Recently powerful direct current (DC) machines were widely used in electric drives for vehicles, vessel propulsion systems and for steel rolling mills. Fast development of alternating current drives, including thyristor or transistor inverters gradually decreases the range of application of DC machines.
Still it seems these machines will be used for certain time period in low-powered equipment because of their ability to ensure simple control of motor speed, low cost and high reliability. It is even possible to create a very simple DC drive with the aid of a motor, power source and rheostats for excitation and armature current control. There exist DC motors which include permanent magnet excitation instead of the electromagnetic one. It should be noted that so far direct current generators have been almost out of use so only DC motors could be of interest in the nearest future.
The most problematical part of a DC motors is its mechanical commutator.
Figure 1 shows a simple DC machine without commutating poles which is typical for low-powered motors. When the motor rotates clockwise its armature coils reach the commutation zone and the current in these coils changes its direction. Several coils are being short-circuited and the current arising in them creates the so-called commutating armature reaction. The corresponding magnetomotive force may significantly affect the total magnetic flux in transient modes taking place in DC machines, particularly at high armature currents.