Q. Explain about Elementary Direct-Current Machines?

A preliminary discussion of dc machines, including Equations. The location of the brushes on the commutator arrangement connected to the armature winding ensures that the rotor and stator mmf axes are at all times at right angles to one another, as shown in Figure. The expression for torque given by Equation is applicable to the dc machine, provided the constant K is adjusted for the nonsinusoidal mmf distributions and sin δ is made equal to unity. Not only are the conditions for constant torque fulfilled, but also the condition for maximum torque and hence for maximum energy conversion is satisfied, since δ = ± π/2, depending upon whether generator or motor action occurs for a given direction of rotation.

Since both armature and field circuits carry direct current in the case of a dcmachine, they can be connected either in series or in parallel. When the armature and field circuits are connected in parallel, the machine is known as a shunt machine. In the shunt machine, the field coils are wound with a large number of turns carrying a relatively small current. When the circuits are connected in series, the machine is known as a series machine. The field winding in the series machine carries the full armature current and is wound with a smaller number of turns. A dc machine provided with both a series-field and a shunt-field winding is known as a compound machine. In the compoundmachine, the series fieldmay be connected either cumulatively, so that itsmmf adds to that of the shunt field, or differentially, so that the mmf opposes. The differential connection is used very rarely. The voltage of both shunt and compound generators (or the speed, for motors) is controlled over reasonable limits by means of a field rheostat in the shunt field. The machines are said to be self-excited when the machine supplies its own excitation of the field windings, as in the cases just defined. The field windings may be excited separately from an external dc source, however. A small amount of power in the field circuit can control a large amount of power in the armature circuit. The dc generator may then be viewed as a power amplifier.