Dc Machines

Lecture 1 & 2

DIRECT CURRENT MACHINE:

Fig. 1: Fluxes in a typical field system The dotted lines represent the path of the main magnetic fluxes and full lines (1, 2, 3 & 4) represent leakage flux. The useful flux shown enters the armature to produce e.m.f and mechanical forces. Electrical Machine: An electrical machine is an electromechanical device or Dynamo-electric machine or more briefly dynamo which converts mechanical energy to electrical energy or vice versa. The machine which converts mechanical energy into electrical energy is known as Generator, while the one converting electrical energy in to mechanical energy is called Motor. However, electrical machine are reversible capable of operating both as a generator and a motor. There are two types of electrical machines according to the nature of the e.m.f and current in the armature conductors. They are: (a) Alternating Current (A.C) Machines. (b) Direct Current (D.C) Machine. The first type has the current and voltage at the armature terminals alternating in direction where as the second type has unidirectional current and voltage at the armature terminals. However, most of the DC machines except homopolar machines are basically similar to AC machines where currents and e.m.fs in the armature conductors are alternating, but furnished with a special device – commutator which makes the current and e.m.f at the armature terminal unidirectional.

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CONSTRUCTIONAL FEATURES AND ARMATURE WINDINGS OF D.C. MACHINES. 1. POLES: Pole cores are usually not laminated and are made of cast iron steel. In some machines, pole cores are laminated and composed of electrical grade steel sheets of about 1mm thickness and insulated with respect to each other. Solid poles are commonly bolted to the yoke. Laminated poles maybe secured in place by dovetail joint. As shown in diagram below

The flux density in the body of the pole core is much higher than that permitted in the air gap. Hence the pole face must have...