594 ROTATING MACHINES
13.4 Direct-Current Machines
Generally speaking, conventional dc generators are becoming obsolete and increasingly often are
being replaced by solid-state rectifiers in most applications for which an ac supply is available.
The same is not true for dc motors. The torque–speed characteristics of dc motors are what makes
them extremely valuable in many industrial applications. The significant features of the dc drives
include adjustable motor speed over wide ranges, constant mechanical power output or torque,
rapid acceleration or deceleration, and responsiveness to feedback signals.
The dc commutator machines are built in a wide range of sizes, from small control devices
with a 1-W power rating up to the enormous motors of 10,000 hp or more used in rolling-mill
applications. The dc machines today are principally applied as industrial drive motors, particularly
when high degrees of flexibility in controlling speed and torque are required. Such motors are
used in steel and aluminum rolling mills, traction motors, overhead cranes, forklift trucks, electric
trains, and golf carts. Commutator machines are also used in portable tools supplied from batteries,
in automobiles as starter motors, in blower motors, and in control applications as actuators and
speed-sensing or position-sensing devices.
In this section, we examine the steady-state performance characteristics of dc machines to
help us understand their applications and limitations, illustrating the versatility of the dc machine.Constructional Features of DC Machines
A dc generator or motor may have as many as four field windings, depending on the type and size of
the machine and the kind of service intended. These field windings consist of two normal exciting
fields, theshuntandserieswindings, and two fields that act in a corrective capacity to combat
the detrimental effects of armature reaction, called thecommutating(compoleorinterpole) and
compensatingwindings, which are connected in series with the armature. The type of machine,
whether shunt, series, or compound, is determined solely by the normal exciting-field circuit
connections, as shown in Figure 13.1.8. Figure 13.4.1 illustrates how various field windings
are arranged with respect to one another in part of a cross section of a dc machine, whereas
Figure 13.4.2 shows the schematic connection diagram of a dc machine. The commutating and
compensating windings, their purpose, as well as the circuit connections are presented later in
this section.Equivalent Circuit of a DC Machine
A review of the material presented with regard to elementary direct-current machines in Section
13.1 can be helpful at this stage to recall the principles of operation for dc machines.
The circuit representations of a dc generator and a dc motor are shown in Figure 13.4.3.
Understeady-state conditionsthe interrelationships between voltage and current are given by
Vf=IfRf (13.4.1)
and
Vt=Ea±IaRa (13.4.2)
where the plus sign signifies a motor and the minus sign a generator.Vfis the voltage applied
to the field circuit,Ifis the field current, andRfis the field-winding resistance.Vtis the terminal
voltage,Eais the generated emf,Iais the armature current, andRais the armature resistance. The
generated emfEais given by Equation (12.2.16) as