TABLE OF CONTENTS Until the 1980s the conventional (brushed) d.c. machine was the automatic choice where speed or torque control is called for, and large numbers remain in service despite a declining market share that reflects the move to inverter-fed induction motors. Applications range from steel rolling mills, railway traction, to a very wide range of industrial drives, robotics, printers, and precision servos. The range of power outputs is correspondingly wide, from several megawatts at the top end down to only a few watts, but except for a few of the small low-performance ones, such as those used in toys, all have the same basic structure, as shown in Figure 3.1.
The motor has two separate circuits. The smaller pair of terminals connect to the field windings, which surround each pole and are normally in series: in the steady state all the input power to the field windings is dissipated as heat none of it is converted to mechanical output power. The main terminals convey the power or work current to the brushes which make sliding contact to the armature winding on the rotor. The supply to the field is separate from that for the armature, hence the description separately excited .
As in any electrical machine it is possible to design a d.c. motor for any desired supply voltage, but for several reasons it is unusual to find rated voltages lower than about 6 V or much higher than 700 V. The lower limit arises...
