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6-2
Safe Torque Off
AC30 series Variable Speed Inverter
STO FUNCTIONAL DESCRIPTION
STO is a means of preventing an inverter from delivering rotational force to its connected electric motor. Please refer to EN61800-5-2:2007 para 4.2.2.2
for the formal definition.
To ensure a high degree of safety, two independent STO control channels are implemented in hardware. The STO circuit in the inverter is designed
such that a fault in one control channel will not affect the other channel’s ability to prevent the drive from starting, i.e. the STO function of the inverter is
tolerant to any single fault. It may not be tolerant to an accumulation of faults. This is in keeping with its declared safety ratings.
STO always overrides any attempt to start the drive. If one or both STO control inputs is requesting the STO function, the drive will not start, even if for
example, the drive’s software malfunctions and tries to cause the motor to turn.
The STO function is implemented in hardware; it overrides all software activities. The only software involvement is to report STO status to the user via a
Graphical Keypad (GKP), serial communications link or user terminal as defined by the drive configuration.
WARNING
THE DECLARED SIL/PL CAPABILITY OF THIS STO PRODUCT CAN BE ACHIEVED ONLY WHEN THE TWO STO USER INPUTS ARE
DRIVEN INDEPENDENTLY. THEY MUST NOT BOTH BE DRIVEN FROM A COMMON SOURCE; OTHERWISE THE SINGLE FAULT
DETECTION WILL BE COMPLETELY INOPERATIVE.
USE OF THE PRODUCT IN THIS “COMMON SOURCE” CONDITION INVALIDATES THE STO PRODUCT SPECIFICATION AND IS
ENTIRELY AT THE USER’S OWN RISK.