Martin Kaufhold

Failure mechanism of the interturn insulation of low voltage electric machines fed by pulse-controlled inverters

Partial discharges (PDs) occur in the air-filled gaps of enameled wires that are touching. They erode the insulation and consequently lead to an interturn breakdown. This paper describes the failure mechanism of low voltage interturn insulation as a consequence of PDs and shows why and how it is influenced by the insulation design, temperature, and the applied voltage. Understanding the failure mechanism leads to better founded maximal permissible stresses and a proper design of the interturn insulation to avoid premature drive failures.

Electrical stress and failure mechanism of the winding insulation in PWM-inverter-fed low-voltage induction motors

The winding insulation of low-voltage induction motors in adjustable-speed drive systems with voltage-fed inverters is substantially more stressed than in line-powered motors. Consequently, this operation is subject to limitations depending on the electrical stress and on the failure behavior of the winding insulation. Actual recommendations do not consider sufficiently the physics behind these phenomena and contain large utilizable reserves.

Behavior of machine insulation systems subjected to accelerated thermal aging test

Accelerated aging tests have been extensively used on motor and generator insulation systems to simulate in a short time period the deterioration mechanisms occurring during normal operation of many years, to identify insulation system designs with longer life time and to support the qualification of a new system. The contribution presents results from extensive investigations on the behavior of two VPI machine test insulation systems (reference and candidate) with thermal class F (155°C), which were subjected to three thermal overstresses in repeated cycles. After each thermal cycle the insulation system is subjected to mechanical stress, moisture, and voltage. In order to check the condition of the insulation systems in a non-destructive manner several properties of insulation like dissipation factor, insulation resistance and partial discharge quantities were measured after each thermal cycle. After that the end of life of the insulation systems were checked with ac voltage and impulse voltage. The results support the qualification by comparative evaluation and they indicate, how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test.

Interface phenomena in stator winding insulation - challenges in design, diagnosis, and service experience

Technically and economically optimized insulation systems can only be offered today if there is complete knowledge of phenomena in the winding insulation that determine its service life. This paper describes how such mechanisms are influenced by operating conditions, how they are detected using modern diagnosis methods, and how they can be intentionally influenced by the design of the insulation.

Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses

This article describes the change in partial discharge (PD) pattern of high voltage rotating machines and the change in the tan /spl delta/ as a function of the applied test voltage during the aging processes as caused by the application of different stresses on stator bars. It also compares the PD patterns associated with internal, slot, and end-winding discharges, which were produced in well-controlled laboratory conditions. In addition, the influence of different temperature conditions on the partial discharge activities are shown. The investigations in this work were performed on model stator bars under laboratory conditions, and the results might be different from those obtained for complete machines, as rotating machines are complex PD test objects, and for example, the detected PD signals in a complete machine significantly depend on the transmission path from the PD source to the measurement device.

High-temperature-superconducting machines- a high-technology step for large rotating electric machines

The presently available high temperature superconducting materials (HTS) transport electric current at high current densities with negligible electric losses. This makes it possible to create higher magnetic fields. Using these features in a rotating electric machine it is possible to reduce the over-all dimensions and weight of the machine, to increase the efficiency and to improve additional features of the machine. To address these goals Siemens started three R&D projects to develop, manufacture and test electric machines with high temperature superconducting field windings. A 400 RW synchronous motor was followed up by a 4000 kVA high speed generator. A 4000 kW high-torque motor will be the third step. Additional to the application in electric machines it is possible to use the substantial advantages of high temperature superconducting materials for passive magnetic bearings. These are operating without any control device of the magnetic field. Siemens started the development of heavy load HTS bearings in parallel to the development of HTS machines

Reliability and statistical lifetime- prognosis of motor winding insulation in low-voltage power drive systems

This paper discusses a statistical approach to calculate the reliability and lifetime of the motor insulation system in complex power drive system application statistical lifetime-prognosiss.

Behavior of a Machine Insulation System during Accelerated Aging Tests

The contribution presents results from extensive investigations on the behavior of a VPI machine test insulation system (thermal class F (155degC)), which was subjected to a thermal overstress (175degC) in repeated cycles. It will represent the thermal deteriorating effects of an insulation system in service on an accelerated basis. After each thermal cycle the insulation system is subjected to mechanical stress, moisture, and voltage. In order to check the condition of the insulation systems in a non-destructive manner several properties of insulation like dissipation factor, capacitance, insulating resistance and partial discharge quantities were measured after each thermal cycle. The results indicate how the changes in electrical and dielectric properties of insulation take place during the accelerated aging test.