13.4 DIRECT-CURRENT MACHINES 603in Figure 13.4.9. Note that the no-load speed is very high; care must be taken to ensure that the
machine always operates on load. In practice, however, the series machine normally has a small
shunt-field winding to limit the no-load speed. The assumption that the flux is proportional to
the armature current is valid only on light load in the linear region of magnetization. In general,
performance characteristics of the series motor must be obtained by using the magnetization
curve. The series motor is ideally suited to traction, when large torques are required at low speeds
and relatively low torques are needed at high speeds.
A schematic diagram for a cumulatively compounded dc motor is shown in Figure 13.4.8(c).
The operating characteristics of such a machine lie between those of the shunt and series motors,
as shown in Figure 13.4.9. (The differentially compounded motor has little application, since
it is inherently unstable, particularly at high loads.) Figure 13.4.10 compares the speed–torque
characteristics of various types of electric motors; 1.0 per unit represents rated values.
Synchronous motorInduction motorShunt motorCompound motor
Series dc motor1.0 (rated value)(rated value)
1.0
Per-unitspeedPer-unit torqueFigure 13.4.10Typical speed–torque
characteristics of various electric motors.EXAMPLE 13.4.2
A 200-V dc shunt motor has a field resistance of 200and an armature resistance of 0.5.
On no load, the machine operates with full field flux at a speed of 1000 r/min with an armature
current of 4 A. Neglect magnetic saturation and armature reaction.
(a) If the motor drives a load requiring a torque of 100 N·m, find the armature current and
speed of the motor.
(b) If the motor is required to develop 10 hp at 1200 r/min, compute the required value of
external series resistance in the field circuit.Solution(a) Full field currentIf= 200 / 200 =1 A. On no load,Ea=Vt−IaRa= 200 −( 4 × 0. 5 )=
198 V. SinceEa=k 1 Ifωm, wherek 1 is a constant,