Primary circuit protection (if necessary) sluggish design – rule of thumb 1.5… 2 x rated current, always fuse secondary current to rated current (starting current of consumers, e.g. motor is to take into account)

0 – 50A on standard transformer terminals, 50 – 340A terminal blocks on head angles, from 340A cable lugs or copper bars. Deviations from this are also possible depending on the design.

Connected load
Specification always as apparent power in VA or KVA at cos φ = 1, otherwise active power in W or KW plus cos φ of the connected machine, calculation 1ph-transformer: P = U x I : cos ϕ,
3ph-Trafo P = U x I x √3 : cos ϕ

Transformers can be designed with tappings on both the input and output side. For example, taps on the input side are used to adapt and use the transformer at different mains voltages.

Conditionally short-circuit-proof transformer
This is a transformer that contains a protective device such as a fuse, overcurrent release or temperature limiter that opens the primary or secondary circuit if the transformer is overloaded or short-circuited.

cos φ
Is determined by consumers, i.e. as motor, contactor. For cos φ = 0.5 the apparent power = 2 x active power), Papp x cos φ = Pactive

Switch-on rush
Designates the inrush current, which arises phase-dependent when switched on. Transformers usually have a switch-on current between 8 and 20 times the rated current. Toroidal cores, on the other hand, have up to 80 times the rated current. As a result, high back-up fuse values are required. Countermeasures can be: inrush current limiter, construction measures for the transformer calculation.

Iron losses
Are remagnetization losses and occur even when the transformer is not loaded during operation. They are depend on induction, mains fluctuations (e.g. mains voltage +/- 10%) and the frequency (e.g. 50 Hz, 60Hz).

Fail-safe transformer
This is a transformer which fails permanently due to improper use, but which does not pose any danger to the user or the environment.

Determines the induction and iron losses, each 50Hz transformer can be operated at 60Hz. But not the other way round!

Separate winding
In transformers with separate windings there is no conductive connection between individual windings, they are galvanically isolated. The type output corresponds to the rated output.

High voltage
Voltages over 1000 volts

Low voltage
Voltages below 50 volts

Copper weight
Can give information about the winding losses and the associated efficiency of the same size transformers.

Short-circuit proof transformer
This is a transformer in where the temperature does not exceed specified limits if the transformer has been overloaded or short-circuited and remains operable after removal of the overload or short circuit.

Short-circuit voltage (uk)
This is the voltage that must be applied to the input winding so that the rated output current flows when the output winding is shorted. It is given in % of the rated input voltage.

Open circuit output voltage (U0)
Is the voltage of an unloaded transformer at rated input voltage and rated frequency.

Open-circuit current (I0)
Is the absorbed current of an unloaded transformer at rated input voltage and rated frequency.

Non-short-circuit proof transformer
This is a transformer without protection against excessive temperature rise. The protective device must be implemented by the user.

Low voltage
Voltages from 51 to 1000 volts

Autotransformer winding
In an economy winding there is a conductive connection between primary and secondary winding. In addition, in autotransformer windings material savings appear.

Vacuum impregnation
Protection against moisture and aggressive atmosphere. The core sheets glued additionally with each other and the windings glued with each other and with their insulation. As a result, a strong noise insulation and better heat coupling of the winding can be achieved.

Power loss of the transformer
The power loss of a transformer is composed of iron losses (caused by induction and mains frequency) and copper losses (caused by the current through the winding and its temperature). Iron losses are no-load losses and are therfore always present. They can be optimized by the structure and type of core sheets. Copper losses are load-dependent, they are always specified at nominal load or nominal current and can be influenced by the quality of the winding and the amount of copper weight.