Articles written in Sadhana
Volume 42 Issue 2 February 2017 pp 153-161
A brief study on a commutation strategy for a current source inverter (CSI)-fed parallel resonant circuit, using switches formed by IGBTs with series diodes, is presented in this paper. The dynamic behaviour of the inverter and different strategies for its reliable operation are discussed here considering parasitic inductance and overlap time between CSI switching devices. The dynamic behaviour of the switches decides the upper frequency limit for the application. IGBTs with the series diodes behave as uni-directional current switches with bi-directional voltage blocking capability. This feature should be taken into account to decide on an appropriate switching strategy for this converter configuration.
Volume 42 Issue 6 June 2017 pp 879-888
Induction heating (IH) converters operate just above resonant frequency, at near unity power factor (UPF), to supply power to the targeted work-piece. Some power electronic converter-fed IH systems use power control strategies based on dynamic tracking of the changing resonant frequency as the work-piece gets heated up (since inductance changes). Therefore, the correct in-process determination of the resonant frequency is essential. A method of dynamically detecting the resonant frequency is by calculating the phase-shift betweencurrent and voltage continuously during the process. In case of CSI- (and VSI-) fed IH, the phase-shift between voltage and current is zero at resonant frequency. Hence one way of identifying the resonant frequency is by varying the frequency until the phase-shift is zero. For controlling this phase-shift between current and voltage waveforms, most of the controllers use a phase-locked loop (PLL) IC. In this paper, a novel method for the dynamic tracking of resonant frequency is proposed and the practical implementation of the same, using a fieldprogrammable gate array (FPGA) based digital-PLL, is presented. The scheme is first simulated with generated off-line signal samples and then implemented on a real-time model of a CSI-fed IH application. Finally, thedigital-PLL logic is implemented on controller hardware and practically tested in a laboratory-made experimental set-up of 2 kW at a nominal frequency of 10 kHz. The switching frequency is auto-synchronising. This fact is practically verified both by varying (i) the geometric dimensions as also (ii) the initial temperature of the work-piece. It is practically observed in the oscillograms that the phase gets locked in few cycles (and hence ensures quick tracking of the dynamically changing resonant frequency for this set-up).
Volume 42 Issue 8 August 2017 pp 1317-1333
This paper presents a novel, reliable and efficient V/f control implementation on a 8-pole, 750 rpm, 5 kW surface-mounted permanent magnet synchronous motor (PMSM) without damper winding. In the absence of a damper winding, open loop V/f control of SM is inherently unstable, particularly at high speeds. Stabilisationcan be done with proper stator frequency modulation in accordance with the change in rotor speed to provide for effect of damping. This has been implemented here without use of any shaft-mounted encoder. The change in rotor speed is observed from power perturbation, thereby eliminating the need for using a speed sensor in the drive. The efficiency of the drive is further increased with appropriate control of the power factor, irrespective of load and frequency variations. Simulated and experimental results are presented for both open loop and the proposed V/f control. These results establish the accuracy of the design of the proposed V/f control strategy and the precision of hardware implementation. A comparative study between the proposed V/f control method and standard vector control method, as implemented on this PMSM, has also been presented here to establish the advantages of the proposed scheme. The PMSM itself was designed and fabricated in the laboratory.
Volume 42 Issue 8 August 2017 pp 1419-1429
This paper deals with the design, analysis and fabrication of a 320 W, 5 m/s, 4-pole permanentmagnet-based linear synchronous machine (LPMSM). The design deals with conventional hand calculations followed by design fine-tuning and analysis using standard FEM packages. Fabrication of the machine has beenalso done at the works of a local small machine manufacturer after procurement of imported permanent magnets (PMs). The work presented here focuses on design and fabrication of LPMSM linor stampings with fixed dimensions and standard available PM tablets with a view to providing simple design guidelines. This prototype forms part of a variable speed linear motor drive that is being developed. The approach presented here might provide new ideas to those researchers who may not have the capability to bear cost of fabrication of lamination and PMs from scratch but would like to manufacture LPMSMs from available stampings and tablet PMs.Additionally, it may serve as a base document for the R&D personnel in this area.
Volume 44 Issue 11 November 2019 Article ID 0221
This paper presents a simple and accurate method for the on-line measurement of the unsaturated phase inductance of a switched reluctance motor (SRM). The basic principle behind the approach is to obtain the instantaneous flux linkage of the phase, which when divided by the instantaneous current of the correspondingphase will dynamically generate the phase inductance for that rotor position. A numerical division process, based on Newton–Raphson (N–R) iterative method, has been implemented using the sensed current and estimated flux linkage on an FPGA platform to generate the inductance value at each instant. It is ensured in the practical implementation that the iron parts do not saturate. It has been also verified through simulation using standard finite-element method (FEM) software packages. Further, it has been established that the effect of mutual inductance is negligibly small. The proposed method will give the true value of the unsaturatedinductance of the phase identified for diagnosis/measurement. Practical results, for the experiments conducted on an existing 4 kW, 1500 rpm Oulton-make SRM, have been also presented. They are further validated against the results obtained from FEM analysis and against previously published results of the phase inductance values for this SRM. The results are found to be in very good agreement.
Volume 45 All articles Published: January 2020 Article ID 0026 Original Article (Electrical Sciences)
This paper presents a novel and exhaustive investigation involving in-depth analysis, performance evaluation and comparative study of two 0.75 hp, 4-pole, 1500 rpm laboratory prototypes of Brushless DC (BLDC) motors of identical nominal ratings with surface and interior permanent magnet rotor structures havingthe same stator and winding (integral slot distributed winding). Both the motors were designed and developed in the lab. The major electrical variables (such as rated power, speed, voltage, current, number of poles, etc.) and the stator (such as core material, stator lamination, stack length, winding pattern and wire gauge) of the fabricated prototypes have also been kept identical to pin-point the direct influence of the two different rotor configurations (viz., surface vs interior permanent magnet) on the parameters, performance and operation ofthese BLDC motors. Additionally, to ensure unbiased basis for appropriate comparison, the overall volumes of magnets/pole in both the motors have also been kept similar. A detailed comparison of different quantities likeair-gap flux density distribution, THD in induced voltage, torque ripple, losses and efficiency, torque–speed characteristics with field-weakening capability, steady state parameters at different operating conditions, etc. has been conducted for the said motors and the salient points duly highlighted. The vulnerability of the permanent magnets to demagnetisation based on armature reaction, particularly during a sudden fault, has also been investigated in both the cases. The theoretically determined parameters and analytically evaluated performancefigures have been verified through standard FEM packages, and later validated experimentally on the fabricated prototypes. Very good mutual agreement has been observed between predicted and experimental values.
Volume 45 All articles Published: February 2020 Article ID 0053 Original Article (Electrical Sciences)
This paper presents a comparative study on the design, modelling, electromagnetic analysis based on finite-element software, fabrication and experiment on rectangular flat (148 g) and C-shaped (148 g) levitationprototypes based on steel plates. No mechanical restrainer has been used in the transverse direction for the levitation. This aspect of the work is an improvement over existing work reported in the published literature. The entire set-up has been designed, fabricated, analytically investigated and experimentally evaluated and verified. The finite-element model (FEM) has been derived using standard commercial package(s). The analytical model has been obtained using specific permeance concepts following Robert Pohl’s method. Excellent correlationbetween the predicted and experimental results is a highlight of the work. The stability against transverse mechanical perturbation has also been investigated. Control system design and implementation is successfully done.
Volume 45 All articles Published: March 2020 Article ID 0069 Original Article (Electrical Sciences)
This paper presents an in-depth analysis, performance evaluation and comparative study of two 5-kW, 8-pole, 750-rpm laboratory prototypes of a permanent magnet synchronous motor (PMSM) of identical nominal ratings with surface and interior permanent magnet (PM) rotor structures having same stator and armature winding (fractional slot distributed winding). The key electrical (such as rated voltage, current, power, speed, number of poles, etc.) and mechanical variables (such as overall volume, air-gap length, rotor diameter, shaft dimensions and magnetic material) of the fabricated prototypes have also been kept same to pin-point thedirect influence of the two different rotor configurations (surface and interior PMSM) on the parameters, performance and operation of these PMSMs. For the two machines, a detailed comparison of air-gap flux density distribution, THD in induced voltage, torque ripple, losses, efficiency, torque–speed characteristics, field weakening capability, steady-state parameters at different operating conditions, etc. has been conducted. The salient observations from this comparative study have been duly highlighted. This paper also includes an indepthcomparison of volume and cost of PM used in the two types of PMSMs. The short-time performance figures of the said motors have also been presented. The possibility of demagnetisation of PMs, during a sudden fault, has also been investigated for both PMSMs. Challenges of making of both rotors have been discussed. The theoretically determined parameters and analytically evaluated performance figures have been verified through standard FEM packages and then validated experimentally on the prototypes.
Volume 45 All articles Published: 25 September 2020 Article ID 0247
This paper presents detailed steps and procedures for the design of a 4-pole, 0.75-hp, 1500-r.p.m. surface-mounted permanent magnet brush-less DC (SPM-BLDC) motor. The motor has been fabricated at the works of a local manufacturer. The parameters of the machine have been analytically evaluated and subsequently compared to the experimentally determined values. Its practical performance on load has been experimentally evaluated in the laboratory and verified against analytical predictions too. Low-cost M45 electrical steel laminations, as used for commercial induction motors (IMs), have been used from considerations of cost and availability. This also enables direct comparison of important parameters (e.g. torque density, power density and efficiency) between the fabricated prototype and commercially available fractional-hp 3-phase and 1-phase IMs of similar rating. This study is significant since electrical motor manufacturers need not change their stator stamping production line for BLDC motor vis-a-vis IM in case of mass production. Such an approach is hardlyreported in the available technical literature. Analytical methods adopted include both conventional hand calculations and finite-element analysis using commercially available software package(s). Excellent agreementsbetween analytical and experimental values uphold the correctness of the design process, precision of fabrication and accuracy of experimental investigations
Volume 45 All articles Published: 31 October 2020 Article ID 0271
This paper presents a novel initial rotor position estimation method for a surface-mounted permanent magnet synchronous motor (SPMSM) using its inherent magnetic features. Here, the initial rotor position has been estimated utilising the variation in phase inductances arising out of the unavoidable but verysmall saliency occurring either out of magnetic saturation of the machine or the magnet shape. Detailed investigations have been carried out to enumerate the actual magnitude of the inductances of the SPMSM with distributed armature winding and the dynamic saturation status with armature current variation. Here, 3-phase balanced voltage of higher frequency (150 Hz) is applied to the motor terminals for a short period of time (for 300 ms) and the corresponding phase currents are indirectly used to determine the rotor position. Innovativesignal processing steps have been used to distinguish small differences in the 3-phase currents caused by the small differences in the phase inductances. The position is determined from the relative values of different phase currents using a novel approach. This still leads to two diametrically opposite (electrical phase) solutions for the instantaneous rotor position. To eliminate this ambiguity, two alternative methods (for pole identification) have been proposed. Extensive co-simulations of finite-element method (FEM)-based electromagnetic simulation andsystem simulation (for logic implementation) have been conducted. The estimation methods have been validated experimentally on a laboratory-developed prototype SPMSM that was designed and fabricated by the authors.The experimental results are found to be in excellent agreement with the FEM-based simulation results. The complete initial position estimation method takes less than 1 s of real time, which is typically less than the prechargingtime of standard commercial inverters.