Kazuo Nishihama

Investigation into loss reduced rotor slot structure by analyzing local behaviors of harmonic magnetic fluxes of inverter feeding induction motor

We analyzed the local behaviors of both time and space harmonic magnetic fluxes in this research to improve the efficiency of inverter feeding induction motors. As a result, we clarified that: (1) the magnetic flux of time harmonics passes in parallel through rotor bars, and (2) the magnetic flux of space harmonics passes radially through the rotor bars from the delay side in the direction of rotation. We propose a new rotor slot structure to reduce harmonic losses by taking these results into consideration. We also fabricated a prototype that was applied to the proposed structure of rotor slots, and measured motor losses in experiments. As a result, the total loss in induction motor with the proposed structure was reduced by 20%.

High-Frequency Interbar Current Losses in Cage Induction Motors Under No-Load Condition

This paper describes a study of the interbar current losses in induction motors under no-load condition by comparing the measured and calculated results. The interbar resistances and losses are measured on the machine of the rotor with end rings removed. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The losses are calculated with three-dimensional (3-D) magnetic field analysis on finite-element model reflecting the measured interbar resistance. The calculated losses agree well with the dependence of the measured losses on the interbar resistances. Moreover, it is confirmed that the core heating treatment caused the loss reduction and the increase of the high-frequency impedances form measurement results.

Starting torque analysis of cage induction motor using permeance model considering magnetic saturation by leakage flux

This paper describes a method to analyze harmonic components of the air-gap flux density distribution and the torque of 3-phase cage induction motors, considering the influence of circumferential slot arrangement, stator winding pattern, and distribution of air-gap permeance due to leakage flux. The proposed method is based on an analytical model of the permeance and magnetomotive force distributions. The proposed method considers magnetic saturation caused by the main and leakage magnetic flux. As a result, the harmonic components of the torque in the starting condition can be determined quantitatively. From the comparison between the calculated and experimental results, the proposed method is shown to be appropriate and useful for quantitatively estimating harmonic torque for cage induction motors.

Magnetic flux density analysis of wound rotor induction motor by permeance model

A fast calculation method for parametric survey is needed when designing motors. We propose a new method for analyzing the harmonic components of air-gap flux density distribution in three-phase wound rotor induction motors. The proposed method is based on analytical equations for permeance and magnetomotive force distribution, considering the influence of the circumference slot arrangement and winding pattern of the motor. Thus, the harmonic components of the air-gap flux density distribution can be analyzed in a short time. The analysis results of the proposed method are in agreement with those of finite element magnetic analysis in on-load condition. Consequently, this method is shown to be suitable for quantitatively estimating the flux density of wound rotor induction motors and useful for parametric survey when designing wound rotor induction motors.

Finite element analysis for field currents and reactances of doubly fed induction generators for wind turbines

Accurate determination of field currents and reactances is important for design of doubly fed induction generators for wind turbines. An analysis method using finite elements for the saturated reactances and field currents at any balanced steady-state conditions of the generators has been developed, dq-axis equations with cross magnetization are shown and all the reactances in them are calculated. A finite element mesh model of the machines has also been proposed. The proposed method and model are applied to a 1 MW machine for a wind turbine. The calculated field currents, voltages and power factors agree well with the measured values. Variation of the reactances due to saturation is also shown

Investigation Into Loss Reduced Rotor Slot Structure by Analyzing Local Behaviors of Harmonic Magnetic Fluxes in Inverter-Fed Induction Motor

We analyzed the local behaviors of both time and space harmonic magnetic fluxes in this research to improve the efficiency of inverter-fed induction motors. As a result, we clarified that: the magnetic flux of time harmonics passes in parallel through rotor bars and the magnetic flux of space harmonics passes radially through the rotor bars from the delay side in the direction of rotation. We propose a new rotor slot structure to reduce harmonic losses by taking these results into consideration. We also manufactured a prototype that was applied to the proposed structure of rotor slots, and measured motor losses. Consequently, although the current was increased by 2%, the total loss under inverter driven condition in induction motor was reduced by 20% with the proposed structure under the equal output power of 1.1 MW.

Interbar current losses in cage induction motors due to harmonic flux

The interbar current losses have been studied through measurements and calculations. However, the quantitative effect of the contact resistance between the rotor bar and the iron core inter-bar current losses is not fully understood because of difficulties both in measurement of contact resistance and its modeling in analysis. This paper describes a study of the inter-bar current losses in induction motors due to harmonic flux and compares the measured and calculated results. The interbar current losses due to harmonic flux are measured on the machine of the rotor with end rings removed. The slot insulation and core heating are applied to make differences in the interbar resistances. The losses decrease as the interbar resistances increase. The increase in interbar resistances caused by the slot insulation treatment is ascertained to decrease the losses effectively. The inter-bar current losses are calculated with three-dimensional magnetic field analysis. The finite element model contains the iron core portion with corrected conductivity reflecting the measured inter-bar resistance. The calculated losses agree well with the dependency of the measured losses on the interbar resistances. Moreover, additional measurements of high-frequency impedances of rotors reveal the interbar loss reduction effects of core heating caused by increasing the high-frequency impedances.

A Basic Study of a Novel Homopolar-Type Magnetic Bearing Unifying Four C-Shaped Cores for High Output and Low Loss

A magnetic bearing (MB) can suspend a rotor shaft with noncontact, so the MB has been used for high-output and high-speed machines. In general, heteropolar-type MB structure is used, because of its simple structure and low cost. However, the heteropolartype MB structure has a problem in terms of rotor iron loss caused by an alternating magnetic field on a rotor core. On the other hand, general homopolar-type MB structure has a low rotor iron loss caused by a dc magnetic field on a rotor core. However, as the general homopolar-type MB has a complicated structure and needs a large magnet for generating the bias flux, it becomes costly. Therefore, this paper discusses a novel homopolar-type MB structure unifying four C-shaped cores for high output and low iron loss. It is shown with three-dimensional finite element analysis that the novel homopolar-type MB has lower iron loss and lower cost than the general homopolar-type MB, which is known for its low iron loss.