Harmonics Induced by Triac-based Soft Starting of an Induction Motor in a Residential Air Conditioner

S. El Ferik, C.A. Belhadj, L. Benamor, and A.S. Hussain (Saudi Arabia)


• The torque oscillations are significantly dampened and mechanical stress is alleviated which will increase the life span of the systems with frequent start-up (like air conditioners). Soft-Starter, Air-Conditioning, Harmonics, Triac.


The authors surveyed the different soft-starter techniques that are used in the minimizations of the energy consumption. They mention the advantages and the disadvantages of the more common soft-starting methods. One of the methods is to employ the inverter that adjusts both the supply frequency and the voltage, so the motor speed can be controlled and the energy usage can be reduced. However, this method is expensive and it does not justify the cost for some application. Besides, the motor can be equipped with the soft starter instead. In this case the supply frequency is fixed and the stator voltage is controlled in dependence of the load to limit the starting current and reduce the motor losses. The authors describe the experimental result of evaluating seven commercial soft-starter used with three motors of different ratings. They came to the following conclusions This work is focusing on the frequency analysis of a window type residential AC unit line current under time based soft-starting control strategy. The control strategy assumes that only source voltage and current measurements are available. The soft-Starter is based on power electronic devises controlled through a firing signal generated by a programmed microcontroller during the first 500 ms. The frequency content shows the effect of the soft starter in exciting high frequency components of the line current. A Fast Fourier Transform as well as a body diagram analysis show that the high frequencies even or odd multiples of the fundamental line frequency are all excited by the soft-starter approach. Some these frequencies may harm the life cycle of the air-conditioner. The paper describes the experimental setup in details. • Soft-Starter reduce the stator current and the developed torque at the starting, allowing reduction of the power rating of the supply system and increasing life expectancy of the drive.

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