Civil Engineering Reference
In-Depth Information
Fig. 7.6
Torque-speed characteristics for electrical machines at given input voltage and current
Rheostatic control, which changes the speed of a motor by introducing a variable
amount of resistance into the stator or into the rotor circuit, always involves waste
of energy when the motor runs at reduced speed. In fact, a voltage drop occurs at the
terminals of the rheostat, thus provoking Joule effect losses in it. Notice that these
systems cannot change the synchronous speed already defined, because they do not
change the frequency of the supply. Once widely used for controlling d.c. and
induction machines with wound rotors, rheostats have gradually become obsolete
because of the introduction of power electronic converters which exercise a better
control with lower losses.
