Speed Control Methods Questions and Answers

Answer: C

The first two follow from the slip relation. The third is the reverse of the truth: a converter slows the machine and therefore reduces the power drawn, while the coupling keeps the machine at full speed and burns the difference.

Answer: D

Correct answer: 1-R, 2-P, 3-Q

Three quite different mechanisms. A fluid coupling varies the slip between input and output, dissipating the difference. A variable-frequency drive varies the supply frequency and so the synchronous speed itself. And stepped pulleys vary the transmission ratio, but only in the discrete steps the pulleys provide.

Answer: A

Correct answer: 1-Q, 2-R, 3-P

Three feedback devices reporting three different things. A tachogenerator produces a voltage proportional to speed, so it feeds a speed loop directly. An incremental encoder produces pulses giving relative position, which must be counted from a reference. And an absolute encoder reports its position outright, with no homing needed.

Answer: B

Resistance control and slipping couplings both dissipate roughly the power not delivered, with the coupling marginally better because its losses are external to the machine. Frequency control reduces the power drawn instead of destroying it.

Answer: C

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. In any slip device the output power is \( (1-s) \) times the input and the remaining \( s \) becomes heat, so running at half speed wastes half the input. A converter drive instead reduces the power drawn from the supply, which is why slip devices have largely been displaced where energy matters.

Answer: D

The shaft can only have one speed, so the loop with the slightly higher reference drives to its positive limit and the other to its negative. Making one a speed master and the other a torque follower resolves it, which is exactly what the reason implies is needed.

Answer: A

The saving depends on the load's own power-speed relation, and the dramatic figures come from fluid loads with their cubic law. A hoist or a conveyor at half speed does half the work in the same time and saves nothing per unit moved.

Answer: B

Correct answer: False, since many loads do not care what speed they run at

Regulation matters where the speed itself is the controlled quantity - a machine tool, a paper machine, a winder. A fan or a pump whose duty is a flow rate is quite indifferent to a per cent or two of droop, and specifying a closed-loop drive for it buys a property nobody needs.

Answer: C

Correct answer: to change the transmission ratio

If the load simply needs to run slower and always at that speed, a pulley or gear change does it at almost no cost and with no loss. The electrical solutions cost money and, in the case of voltage reduction or a fluid coupling, waste power continuously - they earn their place only where the speed must vary.

Answer: D

Correct answer: involves no slip in the power path

A gear or belt transmits torque by contact, trading speed for torque with only the small losses of meshing and bearings. Nothing corresponds to the slip term of a fluid or eddy current coupling, so the efficiency stays high at every ratio - the drawback being that the ratio is fixed once chosen.