Speed Control Methods Questions and Answers

Answer: A

Correct answer: its speed reduced as the diameter increases

Linear speed is the rotational speed times the radius, so holding the web speed constant as the reel grows requires the rotational speed to fall in inverse proportion. The torque must rise at the same time to hold the tension, which is why a winder drive is usually specified in terms of constant power over a diameter range.

Answer: B

Torque is tension times radius, so it grows in proportion to the diameter while the speed falls in inverse proportion. The product, and therefore the power, stays roughly constant, which is why winders are classic constant-power loads.

Answer: C

The pressure the screw must develop is set by the die and the material rather than by the speed, so the torque is nearly flat and the power rises with output. Constant flux over the whole range is therefore what the drive needs.

Answer: D

Each revolution displaces a fixed volume against the discharge pressure, so the torque follows the pressure and not the speed. Such a pump has no cubic law to exploit, and the energy saving from variable speed is proportional rather than dramatic.

Answer: A

A drum full of ore and grinding media has an enormous breakaway requirement and then a steady one, which is why such drives are rated on the start rather than the run. Low-speed synchronous machines on cycloconverters are the usual arrangement.

Answer: B

Almost all the duty is in getting the mass up to speed, and the run-up may last minutes with the drive at its current limit throughout. Thermal rating for that sustained transient, not the steady running load, is what sizes the machine.

Answer: C

Correct answer: the thermal duty of the run-up

Accelerating a large inertia draws heavy current for a long time, and the heat generated during the run-up may far exceed anything the steady load produces. The machine and the drive must be sized on that transient - and on how often it is repeated - rather than on the modest power needed once the load is turning.

Answer: D

Correct answer: one on speed control and the other following its torque

This is the master-slave arrangement, and the reason is the same as before: two speed loops on a common shaft cannot both be satisfied, so they fight, one motoring and one braking. Two torque references with no speed loop leaves the speed uncontrolled altogether.

Answer: A

Correct answer: only the discrete speeds the winding allows

A pole-changing winding gives the two or four speeds the reconnection provides and nothing between them, so it suits a load with a small number of defined duties. Within each of those speeds the efficiency is that of a normal machine, which is its advantage over any slip method.

Answer: B

Each connection has its own synchronous speed and the machine runs a few per cent below it, so the rotor loss is small in both cases. That is quite unlike voltage or resistance control, which obtain low speed by increasing the slip deliberately.