Introduction to D.C. Machines

D.C. machines are characterized by their versatility. By means of various

combinations of shunt-, series-, and separately excited field windings they can be designed

to display a wide variety of volt-ampere or speed-torque characteristics for both dynamic

and steady state operation. Because of the ease with which they can be controlled, systems

of D.C. machines are often used in applications requiring a wide range of motor speeds or

precise control of motor output.

The essential features of a D.C. machine are shown schematically. The stator has

salient poles and is excited by one or more field coils. The air-gap flux distribution created

by the field winding is symmetrical about the centerline of the field poles. This is called the

field axis or direct axis.

As we know, the A.C. voltage generated in each rotating armature coil is converted to

D.C. in the external armature terminals by means of a rotating commutator and stationary

brushes to which the armature leads are connected. The commutator-brush combination

forms a mechanical rectifier, resulting in a D.C. armature voltage as well as an armature

m.m.f. Wave then is 90 electrical degrees from the axis of the field poles, i.e. in the

quadrature axis. In the schematic representation the brushes are shown in quadrature axis

because this is the position of the coils to which they are connected. The armature m.m.f.

Wave then is along the brush axis as shown. (The geometrical position of the brushes in an

actual machine is approximately 90 electrical degrees from their position in the schematic

diagram because of the shape of the end connections to the commutator.)

The magnetic torque and the speed voltage appearing at the brushes are independent

of the spatial waveform of the flux distribution; for convenience we shall continue to

assume a sinusoidal flux-density wave in the air gap. The torque can then be found from

the magnetic field viewpoint.

The torque can be expressed in terms of the interaction of the direct-axis air-gap flux per