Jemimah C. Akiror

On the coefficients of core loss formulas for electrical machines

In this paper the behavior of core loss coefficients is studied using a new method of core loss separation. This method eliminates the assumption of uniform magnetic field distribution in a material especially at high frequencies. With this separation method, the hysteresis and eddy current coefficients are calculated using the original Steinmetz equation and its derivative. The calculated loss from the coefficients of the two formulas is then compared to the loss from the separation. Results and conclusions are also presented.

Consideration of Design and Operation on Rotational Flux Density Distributions in Hydrogenerator Stators

Core losses in an electrical machine vary depending on the type of material, frequency of operation, and flux density magnitude and waveform. Loss prediction using pulsating models is insufficient for estimating core losses in a machine, since it does not take into account rotational core losses. Moreover, superposition of pulsating losses in the radial and tangential direction overestimates losses especially at higher flux densities. The objective of this paper is to study rotational flux distribution in the stator of the generator under different operating conditions. Distribution of rotational flux in a large hydrogenerator stator is studied by plotting the aspect ratio. It was found that increasing the yoke length and reducing the air gap increases the area of the stator with rotational flux. Varying the output power showed slight changes in the rotational flux distribution. Different optimized machine designs are investigated for rotational flux distribution; moreover, rotational core losses are measured and used for a comparative study radially across the stator.

Consideration of design and operation on rotational flux density distributions in hydro generator stators

Core losses in an electrical machine vary depending on the type of material, frequency and flux density. Loss prediction using pulsating models is insufficient for estimating core losses in a machine since it does not take into account rotational core losses. Moreover the superposition of pulsating losses in the radial and tangential direction overestimates losses especially at higher flux densities. The objective of this paper is to study the rotational flux density distribution in the stator of a generator under different operating conditions. The distribution of rotational flux in a large hydro generator stator is studied by plotting the aspect ratio. It was found that increasing the yoke length increases the area experiencing rotational flux. Varying the air gap and output power showed slight changes in the rotational flux distribution. However they have an effect on the flux magnitude and harmonics which potentially increase the total losses. Dimensions and operating conditions of the machine therefore affect the total core losses.

Evaluation of the change in winding pattern of a large hydro generator

Uprating and refurbishment of large hydro generators presents opportunities for increase in power output, efficiency and life time of the machine. Several parts of the machine can be changed depending on the need and predicted performance outcome. For the generator unit in this paper, refurbishment of the stator winding is to be done while maintaining the same stator core design, therefore a new winding pattern is proposed for the machine. This paper numerically compares the proposed new winding pattern and the old winding pattern using flux density distribution and radial electromagnetic forces in the air gap. Emphasis is given to the low frequency harmonics with the potential to excite the natural frequency of the core, causing noise and vibration. The calculated total harmonic distortion (THD) of the new winding pattern is higher than the old winding pattern by 3%. This was reflected in the increased damper bar and core losses in the machine with the new winding pattern.

Effect of Saturation on Rotational Flux Distribution in Hydro Generators

Uprating of hydro generators requires studies to determine the potential areas of failure like hotspots in the machine core. Rotational core losses cause localized heating of the core; therefore, understanding the distribution and behavior of rotational flux in the machine is the key. This paper studies the distribution of rotational flux in a hydro generator as the core material is saturated. The percentage of rotational flux in the stator increases as the material is saturated, especially for aspect ratios above 0.6. This affects the loss distribution and total losses in the machine. Core losses at no load and full load are also computed considering the change in flux distribution, and an increase of over 50% in the total stator core losses is calculated. Moreover, experimental core loss data of nonsinusoidal flux densities in various parts of the machine at no load and full load operation are also presented.

Detection of lamination faults from rotating, magnetic fields

Detection of lamination faults is key in the prevention of more serious generator core damage. Localized stator core faults occur frequently in rotational flux zones. Therefore, this paper studies the behavior of rotational flux in the presence of shorted laminations. Faults result in a redistribution of the magnetic field across the sample and hence the flux density distribution. Different sizes and positions of faults are simulated. Moreover a fault is imposed between two laminations and the associated rotational core losses measured. Experimental results show a distortion in the measured magnetic field with a 5% difference between the normal and faulted rotational core losses samples.

Rotational core loss test setup: Design and measurement

Rotational core loss data is required for core loss estimation in electrical machines. This necessitates the design of 2D magnetizers to accurately measure these losses. However, the accuracy of any 2D magnetizer is contingent on its ability to maintain a uniform field distribution in the sample, at least in the measurement area, for all flux density levels and magnetization directions. Moreover, the measurement area should be large enough to allow measurements over a wide sample area, which effectively represents the material properties. A 2D magnetizer is designed to allow a measurement area of 65 by 65 mm on a 136 mm diameter circular sample. The magnetizer was prototyped and preliminary core loss measurements presented for both pulsating and rotational core losses using M19G29 steel at 60 Hz.

Parameter sensitivity of large electric machines

Machine simulation models allow the use of embedded numerical computation techniques. The numerical model and computational accuracy is usually gauged by comparison with experimental measurements. In this paper a large hydro generator is modeled and the sensitivity of various model parameters is investigated by comparing the simulated results with experimental measurements from four units of the same design. The machine open circuit voltage was found to be very sensitive to the airgap and stator B-H curve. Moreover, for 2D simulations the effective length is also important because it should account for differences in the stator and rotor lengths, in addition to the presence of radial air ducts. A 20% variation in operation airgap between two machines of the same design induced over 18% difference in open circuit voltage. Reduction in permeability of the soft magnetic materials resulted in agreement between the numerical and measured results at saturation. Consequently for large machines, the B-H curves from the Epstein measurements are insufficient and should be adjusted accordingly.

Effect of saturation on rotational flux distribution in hydro generators

Uprating of hydro generators requires studies into the potential areas of failure like hotspots within the machine core. Rotational core losses cause localized heating of the core therefore understanding the distribution and behavior of rotational flux in the machine is key. This paper studies the distribution of rotational flux in a hydro generator as the core material is saturated. The percentage of rotational flux in the stator increases as the material is saturated especially for aspect ratios above 0.6. This can affect the loss distribution in the machine. Moreover experimental core loss data of non-sinusoidal flux densities in various parts of the machine at no load and full load operation are also presented.