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Typical safety functions

Safe torque off (STO)

STO is the required basic foundation for drive-based functional safety, since it brings a drive safely to a no-torque state. STO is typically used for a prevention of an unexpected startup (EN 1037) of machinery or for an emergency stop, fulfilling stop category 0 (EN 60204-1).


Figure STO. Upon activation STO immediately switches off the drive output to the motor. Motor speed then coasts to a stop.

Safe stop 1 (SS1)

This function stops the motor safely, using a controlled ramp stop and then activates the STO function. SS1 is typically used in applications like rolling mills where motion must be stopped in a controlled manner before switching to a no-torque state. In addition to a safe process stop, SS1 can also be used to implement an Emergency stop, fulfilling stop category 1 (EN 60204-1).


Figure SS1. When activated, SS1 will ramp motor speed down to a standstill and then activate the STO function.

Safe stop emergency (SSE)

This safety function is specifically designed for emergency stops. SSE can be configured to execute either STO or SS1 depending on which emergency stop is suitable for the system. For examples of this functionality see figures for STO and SSE

Safely-limited speed (SLS)

This function prevents motors from exceeding a defined speed limit. The SLS safety function can be used in applications such as decanters, mixers, conveyors or paper machines where excess speed can be hazardous during ie. maintenance or cleaning operations.


Figure SLS. Upon activation, SLS will monitor that motor speed does not exceed a defined level. If it is exceeded, SLS will activate STO or SSE to stop the drive.

Safe maximum speed (SMS)

This function is a variant of the SLS-safety function. It provides continuous protection against a motor exceeding a defined maximum speed limit.


Figure SMS. When SMS is used, it is always active and ensures that the set speed limit is not exceeded (ie. maximum allowed speed).

Safe brake control (SBC)

This functions provides a safe output signal to control a mechanical holding brake. Drills, cranes, winches, hoists, vertical conveyors and elevators that need external brake solutions require this type of safety function. Typical use for SBC is when a drive is switched off with STO function and there is an active load affecting the motor (eg. a hanging load on a crane/winder).


Figure SBC. BC provides a safe control signal to operate the mechanical brake. 

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