David Leggate

Pulse-based dead-time compensator for PWM voltage inverters

The dead time necessary to prevent the short circuit of the power supply in pulsewidth-modulated (PWM) voltage inverters results in output voltage errors. Although individually small, when accumulated over an operating cycle, the voltage errors are sufficient to distort the applied PWM signal. This paper presents a method to correct for the dead-time errors. The pulse-based dead-time compensator (PBDTC) is less hardware- and software-intensive than other dead-time compensation methods providing a low-cost solution. The pulse-based technique is developed by analyzing the effects of dead time on a pulse-by-pulse basis and correcting each pulse accordingly. The technique is evaluated through simulation and experimental results. Other compensation methods are evaluated, and the results compared with the pulse-based technique. This comparison indicates previous methods can produce magnitude and phase errors in the applied terminal voltage, whereas the proposed method compensates for the dead time without significant magnitude and phase errors in the terminal voltage of PWM voltage source inverters.

A simple on-line adaptation for indirect field orientation of an induction machine

Three simple methods of adapting for rotor time constant changes of an indirect field-oriented induction machine are proposed. These methods utilize voltages as field-oriented reference models in the basic model reference adaptive control (MRAC) structure. Each of these methods offers some implementation or performance advantage over previously proposed MRAC methods and other adaption techniques. The proposed methods are analyzed along with other previously proposed MRAC methods, and the advantages and limitations of each are discussed. Experimental results are presented for the proposed methods with the most advantages.<<ETX>>

Reflected wave modeling techniques for PWM AC motor drives

Reflected wave transient voltages that are impressed on drive output cables and low voltage AC induction motors are simulated with an excitation source of steep fronted d/spl nu//dt pulse waveforms from a pulse width modulated (PWM) voltage source inverter. Motivation for system simulation arises from a need to correlate reflected wave peak voltage and risetime with the dielectric insulation capability of both motor and cable. Simulations based on an accurate system model also allow investigation into the effects of changing wire gauge, motor HP, cable distance or addition of drive output filters. System parameters of the inverter, cable and motor model are investigated in detail. Special emphasis is given to the importance of modeling cable skin and proximity effects. Simulation, measured lab and field results are compared. The main objective of the paper is to propose a reflected wave building block model that uses existing software on the market, is simple, computationally fast, easily configurable, reasonably accurate and allows investigation with wide variation of system parameters.

Cable characteristics and their influence on motor over-voltages

Overvoltage transients occur on AC induction motors when connected through a power cable of sufficient length to a variable speed AC drive consisting of a PWM inverter and IGBT power devices. Factors contributing to a motor overvoltage transient equal to twice the theoretical DC bus voltage are described using classical transmission line analysis. A critical cable distance (I/sub c/) is defined where twice the DC bus overvoltage occurs. However, literature is lacking on how motor transients greater than twice the DC bus voltage are generated. This phenomenon is observed on all PWM inverters with output cable lengths greater than the I/sub c/ distance. Theoretical calculations of cable frequency and damping are correlated with simulation and experimental results. Finally, the paper presents an external hardware apparatus (the Terminator), based on a cables characteristic impedance, that clamps the motors terminal voltage to levels far below twice the DC bus voltage. All calculations and proposed solutions are verified or demonstrated with experimental results.

Common-Mode Voltage Reduction PWM Algorithm for AC Drives

The impact of common-mode voltage (CMV) generated by pulsewidth modulation (PWM) ac drives on motor bearings is well known. Several algorithms for CMV reduction have been proposed in the literature. While a few algorithms assume ideal switching and fall apart when nonidealities like inverter dead time are considered, some others are effective only in a limited operating range of the drive. In this paper, a previously proposed algorithm is modified for practical implementation to include compensation for dead-time and reflected-wave motor overvoltage stress while still producing output voltage waveforms with reduced common-mode content. Experimental results are provided to show the reduction in CMV over the entire operating range, with other performance attributes such as reflected-wave motor overvoltage that are identical to conventional space-vector PWM. The advantages of applying the algorithm to a fully regenerative ac drive are also demonstrated.

Reflected waves and their associated current

Reflected wave transient voltages that result from fast IGBT voltage source inverters have received considerable investigation. The modeling and simulation of these transients requires sophisticated motor and cable models. Most voltage source PWM adjustable speed drive suppliers now provide combinations of passive and active control techniques to mitigate the adverse effects of overvoltage stress, however, the cost of the passive fixes often exceed the cost of the drive. Another aspect of low rise time devices, heretofore not examined to the extent of the overvoltage problem, is the resulting current from traveling waves. In this paper a historical perspective of the overvoltage problem is presented. Models of system components are reviewed and simulation results are compared with experimental results. These models are then employed to predict the peak currents from voltage source inverters as the cable, load, and IGBT rise time are altered. The paper then examines the consequences of reflected wave currents on current sensing, drive control, and device performance. From these results, a minimum rise time is established.

Indirect field-oriented control of an induction motor in the field-weakening region

A self-organized field-oriented induction machine controller that addresses problems encountered in the field-weakening region is presented. The field-oriented controller (FOC) is based on model reference adaptive principles but relies on samples of the state of the machine to determine control inputs. The proposed controller is compared with other approaches to field-weakening operation, and the advantages and limitations of each are discussed. Experimental results are presented for the classical and the self-organized approaches to field-weakening operation. >

Operation of PWM voltage source-inverters in the overmodulation region

Pulse width modulated (PWM) inverters experience a reduction in gain when overmodulation occurs. The pulse dropping or transition region is examined for continuous and discontinuous modulation strategies. Transition region characteristics for a number of modulation strategies are introduced. The effect of the transition region on field oriented control (FOC) is presented. The adverse effects of bus disturbances on current regulated AC inverters, while in the transition region, are demonstrated by experimental results. The problems encountered are the consequence of the reduced gain of the PWM inverter regardless of the PWM strategy. A compensated modulation technique (CMT) adaptable to continuous and discontinuous modulators eliminates the voltage error and transitions to six-step operation without inducing a voltage transient. The CMT applies to voltage and current regulated PWM inverters employing most of the modern switching strategies. Experimental results presented in the paper demonstrate the CMTs smooth transition to six-step and the improved performance a CMT-PWM algorithm provides.

Pulse based dead time compensator for PWM voltage inverters

The dead time necessary to prevent the short circuit of the power supply in pulse width modulated (PWM) voltage inverters for AC drives results in output voltage deviations. Although individually small, when accumulated over an operating cycle, the voltage deviations are sufficient to distort the applied PWM signal. This paper presents a new method to correct for the dead time deviations. The pulse based compensator is less hardware and software intensive than other dead time compensation methods providing a low cost solution. The pulse based technique is developed by analyzing the effects of dead time on a pulse by pulse basis and correcting each pulse accordingly. The technique is evaluated through simulation and experimental results. Other compensation methods are evaluated and the results compared with the pulse based technique. This comparison indicates previous methods can produce magnitude and phase errors in the applied terminal voltage, whereas the proposed method compensates for the dead time without significant magnitude and phase errors in the terminal voltage of PWM voltage source inverters.

Interaction of drive modulation and cable parameters on AC motor transients

This paper investigates overvoltage transients on AC induction motors when connected through a cable of arbitrary length to a variable frequency drive (VFD) consisting of a pulse width modulation (PWM) inverter with insulated gate bipolar transistor (IGBT) power devices. Factors contributing to a motor overvoltage transient equal to a theoretical twice DC bus voltage are first described using existing transmission line analysis. A critical cable distance l/sub c/ is defined where this 2 pu overvoltage occurs. However, literature is lacking on how motor voltage transients >2 pu bus voltage and up to 3-4 pu are generated. This phenomenon is observed on all PWM inverters with output cable lengths greater than l/sub c/ distance. Contributing factors to the >2 pu overvoltage phenomenon are investigated by exploring the complex interaction between drive modulation techniques, carrier frequency selected, cable natural frequency of oscillation, cable high frequency damping losses and to a lesser extent inverter output rise time. Theoretical calculations of cable frequency and damping are correlated with simulation and experimental results. Novel modifications to the PWM modulator as well as external hardware apparatus are proposed solutions to the >2 pu overvoltage problem, both are simulated and experimentally confirmed.