teresis loss.
- Mechanical losses
- The alternating magnetic field causes fluctuating electromagnetic forces between the coils of wire, the core and any nearby metalwork, causing vibrations and noise which consume power.
- The flux in the core causes it to physically expand and contract slightly with the alternating magnetic field, an effect known as magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores. The familiar hum or buzzing noise heard near transformers is a result of stray fields causing components of the tank to vibrate, and is also due to magnetostriction vibration of the core itself.
- Cooling system
- Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat exchangers designed to remove the heat caused by copper losses and core losses. The power used to operate the cooling system is typically considered part of the losses of the transformer. Small transformers, such as a plug-in "wall wart"/"power brick" used to power small consumer electronics, often have high losses and may be less than 85% efficient.
High frequency operationEdit
The universal transformer emf equation indicates that at higher frequency, the core flux density will be lower for a given voltage. This implies that a core can have a smaller cross-sectional area and thus be physically more compact without reaching saturation. It is for this reason that the aircraft manufacturers and the military use 400 hertz supplies. They are less concerned with efficiency, which is lower at higher frequencies (mostly due to increased hysteresis losses), but are more concerned with saving weight. Similarly, flyback transformers which supply high voltage to cathode ray tubes operate at the frequency of the horizontal oscillator, many times higher than 50 or 60 hertz, which allows for a more compact component.
A transformer usually has:
- two or more insulated windings, to carry current
- a core, in which the mutual magnetic field couples the windings.
In transformers designed to operate at low frequencies, the windings are usually formed around an iron or steel core. This helps to confine the magnetic field within the transformer and increase its efficiency, although the presence of the core causes energy losses. Transformers made to operate at high frequencies may use other lower loss materials, or may use an air core.
Power transformers are further classified by the exact arrangement of the core and windings as "shell type", "core type" and also by the number of "limbs" that carry the flux (3, 4 or 5 for a 3-phase transformer). The differences in the performance of each of these types, while of continuing interest to specialists, is perhaps more detail than is appropriate for a general encyclopedia.
Steel cores Edit
Transformers often have silicon steel cores to channel the magnetic field. This keeps the field more concentrated around the wires, so that the transformer is more compact. The core of a power transformer must be designed so that it does not reach magnetic saturation. Carefully designed gaps are sometimes placed in the magnetic path to help prevent saturation. Practical transformer cores are always made of many stamped pieces of thin steel. The high resistance between layers reduces eddy currents in the cores that waste power by heating the core. These are common in power and audio circuits. A typical laminated core is made from E-shaped and I-shaped pieces, leading to the name "EI transformer". One problem with a steel core is that due to the material's magnetic hysteresis it may retain a static magnetic field when power is removed. When power is then reapplied, the residual field may cause the core to temporarily saturate. This can be a significant problem in transformers of more than a few hundred watts output, since the higher inrush current can cause mains fuses to blow unless current-limiting circuitry is added. More seriously, inrush currents can physically deform and damage the primary windings of large power transformers.
Solid cores Edit
In higher frequ