Parallel Paths = P
No. of Conductors (in series) in 1 path = Z/P Slide 10: 2/11/2009 In General, Generated EMF Slide 11: 2/11/2009 Also, For a given D.C. Machine, Z, P and A are constants Hence, Putting Slide 12: 2/11/2009 Types of D.C. Generators According to the way in which their fields are excited, generators are classified into : Separately excited D.C. Generator
Self- excited D.C. Generator Slide 13: 2/11/2009 Separately excited D.C. Generators are those whose field
magnets are energized from an independent source of D.C. Current Ra = Armature Resistance Slide 14: 2/11/2009 ii) Self excited D.C. Generators are those whose field
magnets are energized by the currents produced by the generators
There are 3 types of self-excited D.C. Generators named according to
the manner in which their field coils are connected to the armature.
These are :
Shunt wound D.C. Generator
Series wound D.C. Generator
Compound wound D.C. Generator Slide 15: 2/11/2009 Shunt wound D.C. Generator Rsh – Shunt Field Resistance
Ra - Armature Resistance Slide 16: 2/11/2009 ii) Series wound D.C. Generator Slide 17: 2/11/2009 iii) Compound wound D.C. Generator Short Shunt Slide 18: 2/11/2009 Long Shunt Slide 19: 2/11/2009 Ex.1 A D.C. Shunt Generator delivers 450A at 230V and the
resistance of the shunt field and armature are 50O and 0.03 O
respectively. Calculate the generated EMF. Slide 20: 2/11/2009 Ex.2 An 8 - pole D.C. Shunt Generator with 778 wave
wound armature conductors running at 500 rpm supplies
a load of 12.5O resistance at terminal voltage of 250V.
The armature resistance is 0.24O and the field resistance is 250O .
Find the armature current, the generated EMF and the flux per pole.