In controlling the fundamental voltage output, the PWM strategy may introduce additional harmonic components, the presence of which detrimental to motor performance and efficiency. Constant-torque operation is obtained below based frequency by modulating the output voltage waveform from the PWM inverter so that the fundamental component of the output voltage is reduced proportionally with frequency, giving the usual constant volts/ hertz mode of operation. If the inverter operates on un-modulated six-step voltage waveform, motor operation at rated voltage and frequency is possible. (1) Variable frequency induction motor drive The ac supply is rectified to a fixed dc voltage by the converter and converted to variable frequency ac by the PWM inverter which also controls the amplitude of the fundamental output voltage. Three Phase a.c Converter PWM Inverter M Fig. For the purposes of comparison, it is assumed that standard 50 or 60-Hz induction motor is fed from a standard ac supply networ by a frequency converter circuit as shown in fig. It is therefore, of interest to compare the different modulation techniques available with regard to the additional harmonic losses in the motor and developed pulsating torque. In an ac motor drive, the modulation strategy which is most appropriate to a particular portion of the speed range is readily selected. Conventional modulation strategies which have been implemented by means of complex analog circuits may now be more effectively realized using a loo up table accessed by a microprocessor or digital hardware. In recent years, more sophisticated techniques have been suggested in which specific low-order harmonics are suppressed or total harmonic content is minimized - These optimized PWM strategies are extremely difficult to realize with conventional analog circuitry, but they can be effectively implemented with modern microprocessor based control techniques.
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Fourier analysis techniques are used in order to allow sin effect phenomena to be taen into consideration and performance criteria are developed to allow comparisons of waveform quality with respect of harmonic copper and iron losses. Pea current is also an important factor in inverter design, and the various modulation strategies are again compared on this basis. The influence of sin effect on motor (I R copper losses) is taen into consideration, and harmonic core losses are compared. The effectiveness of these (PWM) techniques in minimizing harmonic and reducing torque pulsation is investigated analytically, and their performance is compared with that of the usual sinusoidal or sub-harmonic PWM approach. These modulation strategies may suppress specific low order harmonics or minimize total harmonic.
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1 Number 3 Volume 15 September 009 Journal of Engineering DIFFERENCE BETWEEN OF PWM STRATEGIES FOR INVERTER FED INDUCTION MOTOR محمد حسين معهد اعداد المدربين التقنيين ABSTRACT Several sophisticated or "optimum" modulation strategies have been suggested for voltage source pulse with modulated (PWM) inverters for ac motor control.