Speed Control Methods Questions and Answers

Answer: C

Nobody notices a few per cent of speed error in a fan, whereas the other three would produce a torn web, a spoiled thread or an uncomfortable stop. Matching the accuracy specified to the accuracy needed is where a good deal of money is saved or wasted.

Answer: D

Feedback devices, cabling and the commissioning effort all cost money, and an unnecessary encoder is also one more thing to fail. Specifying to the requirement rather than to the state of the art is part of the engineering.

Answer: A

Low speed is obtained by increasing the slip, and the rotor loss is the slip times the air gap power, so the rotor takes a large share of the input. Frequency control obtains the same speed with the slip left small, which is the whole difference.

Answer: B

Correct answer: forced ventilation or a torque derating

A self-ventilated machine carries its fan on its own shaft, so at ten per cent speed it has almost no cooling air while the stator current, and hence the loss, may be at full value. Either a separately driven blower is fitted or the permissible torque is reduced, and the derating curve for continuous low-speed running is part of the machine data.

Answer: C

The cubic law means sixty per cent speed takes about twenty-two per cent of the power, while throttling and recirculation both keep the pump at nearly full power. Intermittent full-speed running avoids some of the waste but subjects the plant to repeated transients.

Answer: D

In each case the speed is the manipulated variable and the process quantity is what is actually being regulated, with the drive taking its reference from a process controller. That is why so many drives sit inside a larger loop rather than being commanded directly.

Answer: A

The process loop is slow and the speed loop fast, which is the same nesting principle that puts the current loop inside the speed loop. Tuning them so that each is well separated in bandwidth from the next is what keeps the whole arrangement stable.

Answer: B

A drive sized on rated power alone may have neither the torque nor the cooling to work at the bottom of its range, which is exactly where the process often needs it. Checking the torque and the thermal duty at the extremes of the range is what prevents it.

Answer: C

Deriving both from one source, with each section's ratio applied digitally, keeps them locked together and lets the ratio be trimmed from the desk. Independent hand settings drift apart and would tear the material between the sections.

Answer: D

A small speed difference between the sections stretches the web slightly, and a load cell or a dancer roll closes the loop on the resulting tension. Exactly equal speeds would leave the tension undefined and the web either slack or torn.