The international efficiency classes for induction motors are designated
Answer: A
The number rises with efficiency, so IE3 premium efficiency exceeds IE2 high efficiency, and many jurisdictions now prohibit the sale of the lower classes. The lettered scale belongs to appliance labelling rather than to motors.
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A motor's efficiency curve against loading is
Answer: B
Iron and friction losses continue whatever the shaft output, so at a quarter load they are a large share and the efficiency collapses. Above about half load the curve is broad, which is why loading anywhere in the upper half is acceptable.
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A motor's power factor against loading
Answer: C
The magnetising current is nearly constant while the active component falls with the load, so the ratio of active to total current collapses. A bank of lightly loaded motors is therefore both inefficient and the usual cause of a poor site power factor.
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An oversized motor costs energy chiefly because it runs
Answer: D
Correct answer: at part load, where its efficiency and power factor are poorer
A motor’s efficiency curve is reasonably flat from about half load to full load and falls away sharply below that, and the power factor falls much faster still, because the magnetising current is drawn whatever the load. An oversized machine therefore spends its life on the part of the curve where both are worst. It does not run faster or hotter - if anything it runs cooler and closer to synchronous speed.
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Motors are commonly found oversized because
Answer: A
The process engineer adds a margin, the pump supplier adds another and the electrical contractor rounds up to the next frame size, and the compounding leaves a machine at a third of its rating. Measuring the actual load is the first step in correcting it.
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The quickest field estimate of a motor's loading uses
Answer: B
Dividing the measured input by the input the motor would take at full load gives the loading directly, and a clamp-on power meter supplies it in a minute. The current alone is misleading, since it does not fall in proportion to the load.
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Estimating loading from the measured current alone understates the true loading because current
Answer: C
The magnetising component persists, so a motor at a quarter load may still draw half its rated current, and the current method therefore overstates the loading rather than understating it. The slip method and the power method are both better.
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The slip method of estimating loading works because slip is nearly proportional to
Answer: D
A machine running close to synchronous speed is lightly loaded and one near its full-load speed is fully loaded, so a tachometer reading gives the loading without any electrical measurement. It is sensitive to voltage, which limits its accuracy.
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Rewinding a burnt-out motor typically
Answer: A
Core damage during stripping, a changed slot fill and a different winding technique each take their toll, and two or three rewinds leave a machine several points down. Comparing the cost of a rewind with a new efficient motor is a routine audit recommendation.
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Replacing a motor rather than rewinding it is favoured when the motor is
Answer: B
A small old machine is cheap to replace and the efficiency gain is large, whereas rewinding a large premium machine is far cheaper than buying another and loses little. Running hours decide whether either is worth doing.
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