Rated Power
Rated power is the product of the rated output voltage and the rated output current.
In three-phase systems, it is calculated as the product of √3 times the rated output voltage times the rated output current, where the voltage is the line voltage.
If the transformer has more than one simultaneously loaded output winding, the rated power is the sum of the individual products.
If only one output winding is loaded, the product with the highest value is taken as the rated power. The same applies to taps. In three-phase systems, the calculation is done analogously.
Compare: Rated power equivalent
Compare: Throughput power
Output Current
The rated output current refers to the current in the output winding at the rated input voltage and frequency for which the transformer was designed and manufactured. When loaded with the rated load impedance, this current is set.
Comment 1:
For the thermal dimensioning of transformers, the effective value of the output alternating current must always be considered. We understand all electrical quantities provided to us to be sinusoidal effective values, unless they are expressly described otherwise.
Comment 2:
Unless otherwise ordered, the rated output current is always assumed to be continuous operation.
Comment 3:
Electronics engineers often give us the DC values behind an input rectifier. We will be happy to calculate the corresponding AC values for you, but then we also need the details of the rectifier circuit and smoothing. The mixed indication of AC voltage and DC current is not useful.
Output Voltage Tolerance
Under rated load, according to EN61558-1, the output voltage may deviate a maximum of 5% from its rated value, 10% on transformers that are inherently short-circuit proof, usually <y 2VA, plus an additional 5% on transformers with rectifiers. For more details - for example, on transformers with multiple output windings - refer to EN611558-1 (2019-12).
Note:
The rated values to which the output voltage tolerance and the no-load deviation refer apply to the ambient temperature ta for which the transformer is rated.
Output Voltage
The rated output voltage is the voltage for which the transformer is designed and manufactured. his is established when the transformer is connected to the rated input voltage at the rated frequency and loaded with the rated output current and power factor.
In the case of multi-phase systems, the so-called "chained" voltage between the outer conductors is referred to.
Note:
The no-load output voltage of an unloaded or partially loaded transformer is always higher than the rated output voltage.
See also: Output voltage tolerances
Tappings
Tappings refer to the windings of transformers where subsets of the total number of windings can be "tapped".
For input windings, this allows for multiple input voltages and input voltage ranges.
For output windings, unless otherwise agreed and labeled, all tappings are maximally loadable with the load current of the highest voltage level. Tappings may only be loaded alternatively unless otherwise agreed.
Ampere (A)
SI base unit of electric current.
Unit: Ampère, defined as 1A = 1W/1V = 1C/1s.
The symbol is I.
Named after the French physicist and mathematician André-Marie Ampère (1775 - 1836).
It is also the unit of magnetic flux (magnetic voltage).
1-Phase / 3-Phase
In principle, the study of n-phase systems is an interesting academic field. In our day-to-day practice - the dimensioning and production of transformers - we only deal with 1- or 3-phase transformers or voltage systems. 3-phase is also referred to as three-phase current.
Remarks:
A so-called core transformer, also called KT or UI transformer, i.e. a transformer with two coils, is also a 1-phase transformer. The primary windings of this transformer are connected either in series or in parallel. The term 2-phase transformer is not in use.
Similarly, a transformer connected between two outer conductors of a 3-phase supply network is a 1-phase transformer. (In the usual public low-voltage grid, this is then, for example, a 1-phase transformer, primary voltage 400V).
It is often desired to distribute a 1-phase load evenly over the 3 phases of a network. This is not possible with a single transformer.
Transformer
Static electromagnetic machine with two* or more windings. Transformers are generally used to transmit electrical energy by converting one alternating voltage into another alternating voltage of the same frequency.
*Exception autotransformer