
Operating Characteristics of Induction Motors 197
Figure 6.14 Torque-speed curve for d.c. injection braking of cage motor
This is in line with what we would expect, since there will
only be induced currents in the rotor (and hence torque)
when the rotor is 'cutting' the flux. As with plugging, injec-
tion (or dynamic) braking is a dissipative process, all the
kinetic energy being turned into heat inside the motor.
SPEED CONTROL
We have seen that to operate efficiently an induction motor
must run with a small slip. It follows that any efficient
method of speed control must be based on varying the syn-
chronous speed of the field, rather than th ...