Speed Control Methods Questions and Answers

Answer: A

A drive quoted as 100 to 1 can be operated from full speed down to a hundredth of it while still meeting its torque and accuracy requirements. It is sometimes called the turndown ratio, and it says nothing about how accurately any particular speed is held.

Answer: B

Expressed as a percentage of the rated speed, it describes how much the speed droops as the load is applied in open loop. A closed-loop drive reduces it to a fraction of a per cent, which is why feedback is fitted wherever the speed matters much.

Answer: C

The controller can only hold the speed as well as it can measure it, so the resolution and the drift of the feedback device set the floor. A good encoder gives a hundredth of a per cent or better, where an analogue tachogenerator gives perhaps a tenth.

Answer: D

It is governed by the available torque and the inertia, and by the bandwidth of the control loops. A servo drive with a low-inertia machine and a fast current loop can reverse in milliseconds, where a large process drive may take seconds.

Answer: A

Below base speed the flux is held and rated current gives rated torque; above it the flux falls and the torque falls with it while the power stays at its ceiling. Every dc, induction and synchronous converter drive behaves this way.

Answer: B

Plotting the two torque-speed curves on the same axes shows at once whether the drive can serve the load throughout, and where the margin is thinnest. A constant-power load below base speed and a constant-torque load above it are the two mismatches that most often catch people out.

Answer: C

Above base speed the available torque falls with speed while the load's demand does not, so the two curves cross and the drive cannot deliver. Either a larger machine or a different gear ratio is needed to bring the duty inside the envelope.

Answer: D

Changing the machine's voltage, frequency, flux or rotor circuit is one family, and interposing a variable-ratio or slipping element between machine and load is the other. The first is now almost always cheaper and more efficient, which is why the second has receded.

Answer: A

Moving the belt between pulley steps changes the ratio in discrete jumps, which is adequate for a bench lathe or a drill press. It is cheap and efficient at each ratio but cannot be adjusted while running or under load.

Answer: B

Correct answer: changing the effective diameters of its driving elements

A continuously variable transmission runs a belt or chain between conical pulleys whose flanges move axially, so the radius at which the belt rides - and therefore the ratio - changes smoothly without steps. It is a mechanical ratio change with no slip in the power path, so it is efficient across its range.