The small transformers used in commercial electrical devices as TV, computers, audio systems can be built from two different core shapes: EI or square shape and toroidal shape. In the past, the EI core shape was more popular. Nowadays, because of their important advantages, the toroidal transformers become very large majority in applications where the lower power is required.
Before assembling, EI core has a shape as letter E and I. The winding is wound around the central leg. Two outside legs make an additional path for magnetic flux. Because it is not possible to assemble it perfectly, the air gap remains between E and I part of the core after assembling. This air gap increases the transformer reluctance and thus the core losses. The magnetic flux flow gives off the electromagnetic inference in an environment which interferes with the other components causing audio distortion. The price is the advantage of this transformer type.
The toroidal transformer core has round or donut shape. The wires are wound around this shape. The core is built by some of the ferromagnetic materials such as laminated iron, iron powder or ferrite. Small transformers with higher operation frequency from tens of kHz to hundreds of MHz usually uses a ferrite core. The toroidal core cross-section can be square, rectangular or circular. The circular cross-section is more expensive.
Just a small amount of magnetic flux can escape because of the symmetrical toroidal core shape. This transformer type has core losses between 10-20 % of the total transformer losses. The other transformer types have core losses 50% of the total transformer losses. Because of this small core loss, the toroidal transformers are smaller and have less weight 20-50% compared with other conventional transformers.
The toroidal construction provides the transformer operation without acoustic noise, while the vibration of iron laminations can produce acoustic noise. But the price of the toroidal transformer is higher than lamination core transformers. The toroidal transformer construction makes them much more expensive to manufacture. Wire winding requires more complex and slower equipment.
The low losses are very important for the instrument transformers and because that this transformer type is usually used for this purpose. Those transformers are a good choice for the audio devices because of their low noise level and EMI. Because of high efficiency, they have an important benefit in amplifier application. They could deliver much more power on the output. The halogen lamp uses the toroidal transformers. Those transformers have been manufacturing with nominal power from 20 VA do 500VA. The thermal switch is usually inbuilt in the case of overheating.
They have an application in industrial electronics as amplifier, charger, rectifier, UPS device. The transformer can be found in the different power range from 20 VA to 7000 VA. They can be provided with a different number of outputs. Because of expensive and complex production, they are practically made for the power range up to 10 kVA (very rarely up to 20 kVA).
Advantages of Toroidal Transformer
The toroidal transformer has a lot of advantages compared to the EI type:
- High electrical efficiency,
- Low noise level,
- Lowelectromagnetic interference (EMI),
- Low signal distortion,
- Minimal magnetic core losses,
- Low hysteresis and magnetizing current,
- Effectively deployed heating,
- Simple housing and protection,
- Small dimension.