Safe Torque Off (STO) is an essential safety feature integrated into motor drives. Its primary function is to halt the motor’s torque generation without completely cutting off the power supply to the motor drive. This ensures both motor and system protection during shutdown, providing a fail-safe mechanism without compromising the configuration or position data. STO plays a significant role in preventing unintended motion and enhancing the safety of machinery and drive systems.
Below is a detailed overview of the STO function, its operation, and its significance in motor drive systems.
What is STO?
STO stands for Safe Torque Off, a safety feature integrated into motor drives such as DC variable speed drives, AC variable frequency drives, servo drives, and stepper drives. When STO is activated, the energy supply responsible for generating motor torque is cut off, ensuring the motor no longer produces any force or motion.
Motor drives are commonly used in a wide range of industrial applications, and safety is a major concern when working with these drives. Traditionally, safety was ensured by interrupting power to the motor drive using contactors. However, this method led to data loss or even damage, particularly under high load conditions. STO overcomes this by integrating safety mechanisms directly into the drive, providing a safer and more reliable solution for stopping motor torque while maintaining essential power for the drive’s control systems.
STO Working Principle
STO works by disabling the motor drive’s Insulated Gate Bipolar Transistor (IGBT) output circuit, which prevents current from reaching the motor. This is accomplished through the opening of the STO safety circuit connected to the gate of the power device. With the circuit open, no torque is produced by the motor, effectively stopping any motion.
The STO function typically utilizes a two-channel normally closed circuit on the drive. If one or both channels open, STO is activated, cutting off the current to the motor and preventing torque production. In this state, the motor will stop due to inertia. For the drive to restart, the STO circuit must be reset, ensuring both channels are closed, followed by a reset or restart sequence.
This design is considered hardware fail-safe because the safety circuit involves no software, relying entirely on hardware mechanisms to stop the motor.
Standards and Compliance for STO
STO functionality is recognized and standardized in several industrial safety standards. Two key standards related to STO include:
- IEC 60204-1: This standard addresses the safety of machinery, specifically focusing on mechanical and electrical equipment. STO is defined as a function for preventing accidental motor startups.
- IEC 61800-5-2: This standard categorizes STO as a safety feature that ensures stopping in category 0, which refers to immediate cessation of motion without delay.
While STO provides a crucial layer of safety, the overall safety system incorporating STO must be analyzed and evaluated according to ISO-13849-1 and ISO-13849-2 to ensure that the required level of safety protection is achieved.
Important Considerations for STO Usage
Though STO stops the motor from generating torque, it is important to remember that the motor drive’s output circuit contains solid-state components. As such, voltage may still be present at the output terminals even after STO has been activated. To avoid electrical shock, it is crucial not to touch the drive or any of its components once the STO function is in effect.
STO is a reliable and efficient method for ensuring motor drive safety, offering a hardware fail-safe solution that helps prevent motion and maintain the system’s integrity. Its integration into modern motor drives has become a standard practice in many industrial applications where safety is of the utmost importance.
At GTAKE, we integrate cutting-edge technologies into industrial variable frequency drives, servo drives, winder power converters, solar inverters for the industrial and clean energy markets. We also offer HEV/EV motor controllers for automobiles with superior efficiency, reliability and passenger comfort.Feel free to get in touch with us. We would love to hear from you.
