Comparison Study of Eddy Current Losses of Induction Motors Fed by SPWM and SVPWM Inverters

In this paper, a comparison study of eddy current losses in three-phase induction motors fed by voltage source inverter with sinusoidal pulse width modulation (SPWM) and voltage space vector pulse width modulation (SVPWM) is presented. The simulation model of SPWM and SVPWM inverter are respectively established in MATLAB. After analyzing the harmonic spectrums of inverter output voltage, both the analytic method and FEA (flnite element analysis) method are adopted to calculate the eddy current losses. The calculation results show that comparing with SPWM, SVPWM control method could decrease harmonic components of output voltage and reduce the iron losses of the motors. PWM (pulse width modulation) inverters are widely applied in speed control of motor devices. SPWM and SVPWM are two common modulation methods. Normally, there are high-order har- monics in the output voltage of PWM inverter, which increase the motor losses (1,2). The paper compared and analyzed the eddy current losses of induction motors fed by SPWM inverter and SVPWM inverter. After introducing the principles of SPWM and SVPWM, Matlab/simulink soft- ware was used to establish the simulation model of SPWM and SVPWM inverter respectively. The harmonic spectrums of inverter output voltage were obtained. Based on the superposition, the eddy current losses of induction motors fed by PWM inverter were calculated. The relation of the eddy current losses with the modulation ratio and carrier wave ratio were discussed. Eddy current losses discrepancy in SPWM and SVPWM was also compared. Finally, the electromagnetic fleld flnite element model of PWM-fed induction motor was established. After time-stepping flnite element calculation, the rules between the eddy current losses and control parameters in SPWM and SVPWM were obtained. The research showed that high-order harmonics had large in∞uence to motor losses and decreased the e-ciency. 2. PRINCIPLE OF SPWM AND SVPWM An inverter bridge that consists of six switches whose state of on or ofi is controlled by the PWM drives is used in a most common way. The outputs are connected to each motor terminal to supply the power for motor. The structure of the voltage-source inverter (VSI) for three-phase induction motor system is shown in Fig. 1. SPWM and SVPWM schemes are used to control the switching devices to generate approximate sinusoidal signals in the stator phases. Fig. 2 shows the general principle of SPWM. An isosceles triangle carrier wave is compared with a sinusoidal modulating wave, and the points of intersection determine the switching point of power devices. The output waveform of the inverter controlled by SPWM is a series of rectangle pulses, whose amplitudes are same and the widths are difierent. The switching angle control is decided by the switching frequency and modulation index, and the output fundamental wave amplitude has a linear relationship with the amplitude modulation index. And the change of the switching frequency can only change the center of the fundamental wave frequency distribution, which does little in∞uence on the amplitude of each harmonic (3). It can be seen that the pulses in the output waveform have a sine weighting equivalent to the reference waveform. This method was realized flrst with analog circuits, and it can be modeled in the MATLAB/simulink to generate a series of simulated PWM wave.