The payback period of a variable-speed drive on a fluid load depends chiefly on
Answer: A
A load that runs at full flow all day saves nothing, while one that spends most of its time at seventy per cent saves a great deal. The duty profile is therefore the first thing to establish before any such investment is justified.
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A drive retrofitted to replace a throttle valve should be checked for
Answer: B
A pump slowed too far cannot generate the static head at all and delivers nothing, whatever the valve position, so there is a floor below which the saving is unavailable. Cooling of the machine at low speed is the other check to make.
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Selecting a speed control method begins with
Answer: C
Everything else follows from what the load demands at each speed and how accurately that speed has to be held. A method adequate for a fan is quite inadequate for a machine tool, and the difference is visible in the load's requirements before any equipment is considered.
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A load requiring rated torque at standstill rules out
Answer: D
Voltage control reduces the torque with the square of the voltage and only produces low speed by increasing slip, so it can offer nothing at standstill. The other three all hold full flux and give rated torque from zero speed.
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A load requiring a very wide speed range with high accuracy at the bottom of it needs
Answer: A
Only feedback of adequate resolution can hold a speed accurately at a hundredth of rated, and only a full-flux method can produce the torque there. The feedback device rather than the machine is usually the limiting component.
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A load with a very high starting torque requirement and no need for speed control at all is best served by
Answer: B
If the speed never has to be varied, paying for a converter or a coupling buys nothing, and a machine with a suitable characteristic and a starter will do. A soft starter may still be worth having to limit the current transient.
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The cheapest adequate speed control method for a simple fan is usually
Answer: C
A fan needs neither accuracy nor torque at low speed, so the simplest converter setting serves and the cubic law delivers the saving. Spending on feedback or on a coupling adds cost without adding anything the load needs.
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Retrofitting a converter to an existing machine requires checking that the machine
Answer: D
Fast switching edges stress winding insulation that was designed for a sinusoidal supply, and running slowly reduces the machine's own ventilation. Inverter-duty windings, output filters and derating curves are the three remedies.
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Bearing currents in an inverter-fed machine arise from
Answer: A
Switching produces a common mode voltage that appears across the thin lubricant film and discharges through it, pitting the races. Insulated bearings, shaft grounding brushes and common mode chokes are the standard countermeasures.
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Consider the following statements about speed control.
1. Speed range is the ratio of highest to lowest usable speed.
2. Speed regulation is the droop from no load to full load.
3. The two are the same quantity.
Which are correct?
Answer: B
The first two are the standard definitions and describe quite different things: how far the speed can be taken, and how well it stays where it is put. Confusing them leads to specifying a drive that meets one requirement and fails the other.
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