The physical connections between dry/moist (conditional) symmetric instability (SI/CSI) off equatorial diabatic thermal forcing, and the onset of the Asian monsoon are investigated using the Goddard Laboratory for Atmospheres general circulation model (GLA GCM). The objective of this study is two-fold: to elucidate the causal relationship between SI/CSI and monsoon onset, and to investigate the threshold behavior of asymmetric monsoonal thermal forcing under condition of SI/CSI. This work corroborates the authors' earlier linear instability analysis results, which show, that SI/CSI in the boreal summer monsoon basic states may be a plausible explanation for the abrupt monsoon transition.Monsoon transitions in the model, as depicted by the abrupt meridional movement of the axis of maximum vertical motion from equator to northern latitudes, occur during 16-20 May for the East Asian Monsoon (EAM) and 1-5 June for the South Asian Monsoon (SAM) regions. The necessary stability criterion for dry (moist) SI over the EAM and SAM regions reveals a sudden cross equatorial advection of negative dry potential vorticity (DPV) and moist potential vorticity (MPV) into the summer hemisphere five to ten days preceding the model monsoon transition. This causes dry and moist SI. Maximum shift of the zero line of DPV and MPV (dry and moist symmetrically unstable regions) happens subsequent to monsoon transition. Simplified analysis of the potential vorticity (PV) budget equation reveals that the lower tropospheric negative PV advection into the summer hemisphere is largely governed by the dominance of vertical differential diabatic heating over horizontal differential diabatic heating.The diabatic heating also shows an abrupt increase from 2-3 K day-1 before the transition, to 12-14 K day-1 at the time of monsoon transition. The genesis of pre-monsoon weak heat source arises primarily due to unstable SI and CSI of the pre-monsoon basic states, which consequently produce moderately large scale lower (upper) tropospheric convergence (divergence) patterns slightly poleward of the zero line of DPV and MPV. Lower tropospheric conditionally unstable tropical atmosphere, in the presence of off equatorial large scale lower (upper) tropospheric convergence (divergence), is conducive to exciting CISK-like processes, which may eventually release large amounts of latent heat and develop a strong heat source at the time of monsoon transition. We have noted that a fully established model meridional circulation originates only when the diabatic forcing magnitude exceeds some threshold value of around 5K day-1 at the time of monsoon transition. The model transition is more pronounced over the EAM region than over the SAM region. The linear steady-state dynamical response of a zonally symmetric atmosphere as a consequence of varying the location and magnitude of an idealized asymmetric thermal forcing reveals that the most intense meridional circulation (maximum efficiency of vertical motion) is accomplished when the thermal forcing is loca...
Brushless DC (BLDC) motors are becoming an increasingly popular motor of choice for its unique characteristics. The BLDC motor drive is assumed to have trapezoidal back-electromotive force (EMF), rectangular phase currents and together produces the desired torque. However, practical back-EMF waveform might not be exactly trapezoidal because of current ripple, design considerations and manufacturing limitations. The adverse effect is the torque ripple generated due to the current ripple that causes mechanical vibration, acoustic noise and affects the accuracy of speed and position control which is not desirable in motor operation. In this paper an algorithm is developed to control and minimize the generated torque ripple using Space Vector Pulse Width Modulation (SVPWM) scheme. The efficiency improvement of slim type BLDC motor is confirmed using MATLAB environment and low cost TI Piccolo F28035 microcontroller (MC). Keyword:Brushless DC motor Matlab Simulink. Voltage source inverter (VSI) Copyright ©2017 Institute of Advanced Engineering and Science.All rights reserved.Corresponding Author:C.Bharatiraja, Department of Electrical and Electronics Engineering, SRM University, Chennai, India. Email: bharatiraja@gmail.com INTRODUCTIONThe BLDC motors are one of the motor types rapidly gaining popularity. As the name implies, BLDC motors do not use brushes for commutation, instead they are electronically commutated. They have many advantages over brushed DC motors in terms of better speed versus torque characteristics, high dynamic response, long operating life and high speed ranges [1]. In addition, the ratio of torque delivered to the size of the motor is higher, making it suitable in applications where space and weight are critical factors [2]. A demand for the motor, especially slim-type BLDC motor, is expected to increase since the minimizing trend of the actuators is getting attention. The phase inductance of the slim-type BLDC motor is large and it causes a lagging of current compared with back EMF in wide range of speed [3]. This causes reduced motor efficiency and increased phase current.As a result, significant torque pulsations may arise due to the back EMF deviation from the ideal rectangular waveform, phase current ripple due to commutation events, PWM and cogging. Thus, torque ripple is generated due to the non-linear increase in the phase current ripple [4]. It is desirable to minimize torque ripple, since it may cause unacceptable speed ripple, mechanical vibration and acoustic noise. Hence various researchers have investigated this commutation torque ripple and proposed several methods. An alternative explanation for the cause of commutation torque ripple was presented in [5]. Furthermore, the behaviour of PMBLDC drive to the vector analysis in the stationary plane in which the current vector follows the petal-shape trajectory is extensively examined [6]. A design is implemented for current control rule in which the duty cycle of PWM is regulated in real time by measuring the wave function
Brushless dc (BLDC) motor drives have gained significant attention due to their merits such as high efficiency, high dynamic response, high reliability etc. From the literatures, it is understood that there has been remarkable developments evolved in application of the feedback controllers for both the BLDC motor drive's speed and current loops. The recent investigation and innovations in the pulse width modulation (PWM) methods are not encompassed in BLDC drives. The conventional PWM results mechanical vibration and acoustic noise, and limit the application scope the BLDC drives. This is due to the incapability of deterministic PWM in spreading the harmonic power. This paper presents random pulse width modulation (RPWM) for BLDC motor drive with better harmonic power distribution. The randomness is introduced in carrier wave by toggling between triangular wave (+fc) and its inverted form (-fc). The performance of the proposed RPWM is validated by using MATLAB software and results such as harmonic spread factor (HSF), total harmonic distortion (THD), and acoustic noise spectrum are presented. The comparison of results with the conventional PWM demonstrates the superiority of the proposed PWM.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.