Principles of D.C. Machines

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DC MACHINES * D.C. machines are the electro mechanical energy converters which work from a dc source and generate mechanical power or convert mechanical power into a dc power. * The direct current (dc) machine can be used as a motor or as a generator. * DC Machine is most often used for a motor. * The major advantages of dc machines are the easy speed and torque regulation. PRINCIPLES OF D.C. MACHINES When a conductor moves in a magnetic field, voltage is induced in the conductor. When a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force. BASIC STRUCTURE OF ELECTRIC MACHINE DC MACHINE CONSTRUCTION MAJOR PARTS OF DC MACHINE * Frame * Yoke * Poles * Armature * Commutator and brush gear * Commutating poles * Compensating winding & Other mechanical parts FRAME * Frame is the stationary part of a machine on which the main poles and commutator poles are bolted and it forms the supporting structure * The ring shaped body portion of the frame which makes the magnetic path for the magnetic fluxes from the main poles and interpoles is called Yoke. Why we use cast steel instead of cast iron for the construction of Yoke? * In early days, Yoke was made up of cast iron but now it is replaced by cast steel. This is because cast iron is saturated by a flux density of 0.8 Wb/sq.m whereas saturation with cast steel is about 1.5 Wb/sq.m. * So for the same magnetic flux density the cross section area needed for cast steel is less than cast iron hence the weight of the machine too. If we use cast iron there may be chances of blow holes in it while casting. So rolled steels are developed. POLES * Pole shoes are generally laminated. Sometimes pole body and pole shoe are formed from the same laminations. * The pole shoes are shaped

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