Speed Control Methods Questions and Answers

Answer: C

Correct answer: circulating oil between an impeller and a runner

The impeller throws oil outward and the runner receives it, torque being transmitted entirely by the momentum of the fluid with no mechanical contact. The two shafts are therefore isolated from shock and torsional vibration, at the cost of a permanent slip of two or three per cent even at full load.

Answer: D

Exactly as in an induction machine's rotor, a fraction equal to the slip is dissipated as heat in the oil. Running a fluid coupling at thirty per cent slip to reduce the speed therefore wastes thirty per cent of the input.

Answer: A

Correct answer: changing the quantity of oil in the working circuit

A scoop tube reaching into the rotating oil ring controls how much oil the working circuit holds, and less oil means less torque transmitted and more slip. The speed is thereby varied smoothly, but the power corresponding to that slip appears as heat in the oil, which must be cooled.

Answer: B

Correct answer: varying the excitation of a field between two rotating members

An eddy current coupling has a driven drum and a driving member separated by an air gap, with a field coil between them. Field current sets the torque transmitted and hence the slip. Like the fluid coupling it is a slip device, so its loss is the slip fraction of the input power, all of it appearing as heat in the drum.

Answer: C

Both work by slipping, so the power not delivered is dissipated inside the coupling and the efficiency falls roughly in proportion to the speed reduction. A variable-frequency drive on the motor achieves the same speed reduction while drawing correspondingly less power.

Answer: D

At fifty per cent slip, half of what enters the coupling becomes heat in it. The motor is still turning at full speed drawing more than the fan's own reduced requirement, which is why replacing such couplings with converters saves so much.

Answer: A

The slip frequency output of the first machine's rotor becomes the supply for the second, whose shaft is coupled to the first. The pair then runs at a speed set by the combined pole number, which gives additional fixed speeds without a converter.

Answer: B

The two machines behave as one with the poles added, so a four-pole and a six-pole pair on 50 Hz runs at 600 rpm. Reversing the phase sequence to the second machine gives the differential connection and a different speed.

Answer: C

Correct answer: 600 rpm

In cumulative cascade the pole numbers add: \[ N_s = \frac{120f}{P_1 + P_2} = \frac{120 \times 50}{4 + 6} = 600\ \mathrm{rpm}. \] The second machine is fed from the slip rings of the first, so the set behaves as a machine of ten poles. The arrangement is largely of historical interest, converter drives having displaced it.

Answer: D

Correct answer: 3,000 rpm

In differential cascade the pole numbers subtract: \[ N_s = \frac{120 \times 50}{|4 - 6|} = \frac{6000}{2} = 3000\ \mathrm{rpm}. \] The negative sign means the second machine’s field opposes, and the set behaves as a two-pole machine. Cascade connection thus offers four speeds - either machine alone, and the two combinations.