Ship propulsion by electric drive is attractive because it
Answer: A
Generators can be placed anywhere convenient and the propulsion motor put in a pod outside the hull, which improves both the arrangement and the manoeuvring. Efficiency at part load is better as well, since generators can be shut down as demand falls.
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A podded ship propulsion drive typically uses
Answer: B
Tens of megawatts at a couple of hundred revolutions a minute is what the propeller needs, and a many-poled synchronous machine on a cycloconverter or a large inverter delivers it. The pod can be rotated to steer as well.
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An electric vehicle traction drive requires
Answer: C
One reduction ratio has to serve from standstill to motorway speed, which means a machine that can be field weakened over a range of three or four to one. Regeneration recovers a useful share of the energy in city driving.
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A permanent magnet machine is usual in electric vehicles because it offers
Answer: D
Correct answer: the best torque and efficiency for a given size
The magnets provide the rotor field without any current, so there are no rotor losses and the torque per unit volume is the highest of any type. In a vehicle, where mass and space are at a premium and every per cent of efficiency is range, that outweighs the cost of the magnet material.
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Match the application with its load characteristic.
1. Centrifugal fan 2. Hoist 3. Winder
P. Constant power Q. Constant torque R. Torque with the square of the speed
Answer: A
The three between them cover most industrial loads, and identifying which applies decides both the machine size and the region of the drive envelope that will be used. A load can also change character with speed, as traction does above base speed.
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Match the application with the number of quadrants it needs.
1. Ventilation fan 2. Downhill conveyor 3. Reversing mill
P. Four Q. One R. Two
Answer: B
Correct answer: 1-Q, 2-R, 3-P
The quadrant count follows from what the load does. A ventilation fan runs one way and never needs braking, so one quadrant. A downhill conveyor runs one way but is driven by its load, so it must motor and regenerate - two quadrants. A reversing mill motors and brakes in both directions, which is all four.
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Match the application with the accuracy it demands.
1. Machine tool axis 2. Paper machine section 3. Ventilation fan
P. A few per cent Q. Micrometres of position R. Hundredths of a per cent of speed
Answer: C
Specifying accuracy to the requirement rather than to the state of the art saves a great deal, since each step up in accuracy costs feedback resolution and commissioning effort. A fan is entirely indifferent to a few per cent of droop.
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Consider the following statements about application requirements.
1. A hoist needs a mechanical brake.
2. A fan drive saves energy through the cubic law.
3. A machine tool spindle is a constant-torque load.
Which are correct?
Answer: D
The first two are correct. A spindle is a constant-power load, because a large workpiece is turned slowly at high torque and a small one quickly at low torque, which is why field weakening matters so much on such a drive.
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Assertion (A): A centrifuge drive is rated on its run-up rather than its running load.
Reason (R): Accelerating the bowl dissipates far more energy than steady running.
Select the correct option.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. The kinetic energy stored in a spinning bowl is considerable, and an equal amount is dissipated in the machine and converter while it is being put there. Repeated every cycle, that heat determines the rating, while the steady running torque is comparatively trivial.
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Assertion (A): An inclined conveyor needs a holdback brake.
Reason (R): The material on the incline can overhaul the drive.
Select the correct option.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Gravity acting on material already on the incline can drive the belt backwards if the drive torque is lost, and the belt would then run away and discharge its load at the tail. A holdback - a backstop or a brake - prevents the reverse motion mechanically, independently of the drive.
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