Dc-link Inductor Research Articles

Current-forced neutral injection in a three-phase rectifier/converter

A new three-phase rectifier/converter with low harmonic input currents and minimal DC voltage ripple is described. The device achieves continuous input line currents by injecting a triangular third harmonic current, derived from the DC link current, into the neutral point of a delta–star transformer at the supply. It uses a conventional six-pulse phase controlled converter, with an additional bridge of four devices in the DC link. The additional bridge also cancels much of the DC voltage ripple, allowing a much smaller DC link inductance to be used. A prototype 10 kW system, suitable for such applications as DC motor drives, load-commutated inverters, exciters for AC generators, and low-power DC links is described. Experimental results obtained from this system are compared with ideal theoretical predictions.

Harmonic interaction between 1500 kVA supply transformer and VFD load at an industrial plant

The application of variable-frequency drives (VFDs) in industrial and commercial facilities has seen a tremendous increase. However, when multiple VFDs are fed from one dedicated power transformer, care should be taken not to drive the transformer into saturation. At one industrial location, nine VFDs rated at 200 HP each, and fed from a single 1500 kVA air-cooled transformer, showed saturation phenomenon, which had the potential of causing serious damage to the transformer. This paper discusses the input AC current waveform asymmetry caused by the interaction of the transformer and the nine 200 HP VFD load fed from it. The term waveform asymmetry is used when the positive half of the input AC current cycle is not equal to the negative half of the input AC current cycle. This results in the existence of DC components along with even harmonics in the input at current waveform. The various tests and the eventual solution to the waveform asymmetry caused by the VFDs is presented. The value of the inductance in the DC smoothing inductor (DC-link choke), which is included as part of the VFD, was found to contribute significantly to the waveform asymmetry. Experimental results showing the performance of a reconfigured DC-link inductor to reduce its interaction with the power transformer are presented.

Linearly controllable boost voltages from Tri-Level PWM current-source inverter

The peak line-to-line inverter AC voltage, in general, cannot exceed the supply DC voltage and transformers have to be used when voltage step-up is required. This paper describes how the necessity of using a Tri-Level PWM strategy in the current source inverter can be turned into an advantage by producing a voltage boost so that transformers are no longer required. During the frequent occurrence of the "shoot-through" states, which is the characteristic of the Tri-Level PWM-controlled inverter, magnetic energy builds up in the DC link inductor. At the end of these periods, the L/sub dc/dI/sub dc//dt voltage across the inductor augments the inverter output voltage in the same way as in the basic boost type DC/DC power converter. The paper describes the circuitry required to ensure linear control. Analytical, simulation, and experimental results are given. Applications are found in all instances where the AC voltage is higher than the available supply DC voltage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Safe and time-optimized power transfer between two induction loads supplied by a single generator

The paper deals with the problem of transferring power between different induction loads supplied by the same current generator, in as short a time as possible. In the reference solution, the system is completely stopped: it needs a long time for the current to reach zero and, afterwards, its new value, because the DC link inductance must be demagnetized and remagnetized; moreover an auxiliary starter is needed. In the proposed solution, the input of the current inverter is short-circuited whenever a transfer must be performed; the inductance remains magnetized and the current at the output of the inverter varies very quickly. The transfer time is minimal, and no starter is needed; the specific inverter control laws used to insure natural commutation are described. The paper also, presents control models of the system, obtained by using the first harmonic modeling method. >

Simple starting scheme for a parallel resonance inverter for induction heating

A starting scheme for thyristor-based parallel resonant current source inverters, with particular reference to induction heating applications, is presented. The input stage to the DC-link is a three-phase phase controlled rectifier. It is shown that instead of using the usual startup circuits providing forced commutation, it is possible to start the system by using a single additional thyristor across the DC-link inductor, together with a special timed gating at the input and output converter. The scheme provides startup for a wide range of loads. The paper explains the principle starting concept and derives a simplified model to quantify the important parameters that govern the startup. The theoretical concept is verified by experimental data demonstrating the start of such an inverter. >

Input characteristics of variable modulation index controlled current source inverters

Standard PWM current source inverters (CSIs) usually operate at fixed modulation index. The proposed modified current source inverter (MCSI) can operate with most pulse width modulation (PWM) techniques and with a variable modulation index, since the DC link inductor current freewheels on itself and not through the CSI. The use of variable modulation index control results in faster response times with no penalty on input power factor as compared to other variable modulation index schemes. This paper confirms this by investigating the input characteristics of the MCSI as seen from the AC mains. The quality of the input AC line currents is examined, and a design procedure for the input filters is given. Power factor and efficiency are discussed. Results are compared to those of other current source inverter topologies. Experimental results obtained from a 5 kVA converter confirm the theoretical considerations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of LCI synchronous motor drives with finite DC link inductance

Steady-state operation of a load commutated inverter (LCI) fed synchronous motor drive is analysed by means of a digital computer based solution of the synchronous machine equations. The solution permits calculation of current, voltage, and torque waveforms and includes the effects of finite DC link inductance, saliency of the rotor and all machine resistances. The continuity constraints for steady-state operation are exploited to enable the proper initial conditions to be calculated directly, without the need for iteration. Operating points are described by the firing angle yo, and the overlap angle μ, and are computed for an assumed DC supply voltage of 1.0 per unit. The linearity of the state equations allows the normalised solutions to be scaled to obtain any desired value of torque, DC link current etc. Sample calculations are provided, showing the influence of the DC link inductance on the torque, DC link current and voltage waveforms. These are then compared with experimentally measured data taken from a 3 kVA LCI fed synchronous motor drive, and a good level of agreement is achieved.

Analysis of the naturally commutated, convertor-fed induction motor drive system using a flux vector method

The paper presents a prediction method for the naturally commutated induction machine drive. Analytical models of the major components of the drive scheme suitable for time-stepping simulation are developed. The adoption of a reference frame for the motor that rotates with the net flux vector enables a full account to be made for the magnetic nonlinearity in the motor's main flux path. Field-oriented d, q (FODQ) equivalent circuits for the induction machine are deduced. Both the capacitor bank and inverter models are expressed in this FODQ frame to produce compatible two-axis equivalent circuits, switching functions being evolved to represent the inverter tristate switch variables. Models for the DC link inductor and the rectifier, the latter also using switching function concepts, complete the drive system model. The system model is validated through comparison between calculated and test results obtained from an experimental rig.

On the behavior of a current fed synchronous machine drive without DC-link inductance

A drive system with synchronous machine and machine-commutated thyristor-inverter is discussed. The current source of this type of drive is realized by a fast switching transistor chopper, and the current is smoothed by the machine reactance and, atypically, by a choke in the DC link. It is shown that operation without link inductance affects the margin angle of the inverter. Hence, the reactive power of the machine has to be increased considerably. To overcome this disadvantage, a special method of control is presented, investigated, and verified by experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Steady-state analysis of three-phase bridge converter supplying an active load

This paper describes the analysis of the steady-state performance of a three phase six pulse thyristor controlled converter feeding a dc back emf load in series with a resistance and a smoothing dc link inductance. A complete current and voltage solution for the balanced three phase converter system is derived together with the harmonic spectrum analysis for both voltage and current on both dc and ac side.A novel method is used in deriving the complete results. The method utilizes the symmetrical property of three-phase ac supply systems in saving computing time by a factor of m which is the pulse number of the converter. Digital computer programs are developed to simulate the steady state performance of such a system. The losses and forward voltage drops of the conducting thyristors are taken into consideration by employing a first order thyristor model. The proposed method can be extended to the analysis of a more generalized system which considers ac system imbalances both in structure and in supply voltages and takes the filters both on dc and ac side into account.

Evaluation of Phase-Commutated Converters for Slip-Power Control in Induction Drives

Three different phase-commutation thyristor bridges are applied to slip-power control of an induction motor. The performance of the drive is experimentally evaluated for each thyristor bridge with respect to input power factor, efficiency, total. input current, size of the dc-link inductor, and regulation. The best results are obtained with the eight-thyristor bridge while the conventional six-thyristor converter offers the worst overall performance.

Simplified Model and Closed-Loop Control of a Commutatorless DC Motor

A simplified dynamic model of a current-fed, self-commutated synchronous motor, operating with a constant angle between the motor counter electromotive force (CEMF) and the input current. The model, which is justified experimentally, includes only two time constants, associated with the rotor inertia and the dc link inductance. This permits the design of the current and speed controlling loops along the same principles used for classical armature-controlled dc motor drives. In this way, the experience which is widely available in industry can be applied, at least with a good approximation, to design of the commutatorless dc motor drives. Various control configurations are discussed. Control loops are designed and closed around the basic motor-inverter block. An experimental result is obtained for each control configuration, thus validating the proposed model.