Q Standard transformers are very efficient. Is there really an advantage in purchasing a high efficiency version?
A It depends on the type of load on the transformer.
Transformer losses are of two types; those that are independent of the load (known as ‘no-load’ losses) and those that vary with load (known as ‘load losses’ or, sometimes, as ‘copper losses’). ‘No-load losses’ are present all the time that the transformer is energised, whether loaded or not. Most of the no-load losses occur in the magnetic steel of the core. ‘Load losses’ are proportional to the square of the load current and so are greatest when the transformer is highly loaded. Transformers are manufactured to meet standard types defined by standards or to meet specific customer requirements.
As a rule of thumb, if the loading is high, such as in an industrial plant, purchase of a transformer with reduced load losses will be beneficial. If the transformer is lightly loaded, such as a hot standby transformer, a transformer with reduced no-load losses will be beneficial. In other cases, it is necessary to calculate the benefit. The SEEDT report gives more information on transformer losses. (2008, 32pp).
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Q What are the benefits of using cast copper rotors in high efficiency motors?
A Induction motor rotors were traditionally made by die casting aluminium around a stack of steel laminations, so forming the rotor bars and end rings. Copper has much better conductivity, which would result in better efficiency, but was not used because it was difficult to die-cast due to its higher melting point. The casting issues have now been solved by improvements in die materials and process control.
Using copper offers a range of options to the motor designer. If the rotor size is unchanged, efficiency can be improved. On the other hand, the rotor size can be reduced while maintaining the original efficiency, so reducing the cost. For further information see ‘Induction Motors with Die-Cast Copper
Rotors’.
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Q
Should I take energy losses in installation cable into account?
A For long circuits, or high power or high duty cycle circuits, energy losses in cable should be considered. Wiring standards normally recommend that the voltage drop in installation wiring is kept to below 2.5%. That represents a power loss of 2.5%, so the cable, as installed, is only 97.5% efficient. Increasing the conductor cross-section reduces voltage drop and energy loss, but increases the cost of the cable. The process of finding the optimum conductor size – for which the lifetime cost is lowest – is described in Pub 116 – Electrical Energy Efficiency. Our cable sizing software allows the optimum size to be calculated easily.
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