An Experimental evaluation of two level and multilevel inverters used for speed control of induction motor and study of common mode voltage present in both cases

Abstract

Variable speed inverter fed Induction Motor (IM) drives are increasingly used in many commercial and industrial applications, the newlinehigh frequency switching operation (2 to 20kHz) has improved the system efficiency, energy saving and ease in control. But with high speed switching device such as MOSFET or IGBT, Inverter fed IM drives have newlinethe drawbacks like 1. Common mode voltage (CM) 2. Rotor shaft voltage and Bearing current etc. The CM voltage is the voltage between the star point of IM stator winding to ground. This voltage always disturbs the newlinenearby measuring and controlling circuits. In a balanced sinusoidal 3-phase system, the 3-phase voltage vector sum is always equal to zero. But in the case of PWM inverter switched from same DC voltage source newlineconverted into 3-phase voltages, it is difficult to make the sum of the 3- newlinephase output voltages equal to zero hence the voltage between star point of stator winding of an IM to ground will exist which has to be minimized.