Two contactors in series ahead of every drive was the old orthodoxy. Integrated safety functions retired it — cheaper, more capable, and better diagnosed — provided you know what each function promises.
The core functions
STO (Safe Torque Off) removes the drive’s ability to generate torque. The motor coasts; nothing rotates actively. It is the foundation everything else builds on and the direct replacement for the contactor pair. Note what STO does not do: brake, hold a suspended load, or stop quickly.
SS1 (Safe Stop 1) brakes actively down a monitored ramp, then triggers STO. This is what most machinery actually wants at a guard door: fast controlled stop, then safe.
SLS (Safely Limited Speed) monitors that speed stays under a limit — enabling setup modes where an operator works near motion legally and sanely. Cousins: SSM (safe speed monitor/signal), SDI (safe direction), SBC (safe brake control) for the hoist world.
Engineering notes from the field
Encoderless variants of the monitoring functions exist and are genuinely useful, with envelope limitations — check the function manual against your worst-case load behavior. Wiring: terminals (F-DI) for simple machines, PROFIsafe for anything with a safety PLC — retrofits love that the safety wiring becomes a telegram. And validation is not optional: every safety function gets a documented functional test with measured stop times feeding back into the risk assessment’s distance calculations.
FAQ
Does STO need output contactors at all anymore? Per the drive’s certification, no — that is the point. Some risk assessments still add one contactor for other reasons (isolation philosophy, brake handling).
Can I retrofit STO onto an old drive? Onto the machine, by replacing the drive with a safety-capable one — usually the moment the whole safety circuit gets the redesign it needed anyway.
Zone Otomasyon designs, implements and validates drive safety as part of retrofits. Safety engineering.