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7. DC Machines > 7.7 - Operation of a dc Machine as a Motor

7.7 Operation of a DC Machine as a Motor

Like dc generators, dc motors are also constructed to work as dc shunt, series, and compound motors. In all such motors, electrical power input, i.e., V × I gets converted into mechanical output. The mechanical output is in the form of torque developed which enables the motor shaft to carry some mechanical load on it. For example, a dc motor will rotate the wheels of an electric train or a trolley bus. The constructional details of a dc motor is similar to that of a dc generator except for certain minor changes in the cooling system.

7.7.1 Working Principle of a DC Motor

A dc motor works on the basic principle that when a current-carrying conductor is placed in a magnetic field it experiences a force. In a dc motor, the armature carries a number of conductors placed in slots and the armature is placed inside the magnetic field created by field magnets as shown in Fig. 7.15. The field magnets are excited by field current passing through them. The field windings may either be excited from a separate dc source or could be excited from the source of supply provided to the armature. The armature is fed from a dc supply source of V Volts as shown. The direction of current flowing through the winding have been shown by cross images and dot images A cross indicates current flowing in the direction perpendicular to the plane of the paper downwards and a dot indicates that current is coming towords the observer looking into the paper.


  

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