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AC VFD Safe Torque Off (STO) Function 



What is the Safe Torque Off Function for on AC VFD's?


Generally used in E-Stop schemes, Safe Torque Off (STO) is a Safety Control method for the Variable Frequency Drives (VFD's), of disabling the VFD, with no current or voltage going to the Motor. The Motor will coast to stop, when the STO disables the VFD output IGBT devices.
Safe Torque Off (STOis an integral part of the Unitronics VFD hardware design, as an Option .
Most implementations of STO do not completely open the VFD outputs. With STO the Variable Frequency Drive (VFD) output current is stopped, causing the Motor to coast. Typically power is still connected to the VFD inputs and the Motor is also connected. 
Note: The Motor power leads do have a connection to dangerous voltages.

The Safe Torque Off (STO) function should be used for stopping the VFD in Emergency Stop situations and Safety Applications!  
In the normal operating mode, the Stop command would be used instead of an E-stop for the the STO function. If a running Variable Frequency Drive is stopped by using the Stop function, the VFD will trip and stop by coasting. 
If this is not acceptable, e.g. causes danger, or is hazardous to Personnel, the VFD and machinery must be stopped, using the appropriate stopping mode such as Dynamic Braking

MD&G+Controls LLC

Ray W Zimbal Jr. 
 
Automation Application Engineering and Sales 
 
 
Dallas-Fort Worth & N TX Area 
1-817-307-1274 
 
Houston & Texas Gulf Coast Area 
1-713-835-9753 answered 24/7 
 
alternate e mails:
 

 
 
Motors, Drives and Gears plus Controls 


VFD Safe Torque Off (STO) Function Details 

The SAFE TORQUE OFF Function provides a means for preventing the Variable Frequency Drive from generating torque in the motor, with a very high level of integrity. It is suitable for incorporation into a Safety System for a machine. It is also suitable for use as a conventional drive enable input. The Safety function is active when the STO input is in the logic-low state as specified in the control terminal specification. The function is defined according to EN 61800-5-2 and IEC 61800-5-2.

The SAFE TORQUE OFF Function makes use of the special property of a VFD with an induction Motor, which is that Torque cannot be generated without the continuous correct active behavior of the VFD circuit. All credible faults in the VFD power circuit cause a loss of Torque generation.

The SAFE TORQUE OFF Function is fail-safe, so when the SAFE TORQUE OFF input is disconnected the Drive will not operate the Motor, even if a combination of components within the Drive has failed. Most component failures are revealed by the Variable Frequency Drive failing to operate. SAFE TORQUE OFF is also independent of the VFD firmware. This meets the requirements of the following IEC standards, for the prevention of operation of the motor. 


SAFE TORQUE OFF can be used to eliminate Electro-Mechanical Contactors, including special Safety Positive Guided AEG Brand of Contactors From: EE Controls, which would otherwise be required for Safety applications. 

STO Function Operational Details:

When the VFD is disabled through SAFE TORQUE OFF, a possible (although highly unlikely) failure mode is for two power devices in the Inverter circuit to conduct incorrectly. This fault cannot produce a steady rotating Torque in any AC Motor. It produces no Torque in a conventional induction Motor with a cage rotor. If the rotor has permanent magnets, then a transient alignment Torque may occur. The Motor may briefly try to rotate by up to 180° electrical, for a permanent magnet Motor, or 90° electrical, for a salient pole induction Motor or reluctance Motor. This possible failure mode must be allowed for in the machine design.

SAFE TORQUE OFF inhibits the operation of the VFD, this includes inhibiting braking. If the VFD is required to provide both braking and SAFE TORQUE OFF in the same operation (e.g. for emergency stop) then a safety timer relay or similar device must be used to ensure that the drive is disabled a suitable time after braking. 
NOTE: The braking function in the Variable Frequency Drive is provided by an electronic circuit, which is not fail-safe and if braking is a safety requirement, it must be supplemented by an independent fail-safe braking mechanism.

SAFE TORQUE OFF does not provide electrical isolation. 
The supply to the VFD must be disconnected by an approved isolation device before gaining access to power connections.

MD&G+Controls LLC

Ray W Zimbal Jr. 
 
Automation Application Engineering and Sales 
 
 
Dallas-Fort Worth & N TX Area 
1-817-307-1274 
 
Houston & Texas Gulf Coast Area 
1-713-835-9753 answered 24/7 
 
alternate e mails: