Speed Control Methods Questions and Answers

Answer: C

Correct answer: limit the torque demanded during a change of speed

Torque is proportional to the rate of change of speed times the inertia, so an unramped step demands infinite torque and the drive simply hits its current limit. A ramp sets the acceleration explicitly, which sets the torque, and so keeps the drive within its rating and the mechanical transmission within its.

Answer: D

Correct answer: limits the rate of change of acceleration

A straight ramp changes acceleration abruptly at each end, which is a step in torque and is felt as a jolt. The S-curve rounds those corners, limiting the jerk. The move takes slightly longer, but the mechanical shock and the excitation of resonances are both greatly reduced.

Answer: A

Correct answer: lifts and cranes carrying people or liquids

A step change in acceleration is felt directly by a passenger and sets a suspended load swinging or a liquid surface sloshing. Jerk limiting is therefore a comfort and safety requirement rather than a refinement, and lift specifications state jerk limits explicitly alongside acceleration.

Answer: B

Taking the slave's reference from the master's actual speed keeps the two in step even while the master is changing speed. A common reference would let them diverge during transients, which on a shared web or load would be unacceptable.

Answer: C

A common digital position reference distributed to every section makes them run in exact relative step without any mechanical connection. It allows each section's ratio to be changed from the control desk, which a gear train never could.

Answer: D

Correct answer: a paper machine or a rolling mill train

Where a continuous web or strip passes through several driven sections, each must run at a set ratio to its neighbours or the material tears or gathers. The ratio changes along the train as the material thins or stretches, so the drives must be coordinated rather than merely set to the same speed.

Answer: A

Correct answer: giving one a speed loop and the other a torque reference

Two speed loops on one shaft fight each other: a slight difference in reference or calibration makes one motor drive the load and the other act as a brake. Putting the second on torque control, following the first’s torque output, makes it contribute without contesting the speed - the master-slave arrangement.

Answer: B

Correct answer: 51 per cent of rated

Power varies as the cube of speed for a centrifugal machine: \[ 0.8^3 = 0.512, \] about 51 per cent. This cube law is the whole economic case for variable-speed pumping and fanning - a twenty per cent reduction in speed halves the power.

Answer: C

Correct answer: wastes the pressure dropped across the valve

A throttle valve reduces the flow by adding resistance, and the pump still develops nearly its full head; the surplus pressure is destroyed across the valve as heat. Slowing the pump reduces the head it develops as well as the flow, so the power falls as the cube of speed instead of barely at all.

Answer: D

Correct answer: 34 kW at that reduced speed

\[ 100 \times 0.7^3 = 100 \times 0.343 = 34.3\ \mathrm{kW}. \] A thirty per cent reduction in speed removes two thirds of the power. Over a year of running the saving pays for a converter drive many times over, which is why fan and pump applications dominate the variable-speed market.