Several Types of Transformer

Laminated Core Transformer

This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics. 1.Widely available in power ratings ranging from mW to MW 2.Insulated lamination minimizes eddy current losses 3.Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings 4.Rectangular core 5.Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used 6.Mu-metal shields can be fitted to reduce EMI (electromagnetic interference) 7.A screen winding is occasionally used between the 2 power windings 8.Small appliance and electronics transformers may have a thermal cut out built in



Toroidal Transformer

Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring (and thus threaded through the ring during winding), and tape for insulation. Toroidal transformers compared to EI transformer: 1.Lower external magnetic field 2.Smaller for a given power rating 3.Higher cost in most cases, as winding requires more complex and slower equipment 4.Less robust 5.Central fixing is either 6.Overtightening the central fixing bolt may short the windings 7.Greater inrush current at switch-on

Autotransformer

An autotransformer has one winding which is tapped at some point along the winding. Voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The power rating of the autotransfomer is lower than the load power rating. For example, an auto transformer used to adapt a 1000 VA load to a 240 volt supply is rated at: (240V-120V)/240V x 1,000VA = 500VA. For voltage ratios not exceeding about 3:1,an autotransformer is cheaper, lighter, smaller and more efficient than an isolating (two-winding) transformer of the same rating.[2] Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 33 kV and 66 kV sub-transmission networks.

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