Transformer characteristics

No load voltage

A transformer always has load losses (copper losses), so the secondary voltage changes with loading. The defined secondary voltage is always reported with a defined load. To calculate changes in secondary voltage, the datasheet reports ΔU. The no-load voltage can be calculated by multiplying the defined voltage by ΔU. (Example: secondary voltage given as 110 V and ΔU=1.05. The no load voltage is thus: 1.05 X 110 V = 115.5 V. Secondary voltage therefore varies from 110 V at normal load to 115.5 V with no load).

Efficiency

Transformer efficiency varies strongly with size—generally, the larger the transformer, the higher the efficiency. The actual efficiency of a transformer is always somewhat better than its apparent efficiency, which can be obtained as the inverse of ΔS.

Deviations from type power

A transformer’s actual power can differ from the data sheet type power for several reasons. A common one is that there are several primary voltages. In this situation the primary winding needs to be designed for full power. This requires more winding space than normal, so generally we need to use more core power. 

Another common reason is the desire to minimise voltage loss, so that windings need to be oversized in order to limit copper losses. Windings also need to be oversized if the ambient temperature is high. Reported type power ratings are for 50 – 60 Hz. For lower frequencies, such as 16 2/3 Hz, the transformer has to be larger, and for higher frequencies, such as 400 Hz, smaller than the standard transformer.

The reported type power is for continual use. If a transformer is used cyclically, it can be made smaller. Thus, it is important to report cyclical use in any order. The usage per cent states what percentage of the usage time the transformer is loaded. Type power can be calculated like this:

Actual power times the square root of the cycle = type power. For example, actual power 100 VA is used for a period of 1 second every fourth second, thus the loading factor is 0.25, its square root is 0.5, thus the type power is 50 VA. In cyclical use, it is to be noted that the loading period needs to be significantly shorter than the transformer’s heating time. Heating time constants are generally 5-50 minutes, depending on the size of the transformer. For large transformers, the time constant is, however, several hours.

3-PHASE VECTOR GROUPS

Full transformers have separate primary and secondary windings, but auto-transformers have only one winding which has taps for different voltages. Auto-transformers deliver more power than the type power reported on the data sheet. Type power can be calculated: the lower voltage is subtracted from the larger and the remainder is divided by the higher voltage. This result is multiplied by the desired power. It can generally be said that the smaller the voltage difference, the greater the efficiency of the auto- transformer when compared with a regular transformer. It is, however, to be noted that safety must be considered in using auto-transformers since primary and secondary windings are not isolated from each other. Auto- transformers are generally used as control and starter transformers.

In 3-phase transformers, windings can be connected to star (Y), delta (D) or tapped Y (Z). The table below illustrates the most common connections.

Encapsulation classes

Encapsulation serves two purposes. It protects the environment from sparks, corona discharge, heat, voltage, and moving parts that could endanger life, health, or property. It also protects the device from environmental factors like moisture, dust, corrosive agents, and other harmful agents that could damage delicate parts. According to the international IEC standard, the encapsulation class is described by the letters IP, followed by two identification numbers. The first number indicates the encasement’s moisture- and foreign-material shielding, and the second its water resistance. If desired, the encapsulation class can be stated with one number or the other with a large X in place of the missing number, such as IP 2X or IP X5. The following table shows the IP classes.

Standard type structures

The E-series transformers are usually wound with a double-section winding and a single-section bobbin. In applications where better coupling between primary and secondary is needed, transformers could also be wound to a single-section bobbin. 

The core is made of 0.5mm cold-rolled generator plate, type EI. Coil temperature rise is generally max 80°C, and the temperature class is B (130°C). If needed, we can also make transformers to meet higher temperature classes. To improve reliability, some of the insulation materials fulfil class F or H requirements. Low-power models have a temperature rise of about 40°C. Products are coated by dipping in resin, which is then cured in an oven. Transformer and choke dimensions are in accordance with DIN norms, and they are produced and tested in accordance with IEC 61558-1. 

Products can be manufactured and tested according to other norms (i.e CSA). Encapsulated models are intended for fixed installation.  

Items to be mentioned in ordering

1. type
2. power
3. primary and secondary voltage; also secondary current as well, if there are to be taps or a separate winding connection
4. connection group, 3-phase transformers
5. possible fuse or overcurrent protection
6. other information such as frequency, cyclic use, auto-transformer, etc.