Electricity and Magnetism

Shunf, Series and Compound Generators In a shunt generator, a fraction of the armature current serves to excite the main magnetic field of the machine (Fig. 27). Shunt generators are widely used as sources of direct current as they need no external source for excitation, which fact markedly simplifies their use. On the other hand, […]
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The Process of Self-Excitation In most d.c. generators the field current is part of the armature current. At starting there is no current in the armature and, as a consequence, in the field winding, but the substantial frame always retains some residual magnetic flux Φr amounting to 1-3% of the normal operating magnetic flux of […]
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The Separately Excited Generator In the case of separate excitation, the field circuit is electrically independent of the armature circuit (Fig. 1.22) . Therefore the field (or excitation) current is independent of the terminal voltage of the generator and, in consequence, of loading. This makes it possible to regulate the magnetic flux and, hence, the […]
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Commutation in D.C. Machines If allowed to occur, sparking under the brushes leads to a rapid destruction of the commutator, because an electric spark has a very high temperature and is capable of severely damaging the hardest metals and alloys. Therefore, the prevention of sparking under the brushes is an essential condition for a machine […]
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Armature Reaction Armature Reaction refers to the effect that the armature current has on the main magnetic field of a d.c, machine. in most cases, . this is an undesirable factor as it distors the main magnetic field and impairs the performance of the machine. That is why measures are provided for during the design […]
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The EMF and the Electromagnetic Torque of a D.C. Machine For any d.c. machine the basic equations are those describing the emf induced in the armature winding and the electromagnetic torque produced by the interaction of the armature current with the main magnetic field of the machine. As a conductor of the armature win­ding moves […]
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Drum Armature Windings , The conductors placed in the armature slots must be interconnect­ed in a proper way so as to make up the armature winding. Most of the present-day d.c. machines use a drum armature which is a cylinder (see Fig. 13.3) assembled from electrical-sheet steel laminations insulated from one another. The sides of […]
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Operation of Brushgear and the Commutator An alternating emf is induced in the rotating armature winding of a d.c. machine, and it needs rectification, which is done by the commutator. The rectification of the emf induced in a d.c. machine can conveniently be traced using as an example a d.c. generator (Fig. 1.9 a) in […]
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1.1 General Direct-current electric machines (generators and motors)  have a wide field of application. The primary advantage of d.c. motors is that their speed can be readily varied as desired and that they can deve¬lop large starting torques. For this reason d.c. motors are widely used for electric traction tramways, trolleybuses and railways), and as […]
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magnetic-flux-pattern-by-plotting-compass-method Magnetic Fields contains Magnetic Flux Welcome and thanks for visiting. In order to make this page loads fast I’ve put the images in the PDF article. You can download this pdf for free, here: Magnetic fields illustration. The space surrounding a magnet in which magnetic force is exerted is called a magnetic field B, […]
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