In many industrial applications, Pulse Width Modulated (PWM) inverter-fed induction motors needs long cables. The PWM inverter output voltage pulses behave like travelling waves on these cables. This phenomenon causes voltage reflection that results in over voltage at the motor terminals. Over voltage at the terminals of the motor is attributed by the rise time of the inverter pulses, length of the power cable that connects the inverter to the motor, or the disparity in impedance between the inverter, cable, and the motor. High dv/dt pulse at the inverter output terminal causes voltage reflection in the cable. One of the methods used to mitigate the voltage reflection at the input terminals of the motor is to increase the rise time of the voltage at these terminals. To accomplish this delay in voltage rise, a passive filter that consists of an LC clamp circuit can be used. This circuit helps to reduce the dv/dt value. Both the common mode and the differential mode issues are addressed by connecting the neutral of the LC clamp filter with diode to the mid-point of DC bus. Since the size of the filter required is small even this filter can be placed inside the inverter enclosure. Thus, the use of LC filter with diode reduces the voltage reflection issue and increases the life of the motor and the reliability of the drive systems.
Index Terms-dv/dt, differential mode voltage, common mode current, voltage reflection, induction motor, IGBT
<p>This paper presents a closed loop Model Reference Adaptive system (MRAS) observer with artificial intelligent Nuero fuzzy controller (NFC) as the adaptation technique to mitigate the low speed estimation issues and to improvise the performance of the Sensorless Direct Torque Controlled (DTC) Induction Motor Drives (IMD). Rotor flux MRAS and reactive power MRAS with NFC is explored and detailed analysis is carried out for low speed estimation. Comparative analysis between rotor flux MRAS and reactive power MRAS with PI as well as NFC as adaptive controller is performed and results are presented in this paper. The comparative analysis among these four speed estimation methods shows that reactive power MRAS with NFC as adaptation mechanism shows reduced speed estimation error and actual speed error at steady state operating conditions when the drive is subjected to low speed operation. Simulation carried out using MATLAB-Simulink software to validate the performance of the drive especially at low speeds with rated and variable load conditions.</p>
Direct Torque controlled (DTC) induction motor drive gives direct control of stator flux and electromagnetic torque. Conventional speed sensors are replaced in sensor less DTC to improve the reliability, noise immunity and to reduce the complexity of the system. In sensor less DTC the rotor speed estimation at low speeds is degraded by the stator resistance parameter variations due to temperature, dead time effects and voltage drop in power electronics devices. The open loop speed estimation used in sensor less DTC depends on various machine parameters. The stator resistance variation at low speeds degrades the speed estimation. In this paper investigation of sensor less DTC controlled induction motor at low speed range is carried out and to improve the speed estimation at low speed closed loop Model Reference Adaptive Scheme (MRAS) is used for speed estimation. Simulation is carried out in Matlab/ Simulink platform and results are compared and presented.
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