TECHNICAL INFORMATION ON TRANSFORMERS AND AUTO-TRANSFORMERS
A transformer is a static electrical machine (it contains no moving parts) belonging to the broader category of converters.
In particular, a transformer makes it possible to convert the voltage (symbol V, the unit of measure for volts) and current (symbols: I, the unit of measure, Amperes [A]) parameters at input with respect to those at output, while maintaining constant the amount of electrical power (unless losses occur due to hysteresis and eddy currents).
A transformer is a machine capable of operating only on alternating current because it exploits the principles of electromagnetism relating to variable fluxes.
A transformer is not able to change the frequency value, therefore the input frequency is always equal to output frequency.
Transformers have a fundamental importance in today’s world. Without them large electricity transmission networks connecting power stations to millions of homes and industrial facilities and systems would not be able to operate.
Until 1965 the general scientific opinion was that it would be impossible to produce DC transformers. In that year a group of researchers at the General Electric company led by Nobel prize winner (1973) Ivar Giaever succeeded in producing such a device as a result of studies on superconductivity at a very low temperature.
In any case the practical and large-scale application of this discovery has not yet been achieved, and in the meantime scientific studies are continuing in the field of superconductivity at high temperature..
The technical references provided here are used to facilitate a correct choice for the use of transformers, auto-transformers, and inductive reactors in the various fields of use.
Here we will only indicate the main definitions contained in the standards CEI 96-1 — CEI 96-2 — CEI 96-3 (EN 61558-1).
Other standards to be referred to are: CEI 14-5 — CEI 14-8 — VDE 0551 — UL 506
For clarification concerning the new provisions of CEI Committee click on regulatory references
- ISOLATION TRANSFORMER – It is advisable to use this kind of transformer when it is necessary to limit the risk of contact between earth and the live parts which can become live in the event of poor insulation.
Max primary and secondary voltage = 1000 V unloaded — with max frequency 500Hz. – Max power = 25KVA if single-phase and 40 KVA if poly-phase — double or reinforced isolation.
- SAFETY TRANSFORMER – his is an isolating transformer, the unloaded secondary voltage of which must not exceed 50 Veff, referred to as ‘safety voltage’. Power levels must not exceed 10 KVA if single-phase and 16 KVA if poly-phase.
- PROTECTION CLASS – n terms of construction this is the characteristic of a device with respect to dangerous currents.
all accessible metal parts of the transformer are isolated from live parts by the basic isolation, and there may be points with a double or reinforced isolation. Furthermore, as an extra safety measure, by means of a ground terminal the accessible conductive parts are connected to a protective earthing conductor forming part of the fixed wiring system.
All accessible metallic parts of the transformer are separated from live parts by a double or reinforced isolation.
In this case the appliance does not have to be grounded..
- ISOLATION CLASS – Corresponding to the isolation classes of insulating materials. A = 105°C — E = 120°C — B = 130°C – F = 155°C — H = 180°C.
The ambient temperature around the transformer, unless otherwise specified, shall be taken as corresponding to the value of 40°C max.
- OVER-TEMPERATURE – he temperature reached during operation, subtracted from the ambient temperature considered as 40°C. The maximum temperatures for the various isolation classes are thus the following: A = 60°C — E = 75°C — 80°C = B — F = 100°C — H = 125°C.
- NOMINAL INPUT VOLTAGE – This is the transformer power supply voltage, which, unless otherwise agreed upon, must not exceed 6% of the rated data-plate value without this involving any damage to the continuous operation of the device.
Care must be taken if, instead of a single input voltage, the requirement is for additional voltages or shunting/branch circuitry. In such cases it becomes necessary to increase the overall sizing of the transformer, which, in relation to the difference between maximum and minimum voltages, may vary from 5% to 30%. The output voltage shunt to other voltages, unless otherwise specified, will have a current value expressed by dividing power by the highest voltage value..
- NO-LOAD CURRENT – This is the current to the primary circuit of the transformer with no load on the secondary at nominal voltage and frequency. It essentially depends on the properties of the magnetic circuit and in practical terms may oscillate, also for transformers of the same series, by +/- 10-15% with respect to the preset nominal value.
- IN-RUSH CURRENT – At the time the transformer is powered up, depending on the degree of residual magnetization of the core and the point where the voltage value is on the sine wave, there is an instantaneous peak current (for several hundredths of a second), which may vary from zero to a value of even 30 and many times the rated current. (A reduction of this phenomenon may occur by increasing the reactance dispersion of the transformer and reducing the magnetic induction value in the transformer core). It is thus advisable to protect the line with the assembly of time-delay fuses and magneto-thermal switches that will avoid any intervention of ENEL (National Entity for Electricity) safety systems.
The equivalent power (referred to as core power in current regulations) is expressed by the formula:
Peq = Pn . Vmax-Vmin
Peq = actual sizing power (core power)
Pn = rated nominal power (shaft power).
Vmax = highest voltage.
Vmin = lowest voltage.
As can be seen, the auto-transformer is particularly convenient when the highest voltage does not exceed more than two or three times the lowest voltage and, obviously, there are no requirements for electrical separation between the input voltage and the output voltage.
- TRI-SINGLE-PHASE TRANSFORMER -A transformer used to mitigate load imbalance generated in a three-phase network by large single-phase use. This type of transformer converts the consumption ratio between the phases from 1 – 0 – 1 to 1 – 2 – 1 and at the same time electrically isolates the network from the single-phase load.
- TRI-HEXAPHASE TRANSFORMER – This is a three-phase transformer with hexa-phase outlets which is found to be particularly convenient when connected to rectifier diodes.