Product Overview

The Allen Bradley 20-750-S PowerFlex 750 Safe Torque-Off Option Module is a safety option for compatible PowerFlex 750-Series AC drives. Its purpose is to prevent the drive from producing motor torque when the Safe Torque Off (STO) function is activated. This allows machine designers to implement safety-related stop functions without relying solely on the drive’s ordinary operational stop command.

In conventional drive control, a stop command instructs the drive to change its operating state according to its configured control logic. STO serves a different purpose: it disables the drive’s ability to generate torque through its motor-control output stage. This distinction matters when machinery must enter a defined safe state during access, maintenance, or a safety-system intervention.

The 20-750-S connects to a supported option port on the host drive and uses dedicated safety input wiring. The module receives signals from an external safety circuit, such as a suitable safety relay or safety controller, and controls the STO function according to its safety architecture. The connected safety circuit must be designed to meet the required performance level and applicable machinery safety standards.

The module supports safety applications up to Category 3, Performance Level e (PL e) under EN ISO 13849-1, and SIL CL3 under applicable functional safety standards. These ratings describe the capability of the safety function under the required conditions; they do not automatically establish the safety rating of the complete machine. The external safety circuit, drive configuration, wiring, validation, and maintenance procedures must all be appropriate for the intended application.

The 20-750-S uses separate safety power and safety-enable connections. The documented interface uses 24V DC signals, with the safety input circuits supplied by a suitable protected extra-low-voltage source. Correct separation of the safety channels, wiring integrity, and appropriate fault monitoring are important to the overall design. A basic PLC output should not be substituted for the required safety-rated control architecture.

The module is intended for use with compatible PowerFlex 750-Series drives, including supported PowerFlex 753 and PowerFlex 755 installations and applicable PowerFlex 755T drive configurations. The host drive’s model, firmware, option-port availability, and application requirements must be checked before selecting the module. Although the 20-750-S can be installed in supported option ports, integrated-motion applications may have specific port-placement requirements.

When STO is activated, the drive prevents the production of motor torque through the safety function. It does not provide electrical isolation from the incoming supply, discharge the DC bus immediately, or guarantee that a rotating load stops instantaneously. A motor may continue to coast, and external mechanical forces can still move the equipment. Applications involving suspended loads, stored mechanical energy, or hazardous motion may require additional braking, isolation, or safety measures.

The 20-750-S is a dedicated safety option rather than a general-purpose communication module. It does not provide the networked safety architecture associated with newer integrated safety options such as the 20-750-S3. Its safety function is implemented through the dedicated safety interface and the compatible drive’s STO capability.

During installation, the drive must be de-energized and the prescribed electrical safety procedures followed. The option module must be seated in a supported port, secured correctly, and wired according to the applicable terminal designations. Safety commissioning must include verification of both the intended stop response and the restoration behavior after the safety circuit is reset.

Typical applications include conveyor systems, packaging machinery, automated assembly equipment, material-handling installations, and other machines where a drive-controlled axis must be brought into a defined torque-disabled state during a safety intervention. The required safety architecture depends on the machine’s hazards, stopping characteristics, and risk assessment.

The Allen Bradley 20-750-S PowerFlex 750 Safe Torque-Off Option Module adds a dedicated drive-based safety function to compatible PowerFlex 750-Series systems, helping engineers implement safety-related torque inhibition while retaining the host drive’s normal motor-control capabilities.

Technical Specifications

Product Features

• Drive-based Safe Torque Off: Disables torque production when the configured safety function is activated.
• Category 3 capability: Supports safety applications designed to the applicable Category 3 requirements.
• PL e and SIL CL3 capability: Supports appropriately engineered safety functions at these performance and integrity levels.
• Dedicated safety inputs: Separates safety-enable and safety-power connections from ordinary drive-control commands.
• 24V DC safety interface: Uses a compatible external safety supply for the documented safety input circuits.
• PowerFlex 750-Series integration: Adds STO functionality to supported host drives without replacing the drive’s motor-control functions.
• Supported option-port installation: Fits compatible drive option ports, subject to the host-drive configuration.
• No network dependency for STO activation: Uses its dedicated safety interface rather than requiring a networked safety command.
• Safety-system integration: Can be incorporated into an appropriately designed safety relay or safety-controller architecture.
• Defined commissioning requirements: Allows the safety response and reset behavior to be verified as part of machine validation.

Working Principle

The 20-750-S operates by controlling the Safe Torque Off function of the compatible PowerFlex drive. It receives safety-related signals through its dedicated terminal interface and, when the STO condition is activated, causes the drive to inhibit torque production.

Safety Device → 20-750-S Safety Inputs → Drive STO Function → Torque Production Inhibited

During normal operation, the external safety circuit supplies the required signals to the module’s safety inputs. The module and host drive operate according to their configured safety conditions, allowing the drive to perform its normal motor-control duties while the safety function remains available.

When a safety device detects a hazardous condition or an operator activates an emergency stop, the external safety circuit changes the relevant safety signals. Depending on the system design, this may result from an emergency-stop device, guard interlock, or safety controller. The 20-750-S responds to the safety input condition and activates the drive’s STO function.

Once STO is active, the drive’s power output is controlled so that it cannot produce motor torque through the normal drive-control process. The motor may coast to a stop because the STO function does not itself apply a mechanical brake. The stopping time therefore depends on factors such as motor inertia, load characteristics, friction, and any additional braking equipment.

The safety-power and safety-enable circuits serve distinct roles in the option’s safety interface. The safety-power input supplies the designated 24V DC circuit, while the safety-enable input participates in the control of the STO function. Their wiring must follow the prescribed terminal assignments and safety architecture. The two circuits should not be treated as interchangeable ordinary digital inputs.

The module works in conjunction with the host drive’s internal safety-related functions. It does not independently measure every machine hazard or determine whether an area is safe for personnel. Those responsibilities belong to the overall machine safety system, including sensors, logic, wiring, and any additional devices required by the risk assessment.

After the hazardous condition has been addressed, the safety circuit may permit the STO function to be reset according to the designed procedure. Resetting STO should not automatically cause unexpected machine movement. The application must account for the drive’s start-command behavior, restart interlocks, and any required manual reset or deliberate restart action.

Commissioning verifies that the safety input transitions produce the intended drive response. Tests should include activation, channel behavior where applicable, fault handling, reset behavior, and confirmation that the drive does not restart unexpectedly. The safety function must be validated as part of the complete machine rather than judged solely by whether the option module appears to operate.

Through this dedicated safety interface, the 20-750-S provides a defined method for inhibiting drive-generated torque while allowing the PowerFlex drive to retain its normal control functions when the safety conditions permit operation.

Role in the Industrial Control Architecture

The 20-750-S occupies the drive-safety layer of an industrial control system. It receives signals from the safety architecture and controls the host drive’s STO function independently of ordinary process-control commands.

Emergency-Stop Devices / Guard Interlocks → Safety Relay or Safety Controller → 20-750-S → PowerFlex Drive → Motor

The safety system and standard PLC have different responsibilities. Ordinary control logic must not replace the required safety-rated architecture, and STO must not be treated as a substitute for electrical isolation during servicing.

Industrial Applications

Installation and Maintenance

1. Verify the exact drive model: Confirm that the host drive supports the 20-750-S and that the selected option port is permitted for the application.
2. Isolate electrical energy: De-energize the drive and follow the prescribed lockout and verification procedures before installing or removing the module.
3. Check the option-port requirements: Confirm the permitted installation position and observe any port restrictions associated with integrated-motion applications.
4. Inspect the module connector: Check the module interface and mounting points for damage before installation.
5. Secure the option module: Seat the connector fully and tighten the retaining screws to the specified torque using the appropriate tool.
6. Use the correct safety supply: Provide the required 24V DC safety power within the documented voltage tolerance and use a suitable protected extra-low-voltage source.
7. Follow the terminal assignments: Wire the safety-power, safety-enable, common, and shield connections according to the applicable installation instructions.
8. Separate safety and power wiring: Maintain the required separation between control wiring and drive power conductors, and follow the prescribed shielding and grounding practices.
9. Verify the safety architecture: Confirm that the external safety relay or controller, input devices, output circuits, and wiring meet the required safety design.
10. Test the STO response: Verify that activation of the safety function inhibits torque as intended and that the resulting machine behavior meets the risk assessment.
11. Check reset and restart behavior: Confirm that restoring the safety inputs does not cause unexpected motion and that the intended restart procedure operates correctly.
12. Document validation results: Record the installed option, wiring configuration, safety test results, drive parameters, and maintenance requirements for future inspections.

If the drive does not respond as expected, check the safety input wiring, external safety supply, module installation, and host-drive compatibility. Do not bypass the safety circuit to restore production. Investigate faults using the prescribed troubleshooting procedure and repeat the required validation after corrective work.

Related Components

Recommended Related Models

Key Advantages

• Provides a dedicated Safe Torque Off function for compatible PowerFlex 750-Series drives.
• Supports appropriately engineered Category 3, PL e, and SIL CL3 safety applications.
• Uses dedicated 24V DC safety inputs rather than relying on ordinary process-control commands.
• Integrates with suitable external safety relays and safety controllers.
• Allows drive torque inhibition without replacing the host drive’s normal motor-control functions.
• Supports safety-related machine stopping architectures when correctly installed and validated.

Technical FAQs

Does the 20-750-S stop the motor immediately when STO is activated?

No. STO inhibits drive-generated torque, but the motor may continue to coast under its existing momentum or external mechanical forces. If a defined stopping time or holding function is required, the machine may need an additional braking or safety arrangement.

Can the 20-750-S replace an emergency-stop switch or safety relay?

No. The module implements the drive-side STO function. Emergency-stop devices, guard interlocks, and the required safety logic must still be selected and configured as part of the complete safety architecture. The overall system must be validated against the applicable risk assessment and standards.

What is the difference between the 20-750-S and the 20-750-S3?

The 20-750-S uses a dedicated safety input interface for the STO function. The 20-750-S3 is an integrated safety option for supported PowerFlex 755/755T drives and uses a different safety architecture and terminal arrangement. They are not interchangeable without checking drive compatibility, wiring, configuration, and safety requirements.

What must be verified after replacing a 20-750-S module?

Confirm the module’s compatibility and installation, inspect the safety wiring, and verify the external safety supply. Then test the STO activation response, fault handling, reset behavior, and prevention of unexpected restart. The safety function must be revalidated after replacement before the machine returns to normal operation.