Stability study of induction motors under variable-frequency modes in the presence of external inductors

With variable-frequency (v.f.) modes of speed control of induction motors, investigation reveals that the low-frequency instability zones exhibited by machines having low inertia constants can be avoided by connecting suitable inductors externally to the rotor circuit (or stator leads). For machines having large inertia constants, which may not exhibit low-frequency instability zones, the inherent low damping during low-frequency operation can be considerably improved with suitable external inductors. The steady-state performances, however, remain practically unaffected with the inclusion of such external inductors.