Braking of Electrical Drives Questions and Answers

Answer: A

Correct answer: decelerate it, hold it, or control a descending load

Braking serves three distinct duties, and they demand different arrangements. Deceleration removes kinetic energy from the rotating parts and ends at zero speed. Holding keeps a stationary load stationary and needs torque at zero speed, which electrical braking cannot give. Controlling a descent restrains a load that gravity would otherwise accelerate, and it may continue for minutes at a steady speed, so the energy involved can be far larger than a stop. Braking has nothing to do with acceleration, power factor or losses at the rated point.

Answer: B

Correct answer: electrical braking gives no torque at zero speed

Every electrical braking method depends on relative motion. Regeneration needs the machine to be generating, dynamic braking needs an induced emf to drive current through the resistor, and an eddy current brake needs flux to be cut. At standstill there is no motion, no emf and no torque, so a suspended load would begin to descend. A mechanical brake, which works by friction and does not care whether anything is moving, is therefore mandatory on any hoist.

Answer: C

Correct answer: applies when the electrical supply is lost

A fail-safe brake is held off by a solenoid or by hydraulic pressure and applied by springs, so any loss of supply, control signal or pressure releases the springs and sets the brake. The load then stops rather than falling. Designing it the other way round - requiring power to apply the brake - would mean that the one event most likely to cause an accident, a power failure, is also the event that releases the load.

Answer: D

Correct answer: holding a load at rest indefinitely

Only a friction brake produces torque at zero speed and continues to produce it for as long as required without drawing power or generating heat. Electrical methods all vanish at standstill. For the other three duties - decelerating a large inertia, recovering energy, and controlling a long descent - electrical braking is far better, since a friction brake would have to dissipate the whole of the energy as heat in the lining.

Answer: A

Correct answer: currents induced in a rotating conducting disc

A conducting disc turning in a magnetic field has eddy currents induced in it, and by Lenz’s law the force on those currents opposes the motion. The braking torque is smooth, contactless and free of wear, and the energy appears as heat in the disc. Because the induced emf is proportional to speed, so is the current and roughly so is the torque - which is why the brake fades to nothing as the machine stops.

Answer: B

Correct answer: falls to zero as the speed falls to zero

The eddy currents are induced by relative motion, so at standstill there is no emf, no current and no torque. The brake is therefore excellent for restraining a descent or absorbing energy at speed, and useless for holding. It shares this limitation with every other electrical method, and it is why a mechanical brake is always fitted alongside where holding is required.

Answer: C

Electrical braking does most of the work down to low speed and the friction brakes take over for the final stop and the hold. The blending controller shares the demand so that the deceleration feels constant to the passengers.

Answer: D

Correct answer: returns to the supply less the losses

In regeneration the machine acts as a generator and the converter passes the power back to the mains or to a battery, so the kinetic energy is recovered rather than wasted - less the copper, iron and converter losses along the way. It is the most efficient method and the only one that recovers anything, but it requires a supply able to accept reverse power flow, which a diode rectifier is not.

Answer: A

Correct answer: the kinetic energy of the rotating parts

In dynamic braking the machine is disconnected from the supply and its armature connected across a resistor, so the only source of energy is the kinetic energy stored in the rotating parts:

All of it ends up as heat, mostly in the external resistor. Plugging is quite different: there the supply continues to deliver energy throughout, so the total dissipated is several times the stored kinetic energy.

Answer: B

The supply continues to deliver energy while the machine also absorbs the stored kinetic energy, and both end up as heat inside the machine. That doubling is why plugging is so hard on the windings and is reserved for occasional use.