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The harmonic spectrum of an inverter output voltage in a PWM inverter can be distributed by using a Random Pulse Width Modulation (RPWM) scheme. RPWM dose not affect the fundamental frequency component of voltage, however it can significantly reduce the acoustic noise and mechanical vibrations of an inverter-fed ac motor drive. Up to now, RPWM has been studied in many researches and various RPWM techniques have been introduced, but none of them have actually shown the effects of the behavior of the random signal on the frequency spectrum of the inverter output voltage and performance of the motor drive. In this paper, three different methods of random signals are employed These different methods and the Probability Density Function (PDF) of the random signal generator will be studied and the performance of the drive is examined Also a new method in utilizing random signal is proposed In this method, the random signal is injected to reference value of the quadrature-axis current. It should be noted that the RPWM technique can not be utilized in hysterisis drives. Here, the random signal with different PDF is injected directly to quadrature axis current. The injection of random signal will directly affect the output responses. In this research, comparison between RSPWM technique and direct injection of random signal has been carried out. Different criteria such as total harmonic distortion (THD), torque ripple and frequency spectrum, efficiency and mechanical vibration have been analyzed The indirect field oriented vector control drive is simulated by using Matlab-Simulink first, and the effects of various methods of employing random signal are compared and the best alternative is introduced For the validation of the simulation results an experimental setup is built. The experimental results confirm the simulation outputs. The simulation and experimental result show superiority of Direct Injection of Random Signal (DIRS) method in comparison with RSPWM technique. Also it is realized that the DIRS method is very simple for implementation and does not need any extra hardware. [ABSTRACT FROM AUTHOR]