Using fast switching power semiconductor devices, such as insulated gate bipolar transistors (IGBTs), a pulse width modulation (PWM) inverter can be operated at high frequency (up to 20 kHz), significantly improving induction motor performance.  Advantages include lower electromagnetic noise, higher efficiency, and more output torque.  Compared with speed feedback-based current-regulated field orientation control, open loop voltage/frequency ("V/Hz") control has the advantages of simplicity and cost effectiveness.  Therefore V/Hz PWM inverter controls are widely used in adjustable speed applications, such as fans, pumps, blowers, cranes, hoists, and so on.  As more and more applications for V/Hz inverter controls are discovered, more and more features are identified as being desirable, such as higher horsepower rating, quieter operation, higher efficiency, wider adjustable speed range, and wider load variation range. 
Under certain operation conditions, some problems with known voltage inverters have been perceived.  One among these is an instability phenomena relating to the obvious and sustained speed oscillation of motors which can occur when they are driven by V/Hz PWM inverters. 
Open loop V/Hz induction motor drives most often suffer instability problems under light load and at low frequency.  These problems can become even worse for large horsepower drives, high efficiency motors, and high PWM frequency operation, thus limiting the applications of such drives.  The inventor and others have conducted extensive experimental investigation to characterize such instability phenomena. 
Instability problems with V/Hz inverters have been observed and analyzed by researchers.  See, e.g.: Kunio Koga, "Stability Analysis and Stabilizing Control of Inverter-Fed Induction Motor," Electrical Engineering in Japan, Vol.  109, No.  3, pp.  130-140, 1989 ("Koga"); Ryuzo Ueda, Toshikatsu Sonoda and Shigeo Takata, "Experimental Results and Their Simplified Analysis on Instability Problems in PWM Inverter Induction Motor Drives," IEEE Trans.  On Industry Applications, Vol.  25, No.  1, Jan./Feb.  1989, pp.  86-95 ("Ueda I"); Morris Lockwood, "Simulation of Unstable Oscillations in PWM Variable-Speed Drives," IEEE Trans.  On Industry Applications, Vol.  24, No.  1, Jan./Feb.  1989, pp.  137-141 ("Lockwood"); Ryuzo Ueda, et al., " Stability Analysis in Induction Motor Driven by V/f Controlled General Purpose Inverter," IEEE Trans.  On Industry Applications, Vol.  28, No.  2, Mar./Apr.,  1992, pp.  472-481 ("Ueda II"). 
Stability analysis of induction motors fed by pure sinusoidal voltage may be performed by using small signal linearization around the operating point.  See, e.g., P. C. Krause, O. Wasynczuk and A. D. Sudhoff, "Analysis of Electric Machinery," IEEE PRESS, 1994.  For V/Hz PWM inverter driven induction motors, the instability analysis becomes more complicated.  An idealized induction motor (IIM) has been proposed to analyze the effects of motor parameters, dead time and dc link capacitor.  See, e.g., Koga.  Later, these effects have also been analyzed based on a practical induction motor model.  See, e.g., Ueda II.  Experimental investigations of the instability problem have been conducted (see, e.g., Ueda I) in which an index was proposed to measure the "degree of instability."  System dynamic simulations have been conducted to investigate these unstable oscillations.  See.,  e.g., Lockwood.  These investigations and analyses have revealed some important facts about this problem.  Some relevant points may be summarized as follows:
(a) The oscillation states depend on motor design, such as motor parameters (resistance, inductance); the number of poles; motor geometry; loss of core material; and the moment of rotor inertia.  PA1 (b) The oscillation states also depend on PWM inverter parameters such as the dead time; dc link capacitor; PWM frequency and strategy.  PA1 (c) The oscillation states further depend on operating conditions such as output frequency; shaft load, applied voltage and even the transition path to the operating point. 
Those of ordinary skill in the art will generally understand that motor oscillations can result from uncontrolled energy exchanges among DC link capacitors, motor magnetic fields, and rotor inertia.  Any mechanism to damp this energy exchange would be expected to stabilize the oscillation. 
On the other hand, it can be difficult to derive an effective stabilization control method, since many variables are involved in the phenomena.  Koga has proposed a voltage vector feedback method to remove the effects of leakage inductance and primary resistance by using the phase current feedback.  In Nobuyoshi Mutoh, et al., " Stabilizing Control Method for Suppressing Oscillations of Induction Motors Driven by PWM Inverter," IEEE Trans.  On Industrial Electronics, Vol.  37, No.  1, Feb. 1990, pp.  48-56 ("Mutoh"), there is proposed a stabilizing control method involving measuring the interval of negative and positive inverter input currents.  This requires a very precise circuit specially designed for high PWM frequency.  Recently, fuzzy inference algorithms have been theoretically investigated for oscillation stabilization.  See, e.g., Marek Budzisz, and Zbigniew Nowacki, "Stabilization Procedures Based on Fuzzy Inference Algorithms for PWM Drives," Conference Record of 1995 IAS Meeting, pp.  1663-1667 ("Budzisz"). 
An instability compensation method should preferably be effective, simple and robust.  The investigated methods mentioned above are believed to have potential drawbacks, and the present invention is believed to offer advantages over prior art methods and apparatuses.