1. Conditions for Obtaining Triple-Frequency Voltage Output
(l) It must be composed of a three-phase five-limb transformer or three single-phase transformers of the same specification, providing a closed uniform magnetic circuit for zero-sequence flux in the iron core. The primary winding of the transformer is connected in star, and the secondary winding is connected in an open delta, with the open ends outputting triple-frequency voltage.
(2) The transformer must be over-excited to deeply saturate the iron core.
(3) When selecting transformer parameters, not only should the requirement for output power be considered, but also whether it meets the limitations of temperature rise and heating, and an appropriate capacity should be chosen.
2. Characteristics
The output capability of the triple-frequency transformer is greatly related to the magnetic flux density in the iron core. Therefore, the generation of the third harmonic is due to the nonlinearity of the core magnetization curve. Only when the flux density works above the knee of the curve can a larger triple-frequency component be generated. When the flux density exceeds the knee point of the magnetization curve, the permeability μ of the core drops sharply, and the internal impedance Z of the triple-frequency generator decreases sharply, resulting in a larger output of third harmonic voltage and current. The deeper the over-excitation, the larger the third harmonic component output, but after reaching a certain extent, the change in permeability μ is not significant, and the decrease in internal impedance Z3 becomes gentle, so the growth of third harmonic output also becomes gentle. Generally speaking, there is no lower limit for over-excitation, as long as the voltage and power requirements are met. The upper limit of over-excitation should consider two points: first, avoid deep over-excitation causing 9th and 15th higher harmonics; second, avoid excessive voltage on the inter-turn insulation of the triple-frequency transformer causing faults, generally not exceeding 2 times the rated voltage. At 1.73 times over-excitation, the third harmonic voltage of the transformer is about 1.1 times the rated voltage, and there is no obvious higher harmonic component.
The voltage and power output of the triple-frequency generating device are greatly related to the nature of the load impedance and whether it matches the load impedance. If the load impedance and the internal impedance of the triple-frequency generating device are equal in magnitude and impedance angle, but opposite in sign, the maximum output can be obtained.
