Empirical Harmonic Analysis Of Induction Motors Fed by Frequency Converters

Clean and efficient energy is one of the greatest goals of our era. It is a known fact that approximately 80% of the motors, in particular industrial zones, are induction motors when most of the countries’ industrial structure is analyzed. The positive reflections of the taken actions for increasing the efficiency of induction motors which have such a large usage rate will be as great as the utilization rate.

Induction motors are frequently used in applications such as fans, pumps and compressors. Consumed power on such applications varies in proportion to the speed of compressor whereas it varies with cubic speed of fans and pumps. Hence, the speed and capacity reductions to be made while there is no need lead to the use of efficient energy by avoiding unnecessary energy loss. As a result of the studies carried out in this way, frequency converters are one step ahead as speed adjusters for induction motors in parallel with the recent semiconductor technologies developed in recent years.

Frequency converters which stand out in energy saving and efficient energy issues, have a nonlinear characteristic due to its AC-DC rectifier unit. Accordingly, the form of the current drawn from the mains is nonsinusoidal. For this reason, they cause harmonics in the network.

In this study, a screw compressor which has a 7,5kW induction motor with star-delta connection was used as experimental setup. Provided that the same conditions are met on the compressor, real-time measurements of harmonic distortion and power are made while a frequency converter is on line to show the exact effect of frequency converters on harmonics. Waveforms of current and voltage, total harmonic distortion and power values were measured with Fluke 438-II Power Quality and Motor Analyzer during the performed tests. Same processes were repeated when the induction motor was fed by frequency converter in the range of 0-50Hz and the difference between two cases is revealed.

1.     Harmonics in case of Star-Delta Starter

The ratios and amplitudes of the fundamental sinewave and harmonics for voltage and current measured on the mains side are given in Table 1.1. In addition, the electrical power consumed by the induction motor is as given in Table 2.2.

It is seen that the harmonics cause a relatively small disruptive effect on the power factor when the values on Table 1.2. is examined. As a result of increased harmonic distortion with the use of frequency converters, the difference between cos? which represents the power factor for the fundamental sinewave and the power factor (PF) which includes the effect of distortion power will be further increased.

2.     Harmonics in case of the Induction Motors Fed by Frequncy Converters

It is observed that the distortion of the current waveform is dramatically increased with the use of frequency converter when Figure 2.1. is examined. The reason for this increment is the rectifier unit of the frequency converter as mentioned before.

As can be seen from Figure 2.2., THD? value increased significantly with the use of frequency converter.

When Table 2.1. is examined, it is observed that the amplitudes of the 5th and 7th harmonics have the highest values as expected for a 6-pulse frequency converters. (The correlation of the order of the harmonics that arise due to the quantity of rectifiers (pulses) in the rectifier unit of frequency converter is h=np±1.)

When Table 2.2. is examined, it is observed that the distortion power due to the high current harmonics increases dramatically compared to the star-delta starter condition. However, due to the capacitors in DC Link unit of frequency converters, the improvement of cos? draws the attention. Since the inductions motor is energized through the capacitors inside the DC Link unit, this can be thought as a kind of reactive power compensation. Therefore, the consumed reactive power is reduced and cos? value approaches to one. However, due to the high current harmonic distortion, the distortion power factor has increased and caused the actual power factor to drop. The actual power factor obtained in induction motors fed by frequency converter is a bit higher than the power factor obtained in case the induction motor which has star-delta starter. The way to increase the actual power factor is to mitigate the harmonic distortion by using harmonic filters. Harmonics can be substantially mitigated according to the type of filter. Experimental studies with some of these filters which are also a part of my master thesis will be shared as well in a different article.

G?khan GüL?EN

MSc. Electrical&Electronics Eng.