Dynamics of Electrical Drives Questions and Answers

Answer: A

Correct answer: the cube of the speed

A centrifugal fan’s torque varies as the square of the speed, and power is torque times speed, so

This cube law is why variable speed control of fans and pumps saves so much energy: reducing the speed by twenty per cent halves the power. It is also why a fan takes so long to run up, since the accelerating torque available shrinks steeply as the speed rises.

Answer: B

Correct answer: constant power

Power is torque times speed, so if the torque falls in inverse proportion to speed their product is constant. Machine tool spindles behave this way in the finishing range, and so does a coiler winding at constant tension and constant linear speed as its diameter grows. It is the characteristic a drive delivers above base speed, where the voltage has reached its ceiling and the field is being weakened.

Answer: C

Correct answer: viscous friction

Viscous friction produces a resisting torque proportional to the velocity, which is the defining property of a damper. Stirring a liquid, an eddy current brake and the damping term in any mechanical model all behave this way. Windage is a square-law effect, the hoist is constant-torque, and the coiler is constant-power - each of the four classes appearing separately in this topic.

Answer: D

Correct answer: the square of the speed

Windage is aerodynamic drag on the rotating parts, and drag varies with the square of the velocity, so the torque does too and the power varies as the cube. This is why windage is negligible in a slow machine and dominant in a high-speed one, and why a flywheel or a turbogenerator rotor is often run in a partial vacuum or in hydrogen.

Answer: A

Correct answer: can drive the motor as well as oppose it

An active load carries its own store of energy and can return it - a suspended weight, a train on a gradient, a wound spring. Its torque acts in the same direction whichever way the drive turns, so it opposes a hoist and drives a lower. A passive load such as friction or a fan always opposes the motion and reverses sign when the motion does, so it can never drive the machine.

Answer: B

Friction, windage and machining forces all reverse with the motion and can never drive the motor round. Only gravitational and tensile loads are active, and it is those that make regenerative braking possible in a hoist.

Answer: C

The friction of the guides and the weight of the balancing rope can exceed the weight of the empty cage, so the motor has to push it downwards. That is why a hoist drive must be capable of operating in all four quadrants.

Answer: D

Plotting torque against speed divides the plane into four, and the sign of their product tells whether power flows into or out of the machine. That product, rather than either quantity alone, is what distinguishes motoring from braking.

Answer: A

Correct answer: forward motoring

With speed and torque both positive the power is positive and flows from the machine to the load, so the drive is motoring in the forward direction. The four quadrants are read from those two signs: same sign is motoring, opposite signs mean the torque opposes the motion and the machine is braking.

Answer: B

Correct answer: forward braking

Positive speed with negative torque means the machine opposes the motion it is undergoing, so the power is negative and the drive is braking while still running forward. This is the quadrant a hoist enters when it slows a rising load, and the one a train enters when it brakes on the level. Reverse braking is the fourth quadrant, with negative speed and positive torque.