Hiroaki Toda

Effect of Si Content on Iron Loss of Electrical Steel Sheet Under Compressive Stress

The magnetic properties of nonoriented electrical steel sheets under compressive stress were investigated. Iron loss under compressive stress decreases with an increase of Si content. In particular, a 6.5% Si steel sheet shows an extremely low deterioration ratio of iron loss under compressive stress. Furthermore, the deterioration ratio of a 6.7% Si steel was negative. These results were explained well by magnetostriction. The deterioration ratio of iron loss under compressive stress decreases with a decrease of magnetostriction. To evaluate the effect of magnetostriction on iron loss in a motor, interior permanent magnet motors were made using 3.0%, 6.5%, and 6.7% Si steel sheets. In the motor using 3.0% Si steel, the iron loss was increased after the heat shrinking. On the contrary, in the motor using 6.5% Si steel, the iron loss showed a little change after heat shrinking and in the motor using 6.7% Si steel, the iron loss decreased after the heat shrinking.

Effect of hardness and thickness of non-oriented electrical steel sheets on iron loss deterioration by shearing process

Non-oriented electrical steels are widely used as the core material of motors. The magnetic properties of non-oriented electrical steel sheets have large influence on a motor efficiency. Punching is general manufacturing process in automatic fabrication of motor cores. This process is known to degrade the magnetic properties of non-oriented electrical steel sheets by plastic strain and elastic strain [1]. Therefore, the influence of the material characteristics such as grain size, hardness, and thickness on the deterioration of magnetic properties needs to be studied. In previous studies, the influence of grain size on the deterioration of magnetic properties was reported [2]. However, it is not clear about the influence of material characteristics except the grain size. The purpose of this study is to clarify the influence of thickness and hardness of non-oriented electrical steel sheets on the deterioration of iron loss by shearing process.

TRV under transformer limited fault condition and frequency-dependent transformer model

The TRV amplitude factors of 4kVA and 300kVA transformers under transformer limited fault conditions were measured. The measured factors were 1.4 and were lower than the IEC standard. The frequency response of the transformer impedance was investigated using a frequency response analysis (FRA). FRA measurement graphs showed that the short-circuit inductance value of test transformers gradually decreases along with frequency. From this result, a frequency dependent transformer model was constructed. Simulated frequency response of transformer impedance was in good agreement with measured values. Simulated amplitude factor using frequency-dependent transformer model was 1.5 and is good agreement with measured values.

Investigation of Transformer Model for TRV Calculation by Using Frequency Dependent Inductance Model

The transient recovery voltage (TRV) of the transformer limited fault (TLF) current interrupting condition has been investigated with several transformers by using current injection (CIJ) method. A transformer model for the TLF condition is treated as leakage impedance and a stray capacitance with an ideal transformer in a computation by EMTP. By using the frequency response analysis (FRA) measurement, the transformer constants were evaluated at high frequency regions. FRA measurement graphs showed that the leakage inductance value of the test transformers gradually decreases along with the frequency. From these results, frequency dependent transformer equivalent circu it was constructed. The frequency response of the model was good agreement with measurement.

Investigation of EMTP transformer model for TRV calculation after fault current interrupting by using FRA measurement

The authors investigated to calculate the transient recovery voltage (TRV) after the transformer limited fault current (TLF) interrupting condition. Several transformers were used to measure TRV by means of a current injection measurement method. To simulate the TRV at TLF interrupting condition alternative transients program-electromagnetic transients program (ATP-EMTP) was used. The transformer equivalent elements, such as leakage inductance, stray capacitance and resistances were derived from frequency versus impedance graph obtained by frequency response analysis (FRA) measurements. Experiment procedure, results, consideration of transformer constants and constructed EMTP transformer models of current interrupting condition for TRV calculation of tested transformer are presented in this paper. The TRV obtained by the EMTP simulation gave agreeable results with the measured values of the experiment in each case, while a limitation was found by using a constant leakage inductance value which depends on a frequency.

Evaluation of stress distribution due to shearing in non-oriented electrical steel by using synchrotron radiation

The influence of the shearing process on the iron loss of non-oriented electrical steels with grain sizes of 10 μm-150 μm was investigated. The deterioration ratio of iron loss was clearly smaller in sample with small grain sizes. The droop height, reflecting the amount of plastic deformation, displayed a good relationship with the deterioration of iron loss under the effect of the material grain size. To clarify the strain distribution around the sheared edge, the elastic strain in a sheet sample with the thickness of 0.30 mm and grain size of 10 μm was evaluated by using synchrotron radiation. The width of the region of elastic strain due to shearing was two or three times of the material thickness. The results of the plastic strain distribution obtained by the measurements were then used to estimate the iron loss deterioration rate in 5 mm width sheared samples. The estimated loss deteriotation coincided with the actual measured iron loss.

Coating of carbon nano-tube filled epoxy resin and its surface conductivity

The aim of this study is to fabricate nanotubes (CNT)/epoxy nano-composites and coat them onto epoxy insulating substrates in order to reduce the surface resistivity. Dispersion agent was uniquely and newly prepared for uniform dispersion of CNT in an epoxy resin as polymer powder. An ultrasonic homogenizer was used for mixing an epoxy resin with commercial CNT and the prepared dispersion agent. The prepared suspension was coated on an epoxy insulating substrate using a spin coater under certain conditions. Cracked surface was observed with the field emission scanning electron microscope (FE-SEM) to check the dispersion of CNT. Surface resistivity was measured by an electrometer with a special test fixture at measurement. As a result, it was elucidated that grafting polymer molecules onto the CNT surface improves the dispersion of CNT in epoxy matrix compared to those containing CNT without functionalization. It was also shown that the surface resistivity of the nano-composites decreases because of more uniform dispersion of CNT. It was found that an addition of just 0.1 wt% CNT with the dispersion agent allows us to decrease the surface resistivity in two-digit.

Flow Visualization of Flow in Nozzle Used in Gas Circuit Breaker.

It is important to understand the aerodynamic characteristics of a circuit breaker nozzle with a complicated inner shape. Therefore, the experimental apparatus was fabricated with air instead of SF6 as the working fluid. Experiments were conducted using an acrylic nozzle model and a glass nozzle. The flow visualization of flow in the acrylic nozzle model was conducted using the laser light sheet technique, and the flow visualization of flow in the glass nozzle model was conducted using the laser shadowgraph method. Moreover, the flow velocity in the glass nozzle model was measured using a laser velocimeter. Judging from the experimental results, the weak shock is formed in front of the electrode in the circuit breaker nozzle.