Multiple Input Voltages
Transformers can be designed for multiple input voltages by means of taps and / or multiple input windings.
However, it should be noted that the additional winding space typically leads to larger core sizes compared to a transformer of similar power designed for a single input voltage. An exception is when the input winding is divided into two equal parts, allowing the user to optionally connect them in series or parallel, requiring only a little additional winding space for insulation.
Note:
A sometimes-observed mistake is that users, in transformers with two input windings intended for optional series or parallel connection, connect only one input winding in applications with a lower network voltage. This leads to overheating and destruction of the primary winding at rated load and should be strictly avoided. This also affects the fusing. (This error may not be immediately noticed at partial load or no-load operation!)
No-Load Operation
The no-load power and no-load current of a transformer refer to its behavior at rated input voltage and frequency when no load is connected. The no-load power is the absorbed active power, while the no-load current is the apparent input current in this state. Due to core sheet tolerances, these values can vary significantly.
The losses of a transformer mainly consist of iron losses and copper losses. To a first approximation, no-load current and no-load power are a measure of iron losses. The copper losses in turn can be estimated by a short circuit test.
If a transformer is expected to be operated mainly at no-load or under small partial load, it is possible and sensible to optimise it with regard to no-load losses.
Note:
The standards for core materials allow for considerable tolerances. The quality of the core materials and thus the idle currents can vary accordingly. Transformer manufacturers and suppliers unfortunately have limited influence on this and limited alternatives on the global market.
No-Load Output Voltage
The no-load output voltage of an unloaded transformer connected to the rated input voltage and rated frequency is always higher than the rated output voltage (voltage under load). In very small transformers, it can be up to twice the rated voltage. The permissible maximum values are specified in EN 61558 in various "Parts 2-.." and are given there as a percentage of the ratio between no-load voltage and output voltage under load.
Important: Always specify a maximum no-load output voltage when using components whose voltage tolerance could be affected. In such cases, it may be advisable to choose a slightly larger transformer type.
Note: The no-load voltage is especially important when the protective low voltage (max. 50V at no-load) or other voltage limits (e.g., 1100V limit) could be reached.
Hint: The rated values, to which the output voltage tolerance and the no-load deviation refer, apply to the ambient temperature for which the transformer is specified.
Short-Circuit Voltage
The short-circuit voltage refers to the necessary voltage that must be applied to a transformer's input winding to achieve the rated input current when the output winding is short-circuited, and the windings are at ambient temperature. It is usually expressed as a percentage of the rated input voltage and represented by the symbol uk.
A low short-circuit voltage indicates that the transformer is designed to have low copper losses at the rated current. However, in some applications, such as with certain converters in drive technology, a higher short-circuit voltage is necessary. In these cases, it might be beneficial to connect a line choke upstream of the transformer.
Short-Circuit Strength
Short-circuit proof according to the definition of DIN EN IEC 61558-1:2019-12 is a transformer in which the temperature, even in the event of an overload or short-circuit, does not exceed the values permitted for this transformer and which continues to meet the requirements of the above standard after the overload or short-circuit has been removed*.
A distinction is made between
- not short-circuit proof transformer,
- absolutely short-circuit proof transformer and
- conditionally short-circuit proof transformer.
A non-short-circuit proof transformer is intended to be protected against overload, short-circuit and otherwise unacceptably high temperatures by a suitable protective device fitted by the user which is not part of the transformer.
With an absolutely short-circuit-proof transformer, compliance with specified limit values for currents and temperatures is ensured by the design. For example, very small transformers up to approx. 2...3VA have such high-impedance windings that these transformers are absolutely short-circuit-proof.
A conditionally short-circuit proof transformer is equipped with a protective device. This interrupts or reduces the current in the input or output circuit in the event of an overload or short circuit. If the protective device is resettable, the original function of the transformer is restored after removing the overload, cooling down the transformer, and resetting the protective device (see resettable temperature limiters).
Addition: Small transformers - especially small print transformers - in a conditionally short-circuit-proof design are often equipped with temperature fuses ("thermal fuse") permanently installed in the transformer. After an overload or short circuit, the transformer must be replaced.
If a BREMER transformer is not explicitly defined as short-circuit proof, it is a non-short-circuit proof transformer. In this case, it is intended that the transformer is protected against overload, short circuit and other inadmissible temperatures by suitable protective devices fitted by the user.
Remarks:
* This does not mean that all types of short-circuit proof transformers are still functional. Among other things, transformers equipped with a non-resettable and non-replaceable protective device must be replaced after it has tripped (thermal fuse).
Insulation Class
Insulating materials in transformers are classified into thermal classes (insulation classes) according to IEC 60085 and IEC 60216. This classification considers the maximum allowable temperatures of the insulating materials, with temperatures being reduced by the so-called hot spot value according to EN61558-1:2019-12.
The following insulation classes result:
Thermal Class A: 100°C
Thermal Class E: 115°C
Thermal Class B: 120°C
Thermal Class F: 140°C
Thermal Class H: 165°C
Transformers should not exceed the temperature values of these classes during intended use, considering also the ambient temperature.
Inductance
Inductance is a measure of the storage capacity of magnetic energy. The inductance is the essential parameter of a choke coil.
Unit: Henry 1H = 1Vs/A
Note 1: Do not confuse with induction.
Note 2: In technical jargon, "AN INDUCTANCE" sometimes refers to an inductive component, i.e., a choke or choke coil.
Induction
Magnetic induction; formula symbol: B
Also called magnetic flux density or flux density.
Induction is one of the elementary internal parameters in the dimensioning of magnetic (inductive) components. Unit: Tesla 1T = 1Vs/m²
Note: Do not confuse with inductance!
Hertz (Hz)
SI unit for the frequency
Unit: 1Hz = 1(1/s) (repetitions / second)
Formula symbol: Hz
Heinrich Rudolf Hertz: German physicist (1857 - 1894)