Mehmet Timur Aydemir

Design of a linear bulk superconductor magnet synchronous motor for electromagnetic aircraft launch systems

High-temperature superconducting (HTS) material in bulk form is used to design a linear synchronous motor for an electromagnetic aircraft launch system. The motor is designed without an iron core. Stator coils are placed in the air while the permanent magnets used in conventional design of linear permanent magnet synchronous motors are replaced by the HTS bulk magnets. The physical, operational, and equivalent circuit parameters of the linear motor with HTS bulk magnets are compared with those of a linear permanent magnet synchronous motor and linear induction motor designed for the same application. Results show that utilizing superconducting magnets is only superior at temperatures below 40 K.

Isolated High Step-Up DC–DC Converter With Low Voltage Stress

Fuel cell stacks and photovoltaic panels generate rather low dc voltages and these voltages need to be boosted before converted to ac voltage. Therefore, high step-up ratio dc-dc converters are preferred in renewable energy systems. A new Z-source-based topology that can boost the input voltage to desired levels with low duty ratios is proposed in this paper. The topology utilizes coupled inductor. The leakage inductance energy can efficiently be discharged. Since the device stresses are low in this topology, low-voltage MOSFETs with small RDS(on) values can be selected to reduce the conduction loss. These features improve the converter efficiency. Also, the converter has a galvanic isolation between source and load. The operating principles and steady-state analysis of continuous and discontinuous conduction modes are discussed in detail. Finally, experimental results are given for a prototype converter that converts 25 V dc to 400 V dc at various power levels with over 90% efficiency to verify the effectiveness of the theoretical analysis.

A critical evaluation of high power hard and soft switched isolated DC-DC converters

Realization of high power isolated DC-DC converters is largely dependent on managing converter parasitics and losses. In this paper, four different DC/DC converter topologies are compared in terms of their losses. The topologies include hard switched half bridge, switched snubber half bridge, zero voltage switching phase shifted full bridge and loss limited (semi soft switched) full bridge converters. These topologies have been designed to obtain 350 V 300 A DC output from a 650 V DC input at 20 kHz switching frequency. Design results and loss evaluations are presented.

Pulse-split concept in series resonant Dc link power conversion for induction motor drives

Three phase simultaneous current pulse control for a high frequency series resonant DC link power converter was achieved by utilizing a previously developed pulse trimming method and current peak limiting with a saturable core. Each resonant pulse is split to flow in three phases almost simultaneously so that very smooth sinusoidal output current waveform is attained. The duality of the series resonant DC link with the parallel resonant link is discussed. Simulation and the basic experiments are performed.<<ETX>>

Utilization of a series resonant DC link for a DC motor drive

A high-frequency series resonant DC link converter is utilized as a DC motor drive. This system generates a resonant current in a series link, and switching is done at zero current instants, reducing switching losses to a minimum value. A pulse density modulation (PDM) strategy, utilizing a current rectangular loop and an external motor speed feedback loop, controls the resonant converter. A sinusoidal input fundamental current and nearly unity input power factor can be observed in different load conditions. The overall characteristics of the system, including such variables as maximum power, input current, start up, and transient responses, are presented by digital simulation and were verified on an actual prototype system. >

Utilization of the series resonant DC link as a DC motor drive

The use of a high-frequency series-resonant DC link power converter as a DC motor drive is described. This system generates a resonant current in a series link, and switching is performed at zero current instants, reducing switching losses to a minimum value. A pulse-density modulation (PDM) strategy, utilizing a current regulator loop and an external motor speed feedback loop, controls the resonant converter. A sinusoidal input fundamental current and nearly unity input power factor are observed in different load conditions. The overall characteristics of the system, including such variables as maximum power, input current, start up, and transient responses, are presented by digital simulation and verified on a prototype system.<<ETX>>

Pulse width modulated series resonant converter

A pulse-width-modulated (PWM) extension of the series resonant converter is presented which reduces the dead time and improves the modulation strategy of this type of converter. With the addition of one thyristor and one diode to the 12 thyristors of a series resonant converter, pulses of link current of arbitrary width can be created. Simulations show that good waveform quality can be achieved, and experimental results also show a reduction in the effective dead time. The proposed control scheme has the following advantages over previous methods: inverter switching and external PWM signals can be synchronized; high-frequency DC link current regulation remains possible, allowing line current power factor control; and device stresses are typically lower than in the normal series resonant DC link converter when a PWM technique is employed.<<ETX>>

Application of adaptive droop method to boost converters operating at the output of fuel cells

Droop method is widely used to operate dc-dc converters in parallel. Although it is very simple and easy to use, poor regulation and uneven load sharing are the major limitations of the method. This paper presents a modified droop method to overcome these limitations. Proposed method is an adaptive technique that re-defines the droop coefficients after an initial period of operation. The method has been tested with three boost converters designed to operate at the output of a fuel cell. Results clearly show that better regulation and load sharing are achieved using the suggested technique.

Active and reactive power sharing in parallel operated inverters

Active and reactive power sharing of parallel operating inverters has been investigated in this paper. The power sharing is achieved by adjusting the frequency and voltage amplitude of each unit. Each unit has its own controller and these controllers make sure that the total active and reactive power is shared by each unit in proportion with their rated powers, without any communication between them. A system consisting of two inverters has been simulated in MATLAB-Simulink

A coupled-inductor Z-source based Dc-Dc converter with high step-up ratio suitable for photovoltaic applications

A new dc-dc converter topology suitable for renewable energy sources such as photovoltaic panels or fuel cells is proposed in this paper. The converter which is a derived from Z-source converters uses coupled inductors to provide isolation and high conversion ratios. A big advantage of this converter is low switch voltage rating. A detailed analysis of a 25 V / 400 V dc, 300 W system is given along with preliminary simulation results.