
100 Electric Motors and Drives
Torque
Field
o
Maximum
Current
7
Speed
a
b
Figure 3.12 Series-connected d.c. motor and steady-state torque-speed
resistances. Hence for all but small motors a starting
resistance is required to limit the current to a safe value.
Returning to Figure 3.12(b), we note that the series motor
differs from most other motors in having no clearly defined
no-load speed, i.e. no speed (other than infinity) at which the
torque produced by the motor falls to zero. This means that,
even when running light, the speed of the motor depends on
the windage and friction torques, equilibrium being reached
when the motor torque equal ...