Vandana Rallabandi

Modeling of a multi-megawatt grid connected PV system with integrated batteries

The multi-megawatt grid connected photovoltaic (PV) system studied in the paper includes parallel arrays and power electronic units, each with their own DC-DC and DC-AC converters. In one configuration, the DC-AC converters of adjacent parallel sections are connected in cascade, in order to effectively operate as a multilevel inverter, thereby reducing the filtering requirements. Grid voltage oriented control is employed for inverters and a battery is incorporated for energy storage and performance improvement. Modeling is performed with the PSCAD/EMTDC software, such that both the power electronics components, controls and subsystem aspects, and the electric grid power system issues, can be studied during steady-state and transient operation. The system simulation is demonstrated on a modified IEEE 14-bus test case.

Multilayer Concentrated Windings for Axial Flux PM Machines

Coreless axial flux machines are of interest because of the absence of stator core losses and cogging torque. These machines generally employ concentrated windings. One of the challenges with such a winding is that the torque producing MMF component that corresponds to the fundamental of the magnet excitation is accompanied by substantial asynchronous components. These harmonics cause losses in the rotor core and magnets, which can become significant at high speeds. This paper proposes a new multilayer winding arrangement to eliminate the non-torque producing MMF components. This winding is applied to a 12-coil 16-pole coreless axial flux machine. The efficacy of the winding is established by 3-D finite-element analysis.

Large-Scale Optimization of Synchronous Reluctance Machines Using CE-FEA and Differential Evolution

A novel automated design optimization procedure based on the application of an ultrafast computationally efficient finite-element analysis (CE-FEA) for current-regulated synchronous reluctance machines supplied from power electronic converters is proposed. The CE-FEA uses only a minimum number of magnetostatic solutions in order to comprehensively evaluate performance, including ripple torque and core losses. The optimization algorithm is based on differential evolution, and uses as independent variables the torque angle and ratios for a generic rotor topology with four flux barriers. Two problems, one with two and the other with three objectives, are studied and results are compared. Global performance indices and objectives incorporate the effect of average torque output, losses, torque ripple, and power factor at fixed cost. It is shown that through optimal studies with more than 5000 candidate designs, high output power, high efficiency, and low torque ripple can be achieved, while the relatively low power factor remains an inherent limitation of synchronous reluctance technology. Simulations are validated versus tests from a 10-hp 1800-r/min prototype.

On the feasibility of carbon nanotube windings for electrical machines — Case study for a coreless axial flux motor

The latest developments in carbon nanotube (CNT) wires and yarns attract great interest for potential application to electromagnetic devices, such as electrical machines and transformers. The CNT material properties are largely different from copper and aluminum in terms of electrical conductivity, mass density, and thermal transfer, creating a new design paradigm for which the traditional rules and device topologies no longer apply. This paper proposes a brushless permanent magnet multidisc axial flux construction with coreless stator and special windings and minimal rotor back iron, as a suitable topology for CNT winding application. Specific analytical closed-form sizing equations, as a function of winding electric conductivity, machine dimensions, and operating speed/frequency, are derived and employed in a systematic comparative study over a range of kW power ratings and speeds between 1,000 and 10,000 rpm. The numerical study is complemented by 3D and 2D electromagnetic FEA. The results show that the designs with CNT windings may have substantially higher specific power per mass, particularly at high rotational speeds and/or supply frequency, where the combined effect of DC and AC conduction losses in the windings is significant.

Current weakening control of coreless AFPM motor drives for solar race cars with a three-port bi-directional DC/DC converter

Solar race cars require compact components operating at high efficiency. This paper proposes the use of a coreless axial flux permanent magnet machine, which has the attributes of low stator mass, negligible core loss and virtually zero cogging torque, as the propulsion motor. A three-phase inverter with its dc bus fed from a three-port DC/DC converter, which accepts inputs from a solar panel and battery powers the propulsion motor. Galium nitride (GaN) devices are used in the three-port converter, allowing very high switching frequencies thereby reducing the size of the transformer which provides galvanic isolation between the two sources and output. The three-port converter ensures operation of the solar panel at its maximum power point and also allows bi-directional power flow between the propulsion motor and battery depending on operating conditions. Operation over a wide range of speeds, which is required by the solar race car application, is achieved by the new approach of current weakening. This method involves raising the dc bus voltage of the motor side inverter at speeds exceeding the rated.

Sensorless control of three phase switched reluctance motor drives using an approximate inductance model

The use of position sensors in switched reluctance motor drives presents disadvantages of decreased reliability and increased cost. This paper proposes a full speed range sensorless control method in order to simply address these issues. The rotor position, during running operation is estimated through a simplified model for the relationship between phase inductance for given current and rotor angle. Means of improving this model are also proposed. For position estimation at standstill, pulse injection is employed. The estimation of the phase standstill inductances is used in order to obtain the initial rotor position. The proposed method does not require a detailed knowledge of magnetic characteristics of the motor nor additional hardware. Simulation and experimental results demonstrate the satisfactory accuracy of rotor position estimation with an error of only 0.5 degrees under high load conditions.

Axial-flux PM synchronous machines with air-gap profiling and very high ratio of spoke rotor poles to stator concentrated coils

Permanent magnet machines including a magnetic gearing effect are attractive for low-speed high-torque applications. This paper proposes a dual-stator axial-flux machine topology with a high ratio for the number of rotor poles to concentrated stator coils. The stator consists of a relatively small number of teeth, with each tooth including multiple smaller/auxiliary teeth. The rotor employs spoke-type permanent magnets, which result in high flux concentration, thereby further improving the torque density. The paper discusses the principle of operation of the proposed machine topology. Finite element results on example designs with 6 stator teeth, each having 2 and 3 auxiliary teeth, and with 20, 32, and 34 rotor poles are presented, respectively. Other feasible slot-pole combinations are also identified.

On the effect of design tolerances on the performance of synchronous PM machines evaluated according to the IEEE Std 1812

In the process of designing an electric machine, the systematic study of tolerances for the design variables and material properties is of the utmost importance. This paper proposes a method by which possible variations of the design variables and material properties can be distinguished. The design of experiments (DOE) technique, open-circuit and short-circuit tests, with minimum instrumentation requirements, have been employed. Virtual tests are conducted based on the recently approved IEEE 1812 testing guide for PM synchronous machines. As a case study, a 100 hp 16-pole 18-slot spoke-type PM machine is discussed. It is shown that variation in magnet remanence, steel grade, as well as dimensional tolerances, may be identified. The ratings and magnetic loading of the machine plays a critical role in identifying manufacturing tolerances.

A comparative study of conventional and coreless axial flux permanent magnet synchronous motors for solar cars

Axial Flux Permanent Magnet (AFPM) motors are suitable options for solar powered vehicles due to their compact structure and high torque density. Furthermore, in NS-type APFM machines, the magnetic stator core may be eliminated, which simplifies the manufacturing and assembly. This paper examines two different machine designs for use in the solar powered vehicle of the challenger class — a single rotor, single stator conventional AFPM machine, and a coreless AFPM machine with multiple stator and rotor discs. The conventional AFPM machine is designed for a one-wheel drive application, while the coreless one is intended for use in a vehicle with two driving wheels. Response surface methodology (RSM) is utilized to select among several hundreds of candidates, in both cases, the designs with minimum losses and mass while meeting the torque requirement. The performance of the selected designs have been studied via 3D finite element analysis (FEA).

Switching frequency selection for ultra-low-inductance machines

Advances in power electronics devices and materials are creating opportunities to improve both machine and drive design in order to obtain better overall system performance by designing each to exploit the advantages available from technology development in the other. In order to combine wide-band-gap devices with novel machine designs that can benefit from their application, guidance about the interactions between the machine and drive in terms of overall system performance is necessary. Herein, a method of evaluating the effect of switching frequency on system losses is presented and demonstrated using conventional and novel machine designs and power electronic devices.