No-load loss refers to the power absorbed by the transformer when the secondary side of the transformer is open-circuited and the sine wave voltage on the primary side is equal to the rated voltage. Generally, only the rated frequency and rated voltage are paid attention to, and sometimes no attention is paid to the tap voltage and voltage waveform, the accuracy of the measurement system, test instruments and test equipment. There is confusion again about the calculated value, standard value, measured value and guaranteed value of loss.
If the voltage is applied to the primary side and there is a tap, if the transformer is constant flux voltage regulation, the applied voltage should be the tap voltage at the tap position corresponding to the power supply. In the case of variable flux voltage regulation, since the no-load loss at each tap position is different, the correct tap position must be selected according to the technical requirements and the specified rated voltage must be applied, because during variable flux voltage regulation, The primary side always applies a voltage to each tap position.
It is generally required that the waveform of the applied voltage must be approximately a sinusoidal waveform. Therefore, one is to use a harmonic analyzer to measure the harmonic components contained in the voltage waveform, and the other is to use a simple method, using an average voltmeter, but with a voltmeter scaled to the effective value, to measure the voltage, and compare it with the effective value voltmeter reading. , when the difference between the two is greater than 3%, it means that the voltage waveform is not a sine wave, and the measured no-load loss should be invalid according to the requirements of the new standard.
For the measurement system, it is necessary to select appropriate test lines, test equipment and instruments. Due to the development of magnetically conductive materials, the wattage lost per kilogram has been greatly reduced. Manufacturers have chosen high-quality, highly conductive grain-oriented silicon steel sheets or even amorphous alloys as magnetically conductive materials. In terms of structure, they have developed new technologies such as ladder connections. The seams and full bevels are non-porous, and the technology of non-stacked iron yoke is adopted. Manufacturers are developing low-loss transformers, especially the no-load loss has been greatly reduced. Therefore, new requirements are put forward for the measurement system. The capacity remains unchanged and the no-load loss decreases, which means the power factor of the transformer decreases when it is no-load. A small power factor requires the manufacturer to change and transform the measurement system. It is advisable to use the three-wattmeter method for measurement, choose a 0.05-0.1 level transformer, and choose a wattmeter with a low power factor. Only in this way can the measurement accuracy be guaranteed. When the power factor is 0.01, the phase difference of the transformer is 1 minute, which will cause a power error of 2.9%.
Therefore, in actual measurement, the current ratio and voltage ratio of the current transformer and voltage transformer must be correctly selected. When the actual current is much smaller than the current connected to the current transformer, the phase difference and current error of the current transformer will be larger, which will lead to larger errors in the actual measurement results. Therefore, the current drawn by the transformer should be close to the rated current transformer. current.
In addition, the no-load loss calculated by referring to the unit loss and process coefficient of the selected silicon steel sheet according to the prescribed procedures in the design is generally called the calculated value. This value should be compared with the standard value specified in the standard or with the standard value or guaranteed value specified in the contract. The calculated value must be smaller than the standard value or guaranteed value, and there must be no margin in calculation, especially for mass-produced transformers. In addition, the calculated value is only valid for designers or design departments and has no legal effect. The calculated value cannot be used to judge the loss level of the product. The standard value specified in the standard or the guaranteed value specified in the contract has legal effect. Products that exceed the standard value plus the allowable deviation, or the guaranteed value (the guaranteed value is equal to the standard value plus the allowable deviation), are considered unqualified products. If there is a loss evaluation system, it will generally be pointed out in the contract, especially for export products, if the loss value exceeds the specified value, there will be a fine, and the fine for no-load loss is the highest. The loss evaluation values of European countries can be found in the 11th issue of "Transformer" magazine in 1994. There are fines of several thousand dollars per kilowatt. This is the legal effect and is directly linked to economic benefits.
The concept of actual measured value must also be understood correctly. Either the reading of the mutual meter (or the reading of the power converter) or the actual measured value must be converted to rated conditions, and must have sufficient accuracy. For the actual measured value of no-load loss, it is mainly that the voltage waveform of the power supply must be sinusoidal, and the difference between the average voltmeter reading and the effective voltage reading is less than 3%.
