There are a few electronic braking systems for AC motors available that work off of battery power in the case of a complete line power loss. They are used quite frequently in the oil well industry to prevent backspin of screw pumps after a power failure. If the power fails, the screw pump "untwists" in the well and allows the oil to flow back from the pipeline down the hole. Once it starts flowing backwards, the shear weight of the oil keeps the shaft spinning and the oil flowing backward. If they hold the shaft for a few seconds with the brake, the shaft untwists and stops, preventing the oil flow. Mechanical brakes fail after one or two of these power failures, so they began using VFDs with battery backed dynamic braking some years ago with great success. VFD manufacturers who specialize in the oilfield industry can usually supply this, try Toshiba, Omron/IDM etc.
That said, it is very dangerous to rely upon any electronic brake method for use on a hoist application unless there is a mechanical brake available to hold the load when braking power is gone. I have seen several manufacturers use a DC injection brake to stop a hoist, then set the mechanical brake to hold it. The DC Injection brake just serves to lengthen the lifespan of the mechanical brake. The problem with this is that when the motor starter is opened, triggerring the DC injection brake to energize, there needs to be a time delay of at least 25-50ms to allow the motor field to colapse before energizing the DC, otherwise the SCRs will short. If the hoist has a high gear ratio, the hoist load can drop during that delay. I have only seen it work on a hoist with a 300:1 gear ratio. That which does not kill me, makes me stronger... and pissed off!
