Discussion
Holding a suspended load at rest cannot be done by electrical braking alone because
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.
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