What Is Safe Torque Off? STO Explained for Industrial Drives.

What Is Safe Torque Off?

Safe Torque Off, or STO, is a safety-related function that prevents a compatible motor drive from generating torque through its normal motor-power control.

STO is commonly used in industrial drives, manufacturing equipment, and robotic systems.

For example, a production machine may use STO as part of a protective function intended to prevent hazardous motor-driven movement.

STO is an important functional safety capability, but it does not physically disconnect every source of electrical power or guarantee that a machine cannot move.

Key Takeaways

• STO prevents a compatible drive from generating motor torque through a safety-related function.
• It does not necessarily remove hazardous electrical voltage or mechanical stored energy.
• STO must be applied according to the machine's risk assessment and the drive manufacturer's safety requirements.

How Does Safe Torque Off Work?

Imagine an industrial machine powered by a motor drive.

The drive controls semiconductor switching devices to produce the electrical signals required for motor torque.

When STO is activated, the drive's safety-related circuitry prevents the normal generation of torque-producing output.

Depending on the drive design, this may be achieved by disabling the control signals required to operate the power switching devices.

The motor can no longer produce commanded torque through that drive function.

However, the machine's mechanical load may continue moving because of inertia, gravity, or external forces.

This is why STO is not equivalent to mechanically locking the machine.

STO vs. Emergency Stop

An emergency stop is a protective function initiated by an emergency-stop device.

STO is a particular drive safety function that may be used to implement part of the required stopping response.

For example, pressing an emergency-stop button might cause the safety system to initiate a defined stop sequence.

STO may then be activated.

The actual sequence depends on the hazards and safety design.

An emergency stop does not always mean STO is activated immediately.

STO vs. Safe Stop 1

Safe Torque Off: Prevents the drive from generating motor torque.

Safe Stop 1 (SS1): Initiates a controlled stopping sequence followed by STO under the applicable function definition.

For example, a moving conveyor may need to decelerate before torque is removed.

Using STO immediately could allow uncontrolled coasting.

SS1 can provide a suitable controlled-stop arrangement when supported and correctly designed.

The appropriate function depends on the machine's stopping requirements.

Why Is STO Not Always Suitable for Vertical Axes?

Consider a robotic mechanism holding a heavy load above the floor.

The motor produces torque to oppose gravity.

If STO removes the ability to generate torque, the load may fall unless another mechanism holds it.

A brake, counterbalance, or other protective arrangement may therefore be required.

STO alone does not guarantee that suspended loads remain stationary.

Does STO Make Electrical Maintenance Safe?

No.

A drive can retain hazardous electrical voltage even when STO is active.

STO is not a substitute for electrical isolation, lockout/tagout, or other required maintenance procedures.

This distinction is particularly important because the word off can create the mistaken impression that the equipment is fully de-energized.

How Is STO Used in Robots?

Robotic systems may use STO in connection with protective stopping arrangements or other safety-related functions.

However, a multi-axis robot can contain several drives and mechanical systems.

Engineers must evaluate how removing torque affects each axis and the complete machine.

For example, some joints may require brakes or additional protective measures to manage gravity loads.

Frequently Asked Questions

Does STO instantly stop a rotating motor?

No. It removes torque-producing capability, but the motor and connected load may continue moving.

Does STO disconnect electrical power from the drive?

No. STO should not be treated as electrical isolation.

The Bottom Line

Safe Torque Off is a defined drive safety function that prevents motor torque generation. Its effectiveness depends on how the machine behaves after torque is removed and whether other hazards, including gravity and stored energy, have been addressed.