Does soft starter suitable for crane?

Soft starters have no Hz regulation capabilities. They can boost current and voltage at start, but it's a voltage boost to initiate movement if the motor bogs at start, after which the boost voltage drops to the accelerated low end ramp settings. My intent was, and I think I accomplished getting the point across, to illustrate simple physics regarding voltage/Hz regulation as variable frequency drives can do and the torque losses incurred when the motor rpm/Hz/FLA apex is varied from, as happens by virtue of how soft starter works. In general, asynchronous motors are the choice in overhead crane, and many if not most conveyor controls we build as the slip helps with generating higher starting torque. The torque curves, and herein may be a point of difference in our viewpoints, directly correlate to the rpm/Hz/FLA efficiency design point of the motor in torque creation at start, as well as in running torque. If the initiating frequency is at 60 Hz and the voltage is reduced from the peak motor design efficiency point, the resulting motor 'starvation' will cause the motor to draw excessive current. That is inescapable and as such current boost is barely a help.

To achieve motion against static resistance, (ie: a machine at idle) using a reduced voltage soft start (and they all work on that principle), rendering a voltage boost is at that point the effective means of getting the wheels turning, as it were. In overhead crane and often in conveyor controls soft starters can work, but the inability of speed regulation is another suffering. In lifting applications, a soft start is out of the question as the rule of thumb is an ability to be able to generate up to a general maximum of 180% to 225% of locked rotor torque at start to initiate motion with a suspended load. If you want to oversize a motor at a several times to one of what is needed for running torque, a soft starter may work, but the initiating voltage would likely have to be approaching half the design motor Hz ratio, or the torque drop after initiation would likely result in a plummet. It also won't be close to cost effective.

This effect also comes into major play when the motor, not a gear-case internal mechanical load brake, is the regulator of the overhauling force against it while lowering loads, another point of operation where soft starts are inapplicable. Regeneration can work, but with finite speed control via Hz/Volt regulating capabilities during deceleration is necessary. Eddy current brakes are another lowering speed regulating method, but typically in wound rotor and generally never in squirrel cage motor hoist systems. In acceleration as well as deceleration in traversal loads, especially in tandem situations where two separate machines (cranes) must travel at synchronous speeds, soft starters are never the choice. If motion start can be accomplished at much lower starting torque levels, and in many crane traversal applications it can work, it's basically all the soft starter can do. As such, soft starters have become an infrequent choice anymore in most crane controls.