Dynamics of Electrical Drives Questions and Answers

Answer: C

Metro systems recover a substantial fraction of their traction energy this way, provided another train is accelerating nearby or the substation can accept the power. Where it cannot, a resistor bank takes the surplus instead.

Answer: D

Correct answer: absorb energy when the supply cannot take it

A regenerative drive returns braking energy to the supply, but the supply may not always accept it - an isolated or lightly loaded network, or a supply failure at the moment braking is called for. The resistor and its chopper give the energy somewhere to go, so the dc link voltage cannot run away and braking remains available regardless of what the network is doing. On a hoist that is a safety requirement rather than a convenience.

Answer: A

Every stage between the supply and the driven machine takes its share, and modest losses at three stages multiply into a noticeable total. It is why a high-efficiency motor behind a poor gearbox is not an efficient drive.

Answer: B

Response depends on the ratio of available torque to inertia, so anything that lowers the inertia or raises the torque helps. Higher gearing, a lighter coupling and a low-inertia rotor are the usual means of getting it.

Answer: C

With the rotor stationary the slip is unity and the machine behaves nearly as a short-circuited transformer, drawing five to seven times rated current. Star-delta starting, soft starters and converters all exist to reduce it.