Transformer cores are built of laminations in order to
Answer: A
Thin sheets separated by insulation break up the paths available to circulating currents, and the loss falls with the square of the thickness. Hysteresis loss depends on the grade of steel rather than on the lamination.
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Cold rolled grain oriented steel is used in transformer cores because it
Answer: B
Aligning the crystal grains with the direction the flux will take reduces the loss and raises the permeability along that axis. Mitred corner joints are used so that the flux stays as nearly as possible in the favourable direction.
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Transformer oil serves as
Answer: C
Its dielectric strength lets the windings be spaced closely, and its circulation carries heat from the core and coils to the radiators. Moisture and particles degrade both functions, which is why oil is tested periodically.
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The dielectric strength of transformer oil is reduced most by
Answer: D
Even a few parts per million of water lowers the breakdown voltage sharply, which is why the breather dries the incoming air and the oil is filtered and dried in service. Particles and acidity from ageing also contribute.
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Dissolved gas analysis of transformer oil is used to
Answer: A
Different faults produce characteristic mixtures of hydrogen, methane, acetylene and the rest, so the ratios point to overheating, partial discharge or arcing. It is the principal condition-monitoring tool for a large transformer.
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The temperature rise limit of a transformer applies to
Answer: B
Correct answer: the winding and the top oil, each with its own limit
Two limits are specified because they measure different things: the top oil temperature governs the ageing of the oil, while the winding temperature, always the higher of the two, governs the ageing of the paper insulation. Insulation life falls roughly by half for each 6 to 8 degrees of sustained excess, which is why both are monitored.
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A transformer's rating can be increased by fitting fans because
Answer: C
The losses are unchanged, but a lower thermal resistance to ambient means the same temperature rise accommodates a larger loss and therefore a larger load. That is what the ONAN and ONAF ratings on a nameplate describe.
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A dry type transformer differs from an oil-filled one in using
Answer: D
Cast resin or open ventilated construction avoids the fire risk and the maintenance of oil, which suits indoor and building installations. Ratings and voltages are more limited than for an oil-filled unit of the same size.
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The efficiency of a transformer is higher than that of a rotating machine of the same rating because it has
Answer: A
Correct answer: no friction, windage or rotor loss
A transformer is static. It has copper loss and core loss like any machine, but nothing rotates, so there is no bearing friction, no windage and no rotor loss, and the cooling requirement is correspondingly smaller. That absence is the whole of the difference, and it is why transformer efficiency exceeds 99 per cent while a motor of the same rating reaches perhaps 95.
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The polarity test on a transformer establishes
Answer: B
Connecting one end of each winding together and measuring across the free ends shows whether the emfs add or oppose, which is subtractive or additive polarity. Getting it wrong when paralleling two units is a short circuit.
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