Mostafa Valavi

Influence of Pole and Slot Combinations on Magnetic Forces and Vibration in Low-Speed PM Wind Generators

In this paper, radial forces and torque ripple characteristics are investigated in permanent magnet (PM) machines having different pole and slot combinations. Using the PM machines with concentrated windings could be beneficial in direct-drive wind generators since it is possible to reduce the size and weight of the generator. The PM machines with concentrated windings having a large number of poles are compared to investigate the effect of pole and slot combinations on force and vibration characteristics in low-speed generators. Cogging torque waveforms and torque ripple are investigated using time-stepping finite-element analysis. Analysis of radial forces is presented, including investigation on radial force density distribution, total forces on teeth, and time-dependent force waveforms on a tooth. Structural analysis and experimental modal analysis are performed on the prototype generator. The main mode of vibration in the prototype machine is observed experimentally and the results are in good agreement with the simulations.

Slot Harmonic Effect on Magnetic Forces and Vibration in Low-Speed Permanent-Magnet Machine With Concentrated Windings

In this paper, the influence of slot harmonics on magnetic forces and vibration is studied in a 120-slot/116-pole low-speed PM machine at no-load. It is shown how the lowest mode of vibration is produced at no-load due to slotting. Comparing the cases of open slots, semi-closed slots and magnetic wedges, the effect of slot closure on radial forces and torque production capability is discussed. Magnetic flux distribution in the airgap is computed using finite element analysis. Spatial harmonics due to slotting are investigated in different cases. Maxwells stress tensor is employed to calculate radial and tangential components of the force density in the airgap. Spatial distribution of the total forces on the teeth and also time-dependent force waveform on one tooth are analyzed and discussed for different cases. It is shown how the magnitude of the lowest mode of vibration is reduced in the case of using semi-closed slots and magnetic wedges. Tangential force density distribution and torque production capability are also discussed. Structural analysis is presented to compute the maximum amplitude of the stator deformations due to the radial forces. Experimental results of the prototype generator are presented verifying the existence of the lowest mode of vibration at no-load because of the slot harmonics.

Magnetic forces and vibration in permanent magnet machines with non-overlapping concentrated windings: A review

Permanent magnet machines with non-overlapping concentrated windings have been gaining importance in the last few years. Significant advantages such as short end-windings, high efficiency and low cogging torque make them an attractive option in several applications. However, due to a large harmonic content in the MMF and also particular pole and slot combinations, the vibration level of these machines is considerably higher than the machines with distributed windings. This paper discusses this problem and reviews the works presented in the literature concerning magnetic forces and vibration in PM machines with concentrated windings.

Effects of Loading and Slot Harmonic on Radial Magnetic Forces in Low-Speed Permanent Magnet Machine With Concentrated Windings

This paper investigates the effects of loading and slot harmonic on radial magnetic forces in a low-speed permanent magnet (PM) machine with non-overlapping concentrated windings. Vibration level of the PM machines with concentrated windings can be potentially higher compared with that of the traditional machines with distributed windings. It is mainly due to the presence of low-order spatial harmonics in radial force density distribution. In this paper, magnetic flux density distribution in the air-gap of a 120-slot/116-pole machine is computed using finite-element method. Maxwells stress tensor method is then employed to calculate radial and tangential forces in the air-gap. Flux density and radial and tangential force distributions are studied in different loading conditions and the effect of d-and q-axis currents are investigated. It is shown how the lowest mode of vibration is produced by different harmonic pairs in the flux density. It is found that d-axis current significantly changes the amplitude of the lowest spatial harmonic of the radial force distribution and consequently the vibration behavior. The main reason is found to be the changes in the amplitude of the slot harmonic, which is heavily affected by loading. Experimental tests are carried out to compare the vibration level of a PM generator supplying resistive, inductive, and capacitive loads. There is a good agreement between simulations and experimental results.

An Investigation of Zeroth-Order Radial Magnetic Forces in Low-Speed Surface-Mounted Permanent Magnet Machines

Zeroth mode of vibration is investigated in low-speed high-torque surface-mounted permanent magnet (PM) machines with non-overlapping concentrated windings. Magnetic field distribution in the air gap is computed using time-stepping finite-element analysis, and Maxwell stress tensor is then employed to calculate the radial force density. Based on the time variation of the mean value of the radial force density spatial distribution, the zeroth-order radial forces are studied. Simple structural equations are used to compare the stator deformations for the machines under investigation. Contribution of the zeroth mode to produce vibration is analyzed and compared with the lowest non-zero mode. The influence of pole and slot combinations and the effect of rectifier load are discussed. This paper is carried out to investigate whether or not the zeroth-order radial forces can be of importance in the presented surface-mounted PM machines that are not connected to pulsewidth modulation (PWM) converters.

Efficiency and loss calculations in design of converter-fed synchronous hydrogenerators

In variable-speed pumped-storage power plants, a promising solution is to connect the synchronous hydro-generator to the grid via a full-rated frequency converter. In this case, the nominal frequency of the hydrogenerator can be chosen independently from the grid. The purpose of this paper is to investigate how the nominal frequency affects generator efficiency using the converter-fed synchronous machine topology. The nominal speed of rotation is varied between 275 rpm and 600 rpm, while the nominal frequency used is between 20 and 85 Hz. An analytical model of the salient pole synchronous generator is used. The weight of active materials, iron losses, copper losses and mechanical losses are calculated for the different nominal frequencies and speeds of rotation. Based on this, generator efficiency curves are calculated and presented.

Influence of slot harmonics on radial magnetic forces in low-speed PM machine with concentrated windings

In this paper, influence of slot harmonics on magnetic vibrational forces are studied in a 120slot/116pole low-speed PM machines in no-load. Different cases including open slots, semi-closed slots and semi-magnetic wedges are discussed. Magnetic flux distribution in the airgap is computed using time-stepping finite element method. Spatial harmonics due to slotting are investigated in different cases. Maxwells stress tensor is employed to calculate radial force density in the airgap. Spatial distribution of the total forces on the teeth and also time-dependent force waveform on one tooth are analyzed and discussed for different cases. It is shown how the amplitude of the lowest mode of vibration is reduced in the case of using semi-closed slots and semi-magnetic wedges. Tangential force density distribution and torque production capability of open and semi-closed slots are also discussed.

Effects of lifting reactance requirements on the optimal design of converter-fed synchronous hydrogenerators

The maximum allowed per unit value of the synchronous reactance of a synchronous generator is normally decided by the grid codes in order to maintain the stability of the system. For the converter-fed synchronous hydrogenerator, the steady state stability can be maintained by the frequency converter. In this paper the constraint on the synchronous reactance is relaxed from 1.2 per unit to 2.0 per unit and then removed altogether. The cost of the active materials and the net present value of the cost of losses are calculated for each case and compared. Three different nominal frequencies are also used to find which one gives the lowest total cost. The total generator cost is reduced when the synchronous reactance is increased. The lowest cost of the 50 Hz designs are lower than the cost of the 25 and 75 Hz designs.

Investigation of magnetic field and radial force harmonics in a hydrogenerator connected to a three-level NPC converter

This paper investigates spatial and time harmonics of flux density and radial force distributions in the airgap of a hydrogenerator with fractional-slot windings. Hydrogenerator is connected to a neutral-point-clamped (NPC) converter, representing a converter-fed synchronous machine (CFSM) in pumped-storage hydropower plant. Magnetic field in the hydrogenerator is computed using time-stepping finite element (FE) analysis, coupled with an external circuit (where PWM converter is modelled). Radial forces are calculated analytically based on Maxwell stress tensor. Three loading conditions are studied (i.e. resistive loading and converter-fed operation with two different carrier frequencies). In the paper, it is investigated how converter-fed operation could change the characteristics of the radial magnetic forces. It is also discussed how vibration of the hydrogenerator is expected to be affected by additional harmonics generated by the PWM converter.

Analysis of radial magnetic forces in hydrogenerators with fractional-slot windings

This paper investigates radial magnetic forces in two hydrogenerators with fractional-slot windings. Using time-stepping finite element analysis, time and space distributions of the flux density in the airgap are computed. Maxwell stress tensor is then employed to calculate the radial forces analytically. Spatial harmonic orders of radial force density at one time instant and time-varying mean value of radial force density are studied. Using simple structural equations, stator deformations due to the zeroth and lowest non-zero modes of vibration are calculated and compared. Simulation results are discussed to identify the major cause of vibration in the hydrogenerators under investigation.