Dynamic control of motor speeds is primarily used for:

Dynamic control of motor speeds is primarily utilized to provide a retarding force under overhauling conditions. When a load is being hoisted with a crane, the dynamic control system can adjust the motor speed to apply an opposing force when the load begins to descend due to its own weight. This is particularly important in preventing mechanical overload and maintaining safety. It ensures that the descent of the load can be controlled effectively, minimizing the risk of accidents or equipment damage.

In situations where the load is heavier than the lifting mechanism can handle, the overhauling condition occurs, necessitating a retarding force to manage speed and prevent uncontrolled movements. By dynamically controlling the motor speed, the system can adapt to the weight of the load and the forces acting upon it, keeping the operation smooth and safe.

The other options focus on aspects like increasing speed during a lift or maintaining constant speed, which, while important in certain contexts, do not capture the primary purpose of dynamic control in conjunction with the challenges posed by an overhauling load. The primary function in this context highlights safety and control rather than just speed adjustments or load changes.