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<\/p>\nPower transformer no-load current is the steady current drawn when a winding is energised at its specified voltage and frequency while the other windings are open-circuited. It is not zero: the transformer still needs magnetic excitation and draws real power to cover its no-load losses. This value is different from load current and the transient inrush current during energisation.<\/p>\n
At Zisheng Electric, a useful technical enquiry identifies the measurement conditions before asking whether a current value is acceptable. This guide helps industrial buyers and maintenance engineers read the parameter without confusing it with energy loss or an automatic fault verdict.<\/p>\n
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For the rated-condition parameter, the reference is the winding’s rated voltage, or the applicable tapping voltage, at rated frequency. RMS means root mean square, the current magnitude convention used here. Schneider Electric’s definition of no-load current<\/a> also identifies the three-phase value as the arithmetic mean of the three phase-current values and notes that a percentage needs a defined winding-current reference.<\/p>\n The other windings being open-circuited means that they are not supplying an external load. It does not mean their terminals are voltage-free. This article concerns interpreting specifications and results; test connections and energisation require an approved procedure and qualified personnel.<\/p>\n For the wider operating principle, see our power transformer guide<\/a>. The important distinction here is the operating condition under which the current is stated.<\/p>\nCurrent, Loss and Inrush Are Separate Quantities<\/h2>\n
| Param\u00e8tre<\/th>\n | Usual expression<\/th>\n | What a buyer should establish<\/th>\n<\/tr>\n<\/thead>\n |
|---|---|---|
| Courant \u00e0 vide<\/td>\n | Amperes or a percentage on a stated base<\/td>\n | Winding, voltage, frequency, tap and measurement method<\/td>\n<\/tr>\n |
| Les pertes \u00e0 vide<\/td>\n | Watts or kilowatts<\/td>\n | Real input power under the specified no-load test conditions<\/td>\n<\/tr>\n |
| Inrush current<\/td>\n | A transient waveform, magnitude and duration<\/td>\n | The energisation scenario and its relevance to protection<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Do not read a 0.60% current entry as a 0.60% energy loss. Current magnitude includes the magnetising demand; it is not a direct measurement of real power consumption. Likewise, an acceptable steady no-load current does not establish the peak or duration of an energisation transient.<\/p>\n When evaluating standby operating cost, obtain the guaranteed no-load loss and its stated basis. When reviewing an unexpected protection operation, request the event records. Replacing either investigation with a no-load-current percentage answers the wrong question.<\/p>\n How to Read a Percentage Without Changing Its Meaning<\/h2>\nA power transformer no-load current percentage is useful only if its base is clear. For a stated three-phase winding rating, the rated line current is:<\/p>\n Consider an illustrative 6.3 MVA, 33 kV winding with one stated power-rating base. Its rated line current is approximately 110.2 A. If the defined mean RMS no-load line current on that same winding is 0.66 A, the arithmetic is:<\/p>\n These are calculation inputs, not factory measurements, a Zisheng delivery record or an acceptance limit. They show why the winding and power-rating base must accompany the number. Multi-winding or multiple-cooling-rating equipment needs the convention specified by the applicable test code and contract.<\/p>\n A diagnostic excitation-current test may use a different voltage and connection arrangement from the rated-condition factory test. A number in milliamperes from that test cannot be compared directly with a factory percentage without establishing what each measurement represents.<\/p>\n |
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