Gary L. Skibinski

Zero-switching-loss inverters for high-power applications

The development of zero switching loss inverters has attracted much interest for industrial applications. Two topologies for realizing zero switching losses in high-power converters are proposed. The actively clamped resonant DC link inverter uses the concept of a lossless active clamp to restrict voltage stresses to only 1.3-1.5 supply voltage. For applications demanding substantially better spectral performance, the resonant pole inverter (RPI), also called the quasi-resonant current mode inverter, is proposed as a viable topology. Using only six devices rated at supply voltage, this circuit transfers the resonant components to the AC side of each phase and thus requires additional inductor and capacitor (LC) components. On the other hand, the RPI is capable of true pulsewidth modulation (PWM) operation at high frequency as opposed to discrete pulse modulation operation found in resonant DC link invertors. >

An evaluation of the electrostatic shielded induction motor: a solution for rotor shaft voltage buildup and bearing current

This paper evaluates a new induction motor, the electrostatic shielded induction motor (ESIM). An ESIM reduces rotor shaft voltages to levels below the bearing lubricants electric field intensity breakdown level and offers one solution to accelerated bearing wear caused by fluting induced by pulse width modulated (PWM) inverters. The paper begins by reviewing shaft voltages and the resulting bearing currents when operated on PWM voltage source inverters. An example of bearing fluting is shown and system models are presented and discussed. The construction details and test results for several ESIMs are presented. Experimental results show the ESIM solves the electrostatically induced rotor shaft voltage and bearing problem without degrading the electromagnetic performance of the motor.

Thermal parameter estimation using recursive identification

A novel method that converts a semiconductor transient thermal impedance curve (TTIC) into an equivalent thermal RC network model is presented. Thermal resistance (R) and thermal capacitance (C) parameters of the model are identified using manufacturers data and offline recursive least square techniques. Relevant estimation theory concepts and the formulation of an appropriate model for the identification process are given. Model synthesis is illustrated using an isolated base power transistor module. The application of time decoupled theory for high order thermal models is outlined. Simulation of junction temperature responses using model and manufacturer TTICs are compared. Estimated parameter validity is further confirmed by parameter calculation obtained from module physical dimensions. >

Experience with variable frequency drives and motor bearing reliability

Motor bearing life historically has been difficult to predict. Many variables influence bearing life in motors including operating speed, mechanical fit, loading, lubrication, vibration and environmental conditions. A large synthetic fibers producer recently discovered that motor shaft voltage induced by variable frequency drives (VFD) can add another significant variable to the formula. Based upon this experience, this paper presents theory, test data and actual results quantifying electrical discharge machining (EDM) damage to bearings initiated by variable frequency drives. The paper presents several mitigation techniques and quantify their relative effectiveness and attributes. Other factors and interactions of the above mentioned variables are also discussed in relation to the total goal of improving motor bearing life.

The effects of PWM voltage source inverters on the mechanical performance of rolling bearings

Modern power inverters provide the industrial control industry with significant advantages. Faster switching devices have increased drive performance, but with some recently discovered disadvantages. One disadvantage, rotor shaft voltage and resulting bearing current, has become an industry concern. The oil film in a bearing acts as a capacitor and provides a charging mechanism for rotor shaft voltage buildup. Electrical breakdown of the film can damage the bearing. The paper examines the mechanical and electrical characteristics of the bearing and converts them into models. The mechanical model for the bearing contact area establishes an allowable bearing current density, which is used to estimate the effect of electrical life of a mechanical bearing. The electrical model for the bearing provides a significant advancement to aid bearing design and in the analysis of electrically induced bearing damage. Finally, the paper presents quantitative results on one solution to the shaft voltage and bearing current problem.

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.

Characterization of GTOs for soft switching applications

An examination is made of modifications possible in GTO (gate-turn-off) specifications as a result of application in soft switching topologies. Turn-off is characterized as a function of (dv/dt, di/dt) controlled by the external resonant elements and gate drive circuits. Turn-on is characterized for low-voltage switching and external gate drive circuit influence. Device interaction with the various operating modes of the soft switching topologies is examined.<<ETX>>

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.

Pre-charge circuit utilizing non-linear firing angle control

A new capacitor pre-charge circuit is described that limits both peak DC link and peak AC input line current to any desired magnitude, thereby providing for optimum pre charge control and robust operation. Pre-charge control is optimum since the link capacitor is charged without voltage overshoot in the shortest possible time with maximum allowable power converter current. The new control is robust in applications containing partially precharged bus capacitors, ground and bus faults, input AC line transients or momentary AC line loss. The precharge circuit employs a microprocessor which implements a novel nonlinear discontinuous current control scheme without the need for current sensors. The pre-charge circuit is well suited for use with an AC to DC power converter having a link inductor and a DC bus capacitor which requires pre-charging before full power is applied to the power converter. The paper describes the problem with existing pre-charge circuits while emphasizing the advantages of the new control. Governing equations and a control block diagram of the new nonlinear control algorithm is given along with implementation suggestions.