Sim's Electrical Companion
Transformer Construction, Types & Calculations
Single Phase Transformers
A transformer is a simple device that changes a primary voltage / current into a secondary voltage / current. The method of achieving this is so simple, a primary source conductor current produces a magnetic fields and is wound around an iron core to increase its subsequent magnetic field. The lines of flux cut across a secondary winding which induces an emf in the secondary circuit. The secondary voltage will be dependant upon the ratio of windings primary and secondary. If there are an equal amount of windings on both sides the transformers output and input will be the same (excluding loses). If the secondary windings are lower the transformer is considered to be a step down transformer. If the secondary windings are greater than the primary windings it is a step up transformer.
Transformers have a ferrous core which increases the magnetic field produced by the primary coil. There are three common types of core construction Shell, Core and Toroid (pictured right). The toroid has the secondary windings wound over/under the primary windings. Shell and core types the windings are clearly separated.
Transformer Construction, Types & Calculations
Single Phase Transformers
A transformer is a simple device that changes a primary voltage / current into a secondary voltage / current. The method of achieving this is so simple, a primary source conductor current produces a magnetic fields and is wound around an iron core to increase its subsequent magnetic field. The lines of flux cut across a secondary winding which induces an emf in the secondary circuit. The secondary voltage will be dependent upon the ratio of windings primary and secondary. If there are an equal amount of windings on both sides the transformers output and input will be the same (excluding loses). If the secondary windings are lower the transformer is considered to be a step down transformer. If the secondary windings are greater than the primary windings it is a step up transformer.
Transformers have a ferrous core which increases the magnetic field produced by the primary coil. There are three common types of core construction Shell, Core and Toroid (pictured right). The toroid has the secondary windings wound over/under the primary windings. Shell and core types the windings are clearly separated.
Transformer Losses & Three Phase Transformers
As stated in the previous page VA on both sides of a transformer should be equal. The first law of thermodynamics states that energy cannot be created or destroyed it can only be converted into another form of energy.
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If voltage is decreased like in the example right then the current is increased to satisfy this law VA must be equal on both sides.
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Transformers are on average about 95-99% efficient. Other than active cooling there are ordinarily no moving parts for the transference of electrical energy but there are loses that should be considered.
The two types of transformer losses are iron losses and copper losses. Copper loss is heat generated by carrying a current. When a conductor carries a current the atoms are exited and give out heat in proportion to the amount of current that is flowing. The heat generated causes less coherence in the arrangement of the atoms which is evidenced by an increase in resistance.
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Iron loses are regarding the cores reaction to the intended magnetic fields. When a magnetic field cuts across a conductor it generates an emf. As well as the emf induced in the secondary coil the core is also a conductor. The current induced into the core are called eddy currents. This is an undesirable side effect and is minimised by efficient core design. Methods of reducing the eddy currents include cutting circular routes to break electrical continuity and laminating the core with insulating material to reduce the size of each ferrous part of the core (See picture above right - operative assembling a laminated core). Each of the laminations is sprayed with a thin coat of insulating lacquer to minimise weight and bulk.
Transformers are subject to an alternating current which effectively makes a N-S / S-N magnet for every full cycle of the sine. Hysteresis is a copper loss associated with the magnetisation and demagnetisation of the core relative to the AC frequency. The ferrous core which has a small amount of remainance which will oppose the next change in the AC be it N-S / S-N the small amount of remainance will oppose the next magnetic field. The eddy currents in the core will have a magnetic field that opposes the field that induced the emf this is Lenz's law mentioned earlier in this chapter.
The illustration right is a hysteresis graph. This shows the amount of magnetic field generated dependent upon an applied magnetic field. The saturation point is where the material is fully magnetised (N/S or S/N).
The greater the applied field (H) the greater the flux density (B). The dashed line shows magnetisation on a material that has no existing magnetic field. If the current was then removed the material will retain some of the magnetic flux (retentivity). This can be undone by applying an opposing magnetic field. The point of net zero magnetic field is the coercivity point. At this point the material is now no longer magnetic.
If the reverse field is applied beyond that point, dependent upon the amount of current and strength of magnetic flux, the material will be magnetised in the opposite direction to its opposing point of saturation (coloured blue).
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Three Phase Transformer Types
The two main types of three phase transformers are the shell and core types illustrated below.
