What is Safe Torque Off in Variable frequency drive?

Safe Torque Off (STO) is a method the variable frequency drive (VFD) is disabled with no current or voltage going to the motor. Generally used in E-Stop schemes. The motor will coast to stop. Also the STO circuit has been qualified by a certified agency (the variable frequency drive has to meet certain requirements). The STO disables the output IGBT devices and is an integral part of the VFD hardware design.

Most implementations of STO do not completely open the vfd outputs. With STO the variable frequency drive output current is stopped, causing the motor to coast. Usually power is still connected to the variable frequency drive inputs and the motor is connected. The motor power leads do have a connection to dangerous voltages. The standard H-bridge output has power on the top and bottom rails. The built in fly back diodes are connected to the rails. So if you touched the power lead you would receive a shock and if you shorted a motor power lead to ground you would be creating a large short.

Note: The Safe torque off function can be used for stopping the vfd in emergency stop situations. In the normal operating mode, use the Stop command instead. If a running variable frequency drive is stopped by using the function, the variable frequency drive will trip and stop by coasting. If this is not acceptable, e.g. causes danger, the VFD and machinery must be stopped using the appropriate stopping mode before using this function.