No-load test of transformer is a test of applying rated voltage of sine wave rated frequency from any winding of transformer, opening other windings and measuring no-load loss and no-load current of transformer. No-load current is expressed as the percentage of measured no-load current I0 to rated current Ie, and it is recorded as IO.
When there is a significant difference between the value measured by the test and the design, ex-factory value, the same type of transformer or the calculated value before overhaul, the reason should be found out.
No-load loss is mainly iron loss, that is, hysteresis loss and eddy current loss consumed in iron core. When the excitation current flows through the primary winding at no load, resistance loss will also occur. If the excitation current is small, it can be ignored. No-load loss and no-load current depend on transformer capacity, iron core structure, silicon steel sheet manufacturing and iron core manufacturing process.
Extended data:
The loss of transformer is an important performance parameter of transformer. On the one hand, it indicates the efficiency of transformer in operation, on the other hand, it indicates whether the performance of transformer in design and manufacture meets the requirements. No-load loss and no-load current measurement, load loss and short-circuit impedance measurement of transformers are routine tests of transformers.
No-load test of transformer is to measure the no-load loss and no-load current of transformer when rated voltage is applied to any group of windings and other windings are open. No-load current is expressed as a percentage of rated current.
1, no-load test is to measure no-load loss and no-load current at rated voltage. During the test, the high voltage side is open and the low voltage side is pressurized. The test voltage is the rated voltage of the low voltage side, and the test voltage is low. The test current is several percent or several thousandths of the rated current.
2. Selection of power supply capacity for transformer no-load test: ensure that the waveform distortion of power supply does not exceed 5%, and the sample no-load capacity should be below 50% of the power supply capacity; No-load capacity should be less than 50% of the voltage regulator capacity; When the generator set is used for test, the no-load capacity shall be less than 25% of the generator capacity.
The test voltage of no-load test is the rated voltage of low voltage side, and the no-load test of transformer mainly measures the no-load loss. The no-load loss is mainly iron loss. The size of the iron loss can be considered to be independent of the size of the load, that is, the loss at no load is equal to the iron loss at load, but this refers to the situation at rated voltage.
If the voltage deviates from the rated value, the no-load loss and no-load current will change sharply because the magnetic induction intensity in the transformer core is in the saturation section of the magnetization curve. Therefore, no-load test should be carried out at rated voltage.
Note: When measuring no-load or load loss of large transformer, cosφ can be less than or equal to 0. 1 due to low power factor. Therefore, it is necessary to use a wattmeter with low power factor.
3. Through the no-load test, it can be found that the transformer has the following defects: poor insulation between silicon steel sheets. The local short circuit between the magnetic pole and the core piece is burned, and the core bolt is pierced or the insulation parts such as steel belt, pressure plate and upper yoke are bound. Be damaged to form a short circuit.
In the magnetic circuit, the silicon steel sheets are loose and dislocated, the air gap is too large, the iron core is grounded at multiple points, the coils are short-circuited between turns and layers, the number of turns in parallel branches is unequal, and the ampere-turns are unbalanced. Misuse of high-consumption and inferior silicon steel sheets or design calculation errors.
References:
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