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Efficiency Investigation of an Induction Motor drive system with Three Different Types of Frequency Converters with focus on HVAC applications

SUMMARY This paper discusses the efficiency of induction motor drives for HVAC applications, based on measurements made on three 4kW induction motor drives. Both converter efficiency as well as induction motor efficiency is studied and in particular the total system efficiency is determined. Measurements have been performed on a 4kW induction motor fed by three different types of frequency converters. Converters A and B use an open loop, constant flux control, where B has a L-C filter on its output. Converter C uses an open loop, field weakening algorithm but also a special pulse width modulation (PWM) technique in order to reduce the switching losses in the converter. Simulations and measurements have shown that an efficiency improvement of the system can be achieved using a field weakening algorithm which improves the efficiency at light load with 18% at 30Hz which in particular is of importance in HVAC applications. At full load the difference in efficiency is negligible between system A and C but 3% lower using system B due to the sinus filter. Efficiency improvements were also accomplished by using the special PWM technique. It was also found that it is important to make a complete optimization of the energy efficiency. Using system B, with L-C filter, gives higher machine efficiency but a low overall efficiency due to high converter losses. As a general conclusion, it has been shown that system C was the best solution from an efficiency point of view for a typical HVAC application.