Effects of momentary voltage dips on the operation of induction and synchronous motors

The author presents simplified stability calculations of induction and synchronous motors to voltage dips. The voltage dip withstand capabilities of induction and synchronous motors of the same horsepower, driving similar inertia loads, are calculated. The motor characteristics influencing stability are type of motor selected (synchronous or induction); the pull-out torque of synchronous motors and breakdown torque of induction motors; a constant excitation controller or an excitation boost on voltage dips for the synchronous motors; and load and motor inertia and load characteristics. Synchronous motors are not suitable for fast autoclosing or bus transfer, though these can be auto-resynchronized. Transient torque and current surges must be considered when induction motors are connected for fast bus transfer. It is noted that due to varying power system, motor and load characteristics and much interaction between these, an analysis of avoiding a shut-down on voltage dips has to be performed on a case-by-case basis