Michael J. Melfi

Low-Voltage PWM inverter-fed motor insulation issues

The topic of how low-voltage insulated-gate-bipolartransistor (IGBT)-based pulsewidth modulation (PWM) inverters create additional insulation stress through voltage reflections (due to high dV/dt) has been documented for several years. First, this paper explains and summarizes the basic causes and effects of the high dV/dt environment. Second, the methodologies of various approaches to solutions are critically reviewed. Finally, standards (such as NEMA MG1, parts 30, 31) (NEMA Standards Publication MG 1, 2003) have attempted to partially address this issue. However, there are still some areas that require important clarification. This paper makes the case for what can be done to achieve the needed clarity. Testing of motor insulation systems to verify compliance with the proposed requirements is also covered.

Viability of highly-efficient multi-horsepower line-start permanent-magnet motors

The incremental efficiency improvements achieved with induction motors have been mostly accomplished by adding material to the designs of the prior efficiency level. In order to achieve levels beyond NEMA Premium (or beyond lE3), it has been recognized that a different technology must be employed. The working groups for IEC efficiency levels have stated that inverter-fed permanent-magnet motors are the likely technology to reach lE4 levels. Other motor technologies (such as synchronous reluctance) which also require inverters for operation have also been proposed for higher motor efficiency. This paper will discuss a technology which can be utilized without requiring an inverter, and that can achieve beyond-lE4 levels of efficiency while using less active material in the motor. This discussion is based on testing of technology demonstration prototypes in the range of 30-250 HP. Capabilities as well as limitations of this technology will be discussed.

Effect of surge voltage risetime on the insulation of low voltage machines fed by PWM converters

This paper investigates the repetitive surge voltage withstand of low voltage mush wound machines operated on adjustable speed drives (ASD) using insulated gate bipolar transistor (IGBT) semiconductor technology. Historical work on surge testing of motor insulation has focused on one or more of the following aspects; large HP motors, medium voltage form wound motors, single shot impulse type transients or low voltage machines with surge risetimes >200 ns. IGBT drives can have risetimes of 50 ns to 200 ns. Thus, a new study on electrical stress of insulation systems due to the nonlinear voltage distribution of mush wound motors when subjected to repetitive steep dV/dt square pulse waveforms (rather than impulse wave testing) is presented. Magnitude and risetime of the repetitive ASD surge voltage transient induced on the machine terminals is reviewed first. Next, surge propagation into the winding was investigated to identify maximum voltage stress points on the conductor insulation. Potential failure mechanisms observed at these points are then discussed. The significance of decreasing surge risetime and increasing cable lengths on internal nonlinear voltage distribution is studied with experimental results from a 7.5 HP motor with a tapped stator winding.

Low voltage PWM inverter-fed motor insulation issues

The topic of how low voltage IGBT-based PWM inverters create additional insulation stress through voltage reflections (due to high dV/dt) has been documented for several years. This paper first explains and summarizes the basic causes and effects of the high dV/dt environment. Second, the methodologies of various approaches to solutions are critically reviewed. Finally, standards (such as NEMA MG1, parts 30, 31) (2003) have attempted to partially address this issue. However, there are still some areas, which require important clarification. This paper makes the case for what can be done to achieve the needed clarity. Testing of motor insulation systems to verify compliance with the proposed requirements is also covered.

Application considerations for class-1 div-2 inverter-fed motors

Application considerations for fixed-speed motors utilized in hazardous locations are not new to the electrical community and have been discussed previously and are relatively well known in the petroleum industry. Documented guidelines for these applications are currently established, are primarily centered on motor-temperature-rise limitations and the use of nonsparking fans in class-1 division-2/zone-2 locations. However, there are additional considerations when motors and their associated loads are on inverter supply. Items such as the effect on motor temperature rise when operating at reduced speeds for units without independently powered blowers, the effect of nonsinusoidal supply at the motor terminals on motor temperature rise as well as rotor voltage related phenomenon are relevant items. This paper identifies and provides guidelines particular to inverter-fed applications for class-1 division-2 installations.

Application considerations for class 1 div 2 inverter-fed motors

Application considerations for fixed-speed motors utilized in hazardous locations are not new to the electrical community and have been discussed previously and are relatively well known in the petroleum industry. Documented guidelines for these applications are currently established, are primarily centered on motor-temperature-rise limitations and the use of nonsparking fans in class-1 division-2/zone-2 locations. However, there are additional considerations when motors and their associated loads are on inverter supply. Items such as the effect on motor temperature rise when operating at reduced speeds for units without independently powered blowers, the effect of nonsinusoidal supply at the motor terminals on motor temperature rise as well as rotor voltage related phenomenon are relevant items. This paper identifies and provides guidelines particular to inverter-fed applications for class-1 division-2 installations.

Can a Shaft Brush be Safely Applied on a Motor in a Class I Hazardous Location

Some inverter-fed motors, particularly at higher power ratings, are supplied with a shaft brush in order to help mitigate bearing currents. As more inverters are applied to motors in Class I Division 1/Zone 1 and Class I Division 2/Zone 2 locations, there is a need to understand whether a shaft brush can be safely applied in such an environment. This paper addresses the physics of why this is a concern and a methodology to evaluate the relative safety of two example cases. These issues are considered from the perspective of users, manufacturers of inverters and motors, and certifying bodies. The question posed in the title of this paper is answered by considering whether the addition of a shaft brush to an inverter-fed motor in a Class I hazardous location makes the system more safe or less safe.

A comparison of current motor technologies appropriate for petroleum and chemical applications

A number of different motor configurations have been proposed to either replace or augment the commonly applied three phase induction motor. Motors with permanent magnets, segmented stators, concentrated windings, five or six phase windings, cast copper rotors, salient pole rotors, solid rotors, and hybrid combinations of these have all been touted as having superior characteristics in one way or another. This paper will gather a wide range of competing technologies and compare and contrast them on a number of attributes. In addition to comparisons of efficiency, power factor, relative cost, and power density, these technologies will also be contrasted in regard to other specific benefits they may bring to an application. They will be compared as to how the technology scales to different sizes and speeds. They will also be compared as to their applicability to fixed and variable speed applications, including what simplicity or complexity that they may bring to an electronic controller that they may need to be matched with. In the end, this paper will provide a clean and consistent comparison of a range of possible technologies that may be used in applications in the petroleum and chemical industries.