Manoj Ramprasad Shah

On-line capacitance and dissipation factor monitoring of AC stator insulation

A new on-line technique for monitoring the insulation condition of ac motor and generator stator windings is proposed. The approach uses a newly developed High-Sensitivity Current Transformer (HSCT) to precisely and non-invasively measure the differential current (e.g., the insulation current) of each phase winding from the motor junction box. Conventional differential current transformers (CT) used for fault protection can be replaced with the new HSCT to measure the winding insulation current with higher sensitivity and accuracy. The HSCT can serve both motor health monitoring and motor protection functions. Presently, indicators for insulation condition such as capacitance (C), dissipation factor (DF), or insulation power factor (PF) are only obtainable off-line. The new approach can provide a low-cost solution for on-line motor insulation condition assessment. Validation of the new HSCT technology was carried out during an accelerated life testing of a 460 V, 100 HP, 1200 RPM form wound induction motor. The motor discussed in this paper was aged at high temperature (255 °C) as the load cycled between 0 % and 200 % every 5 minutes. Although this highly accelerated life test does not represent how a motor ages in service under real operating conditions precisely, the principal goal was to prove the capability of the new HSCT to accurately detect the insulation current and quantitatively monitor motor insulation gradual aging and health. On-line data from the HSCT correlated well with off-line data from a commercial capacitance and DF bridge. It is hoped that the benefits of the on-line motor health monitoring are fully realized and the method extended to other electrical assets as well.

Eddy-Current Loss Minimization in Conducting Sleeves of Surface PM Machine Rotors With Fractional-Slot Concentrated Armature Windings by Optimal Axial Segmentation and Copper Cladding

When a nonmagnetic high-strength metallic retaining sleeve offers advantages over a nonmetallic (e.g., carbon fiber) one, it is possible to consider the application of a high-conductivity shield ldquocoatingrdquo on this sleeve to reduce the surface eddy-current losses due to nonsynchronous fields. One can start by using a Maxwells equation-based analytical model to ldquoscreenrdquo for the optimal shield thickness and then employ a ldquo2.5 Drdquo finite-element method that accounts for periodic fields and finite rotor length, including axial segmentation and/or copper cladding. These are quantified to help design a low-loss rotor sleeve for a surface permanent-magnet machine with fractional-slot concentrated armature winding. With this type of winding, the sleeve losses can be significant due to its rich (read parasitic) asynchronous harmonic armature reaction MMF content.

Effect of Number of Phases on Losses in Conducting Sleeves of Surface PM Machine Rotors Equipped With Fractional-Slot Concentrated Windings

High-speed machines with a solid rotor or a high- strength retaining sleeve could offer design and performance advantages. For a specific application, if the use of a high-strength nonmagnetic metallic retaining sleeve is more advantageous than a nonmetallic (e.g., carbon fiber) one, one needs to evaluate the eddy-current losses due to armature reaction space and time harmonics and/or tooth ripple, as they can be significant. This problem is aggravated furthermore in fractional-slot concentrated-winding machines due to their inherent sub- and super nonsynchronous MMF harmonic components. In this paper, the impact of the number of phases is quantified to help design a lower eddy loss rotor sleeve for a high-speed surface-mounted permanent-magnet rotor machine with fractional-slot concentrated armature winding, FSPCW-SPM. The goal of this paper is to provide a general method for laying out preferred FSPCW configurations. Also, a general method for screening and choosing the optimal slot/pole combinations is presented.

Development of a High Speed HTS Generator for Airborne Applications

General Electric, under contract with the Air Force Research Labs (AFRL), has successfully developed and tested a high speed, multimegawatt superconducting generator. The generator was built to demonstrate high temperature superconducting (HTS) generator technology for application in a high power density Multimegawatt Electric Power System (MEPS) for the Air Force. The demonstration tested the generator under load conditions up to 1.3 MW at over 10,000 rpm. The new MEPS generator achieved 97% efficiency including cryocooler losses. All test results indicate that the generator has a significant margin over the test points and that its performance is consistent with program specifications. This demonstration is the first successful full-load test of a superconducting generator for the Air Force. In this paper we describe the development of the generator and present some key test results used to validate the design. Extrapolation to a higher power density generator is also discussed.

Automated Detection of Rotor Faults for Inverter-Fed Induction Machines Under Standstill Conditions

It is difficult to apply conventional online motor current signature analysis techniques for diagnosis of rotor faults for closed-loop induction motor drives for many applications due to the masking effect of the feedback current controller and/or variable frequency or load operation. Relying solely on traditional offline inspection techniques during regular maintenance does not allow frequent monitoring of rotor problems and is inconvenient since it requires rotor disassembly and/or manual rotor rotation. In this paper, a new automated technique for testing voltage source inverter-fed squirrel-cage induction machines at a standstill for rotor faults is proposed. The main concept is to use the inverter to excite the machine with a pulsating field at a number of angular positions to observe the variation in the impedance pattern due to broken rotor bars, whenever the motor is stopped. An experimental study on a 7.5-hp induction motor verifies that broken bars can be detected with high sensitivity and reliability. It will be shown that the proposed method can provide automated and reliable assessment of rotor condition frequently without motor disassembly, manual rotation, or additional instrumentation. The proposed test can also provide rotor quality assessment independent of variations in motor or load operating conditions since it is a standstill test.

Optimization of Shield Thickness of Finite-Length Solid Rotors for Eddy-Current Loss Minimization

A high-conductivity shield is often used for coating the rotor of solid-rotor synchronous machines for reducing the surface eddy-current losses due to armature-reaction space/time harmonics and/or tooth ripple. Since the design process for determining the optimal shield thickness can be complicated and time consuming, a simple analytical model based on Maxwells equations was developed and presented in a previous paper to simplify the process. It has been shown that such an analytical tool can be used as a quick and effective ldquoscreening toolrdquo for determining the range of the optimal shield thickness for minimizing rotor surface losses; however, the influence of finite rotor axial length including the end-face losses was not taken into account. In this paper, an additional step is introduced in the shield design process where a special finite-element (FE) method that accounts for the impact of finite rotor axial length is employed for refining the design obtained from the analytical solution. Comparisons are made for a number of shield thicknesses and rotor lengths for significant space and time harmonic combinations to verify the validity of the proposed two-step design process (analytical and FE) and to evaluate the impact of the finite length of solid rotors.

Comparison of Induction Machine Performance with Distributed and Fractional-Slot Concentrated Windings

Fractional-slot concentrated windings have been gaining a lot of interest in permanent magnet synchronous machines. This is due to the several advantages provided by this type of windings which include: shorter non-overlapping end turns, higher efficiency, higher power density, higher slot fill factor, lower manufacturing cost, better flux-weakening capability resulting in wider constant power speed range, and fault-tolerance. However, the tradeoffs involved in using this type of windings in induction machines have not been addressed in literature. This paper examines induction machine performance with fractional-slot concentrated windings using the standard distributed lap windings as reference. Four designs are compared and various performance tradeoffs highlighted. The first machine has integral-slot distributed 2 slots/pole/phase lap winding and it serves as the reference winding. The second machine has a double-layer 1/2 slot/pole/phase winding, a workhorse for brushless DC machines. The third machine has double-layer 2/5 slot/pole/phase winding. Lastly, the fourth machine has single-layer 2/5 slot/pole/phase windings. The comparison includes torque-speed curves (including the effects of major space harmonic components), rotor bar losses, and ripple torque levels.

An iron core probe based inter-laminar core fault detection technique for generator stator cores

A new technique for detecting incipient inter-laminar insulation failure of laminated stator cores of large generators is proposed in this paper. The proposed scheme is a low flux induction method that employs a novel probe for core testing. The new probe configuration, which uses magnetic material and is scanned in the wedge depression area, significantly improves the sensitivity of fault detection as well as user convenience compared to existing methods. Experimental results from various test generators tested in factory, field, and lab environments under a number of fault conditions are presented to verify the sensitivity and reliability of the proposed scheme.

Rapid analytical optimization of eddy current shield thickness for associated loss minimization in electrical machines

A copper or another high-conductivity shield is often used for coating the solid rotor for reducing the armature-reaction space and time-harmonic-induced surface eddy-current losses in a solid-rotor synchronous machine. Since finite-element-simulation-based surface-loss evaluation for shield design can be very time consuming and complicated; a simple analytical model for calculating the surface losses is derived in this paper. A set of equations is derived based on Maxwells equations for a general case and applied to a solid-rotor synchronous machine. Simulation results are provided to show that the proposed analytical model can serve as an effective screening tool for determining the optimal shield thickness for minimizing the rotor surface losses. The model is useful for assisting the shield-design process for synchronous machines with solid rotors, especially for high-speed machines operating in conjunction with power electronic converters.

Experimental study of inter-laminar core fault detection techniques based on low flux core excitation

A comparison between two inter-laminar stator core insulation failure detection techniques for generators based on low flux core excitation is presented in this paper. The two techniques under consideration are: 1) the iron core probe based method developed recently and 2) the existing air core probe based method. A qualitative comparison of the two techniques is presented along with an experimental comparison on a 120-MW generator stator core. The test results are compared in terms of fault detection sensitivity, signal to noise ratio, and ease of interpretation, which are the main requirements for stator core inspection. In addition to the comparison, the performance of the iron core probe technique for machines with short wedge depression depth is presented along with the recent improvements in the algorithm. It is shown that the main requirements for stator core inspection are significantly enhanced with the new iron core probe-based core fault detector.