Variable frequency drive insights

• Certified safety variable frequency drives (VFDs) include features that can be implemented within the design of a production line to improve productivity.
• It’s important to conduct a safety risk assessment ahead of installing a safety VFD.
• Sensors must be selected to directly monitor equipment at the specific locations where failure could lead to unsafe conditions.

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Variable frequency drives (VFDs) are widely used within industrial manufacturing because of the ability to precisely and dynamically control a motor’s speed and torque. This capability results in improved energy efficiency and reduced operational cost, as well as process adaptability.

However, the capabilities of VFDs go far beyond these conventional improvements. Certified safety VFDs include additional features that can be implemented within the design of a production line to significantly improve productivity. These VFDs allow for the creation of functionally safe states, allowing equipment to operate safely even when personnel are nearby. This optimizes productivity and has the potential to reduce the overall footprint of the system.

The unique features of safety VFDs

Without a VFD, a motor’s only safe state is the off position — a restriction that may suffice for some applications but is limiting in others. Powering down a motor to ensure personnel safety can be disruptive and reduce productivity, especially when done frequently. This issue is minimized by using safety VFDs to design safe operating states for motor-driven applications.

Industry divides a safety VFD’s features into three broad categories:

• Safe stop
• Safe brake
• Safe motion

Some features of safety VFDs and the potential applications include:

Safe torque off (STO)

Important for the prevention of accidental starts, STO directly interrupts the power flow to the motor, immediately disabling its ability to generate torque. This is equivalent to Safe Stop Category 0. The motor will continue to turn due to its own inertia until it naturally comes to a stop. STO keeps the power supply to the VFD intact while disabling the insulated-gate bipolar transistors (IGBTs) within the VFD’s inverter such that no alternating current (ac) output signal is generated. This operation physically disconnects the motor, keeping it nonoperational while also ensuring the system is ready for restart. This offers an advantage over standard VFDs, which may require a complete power cut-off and reboot time.

Safe stop 1 (SS1)

When SS1 is initiated, the VFD actively controls the deceleration of the motor from its operational speed to a predefined safe speed before removing power via STO. This is equivalent to safe stop category 1. This is done by adjusting the frequency and voltage supplied to the motor, based on its current speed and torque from encoder feedback. This feature is particularly useful for conveyor systems where gentle stops prevent damage to transported materials. 

Safe stop 2 (SS2)

Like SS1, SS2 also controls the motor’s deceleration to a stop. However, unlike SS1, after the motor reaches the stop, the VFD maintains power to the motor, keeping it energized while monitoring the motor’s speed to ensure it remains stationary. This is equivalent to safe stop category 2. SS2 controls the frequency and voltage applied to the motor, based on speed and position information from an encoder. It can maintain the motor in a ready state without motion, allowing for rapid resumption of activity when necessary. This precise control and continuous monitoring make SS2 particularly effective for applications where maintaining exact positioning is critical, such as in robotics.

Safe braking

This safety feature manages a motor’s deceleration to ensure a controlled and safe stop. It controls the frequency and voltage supplied to the motor for deceleration, but additionally controls external braking resistors and/or regenerative braking circuitry for faster deceleration and to dissipate or transfer energy efficiently. It is always used with STO. As an example, safe braking is used in elevator applications. It keeps the brake engaged to hold the elevator in position while a motor comes up to torque before releasing the brake.

Safe-limited speed (SLS)

This function allows the VFD to limit the maximum speed of the motor to a predetermined safe level based on encoder feedback. This feature is particularly useful in conveyor, mixer and automated guided vehicle applications, where excessive speed can lead to safety hazards or equipment damage.

Safe direction

Safe direction prevents the motor from running in an unintended direction. The VFD uses encoder feedback to monitor and then control the motor’s direction, ensuring it only rotates in the specified direction. This feature is used for controlling winding/unwinding operations where reversing direction unintentionally could cause accidents or material damage.

Safe-limited torque (SLT)

This function limits the motor’s torque to a predefined safe quantity, ensuring that the motor does not exert excessive force. Feedback from the motor and possibly an external torque sensor is used to continuously adjust the power output. In the context of safety, it is useful in collaborative environments where humans and machines interact closely. By implementing SLT in a conveyor application, for example, the torque can be sufficiently limited so personnel can work within its vicinity without an entanglement risk.

Safe-limited position (SLP)

This safety feature constrains the motor to operate within predefined positional limits, preventing it from exceeding or falling short of specific locations. SLP is used in applications where positional accuracy is critical to the process’s integrity, such as robotics. In the context of safety, SLP can be used to prevent machinery from entering hazardous zones or performing movements that could endanger personnel.

How to design VFDs for functional safety

In many circumstances, a safe operating state can effectively address a hazard without stopping the line. The features of safety VFDs are often foundational to the system’s design.

Designing functionally safe states of system operation generally begins with a safety risk assessment, systematically identifying all hazards, assessing the likelihood and evaluating the potential consequences. Each identified hazard is then analyzed to define the required safety measures that mitigate the hazard to an acceptable level. This information is documented within the safety requirement specification.

In many cases, a safe state of operation can address an identified hazard, rendering it safe while maintaining motion. For example, setting limits on speed or torque or implementing a slow start (limited acceleration) all offer the potential to effectively render specific hazards safe without stopping production. Safety standards, such as ISO 13849, can provide design guidance to ensure that hazards addressed in a functional manner remain in compliance with safety regulations. By adhering to them, engineers can develop robust safety measures tailored to specific operational states and hazard levels.

Some examples that employ the features built into a safety VFD are listed below. In many circumstances, multiple safety features may be needed to effectively address a hazard.

• Maximum safe speed: While application-specific, 250 millimeters/second is frequently considered a safe maximum operating speed for most equipment within human proximity. The safe limited speed feature within a safety VFD can be used to design a system where equipment remains in motion at reduced rates while in the presence of personnel.
• Safe limited torque: Applicable to various machinery, safe limited torque is considered a critical parameter for ensuring the safety of personnel working close to moving equipment to avoid entanglement. A safety VFD can restrict the maximum torque to recognized safe levels that allow a human to stop a machine, such that a system remains functional yet safe in circumstances where direct human interaction with the machinery is necessary.

Choosing the right safety controller for specific tasks

The safety features of VFDs can be integrated into a system using three main approaches, each suitable for different levels of system complexity:

1. Safety programmable logic controllers (PLC) for comprehensive control

A safety PLC is a common means of integrating the safety features of a VFD within a larger system. The safety PLC can be programmed to monitor input from several monitoring devices including encoders, torque sensors and area proximity sensors. It can then interface with one or several VFDs and other devices to maintain safe system operation.

2. Safety controllers for moderate complexity

A safety controller can effectively manage mid-level complexity applications. It can interface with a limited number of monitoring devices to manage the system safety. 

3. Safety relays for basic safety functionality

While a safety relay offers limited functionality, it may be suitable for meeting simple safety requirements. For example, a company recently installed a safety VFD that required the SS1 feature to allow one second for the drive to come to a full stop and then halt power upon detecting a safety trigger. Because the application was simple, the company specified a safety relay to meet its needs.

How to test and verify a functional system’s safety

The placement of sensors must be strategically selected to directly monitor equipment at the specific locations where failure could lead to unsafe conditions. For instance, if an encoder is used to monitor the speed of a motor driving a shaft, it likely should not be mounted on the motor itself, as doing so may lead to a missed hazard. Should the shaft break, the motor-mounted encoder would continue to indicate safe conditions, leaving the safety logic of the system blind to the unfolding hazardous situation.

Monitoring torque at an endpoint also requires a careful approach. Depending on the system’s design, the endpoint torque may be different from that of the motor. Torque levels can be monitored with torque sensors to verify that the limits set within the safety VFD meet the safety requirements at the endpoint.

 The reliability of certified safety VFDs

Like other equipment, safety VFDs that have been certified by a Nationally Recognized Testing Laboratory (NRTL) have undergone rigorous testing to verify that they reliably operate as specified. Successfully tested products receive a certification mark, such as the commonly seen TÜV stamp, which attests to the safety feature’s reliability and compliance with regulatory standards. Since many VFD manufacturers originate in Europe, the NRTL certification is particularly important for this equipment as it ensures compliance with North American safety regulations, which may differ from European standards.

Example 1: A safety VFD controls a calendar machine, minimizing downtime and eliminating fencing.

A tire company recently improved the productivity of a large calendar machine using the safe limited speed feature of a safety VFD. The design also eliminated the need for safety fencing, therefore reducing its overall footprint.

The calendar machine was originally driven by a motor without a VFD and had a large fenced-off area surrounding it to maintain safety. Personnel were prohibited unless they requested entry, which then required a full power down of the system. Additionally, the system’s restarting process was lengthy, resulting in significant downtime whenever someone needed access to the equipment.

Using the safe limited speed feature of a safety VFD, a more flexible safety solution for the calendar machine was designed and implemented. If the calendar machine could always be stopped such that an individual within its proximity would never be injured, then safety fencing would not be required (see Figure 1).

The safe speed of operation was therefore defined dynamically, based on the current proximity of personnel to the machinery. Production could continue at reduced rates as defined by the proximity of personnel to the machine and its stopping time based on its current speed of operation. A proximity detection sensor was implemented so that a person’s distance from the hazard could be continuously monitored.

As an individual approaches the machine, the safe limited speed quantity is reduced to allow sufficient stopping time if this individual begins running toward it. Once an individual is sufficiently close, the machine stops entirely. As an individual moved away from the machine, the machine safety returned to production speed.

By allowing the equipment to run at reduced speeds in the presence of personnel, this setup minimized downtime therefore increasing overall productivity. In addition, the need for physical barriers was eliminated, freeing up floor space.

Example 2: A modular safety VFD system controls an extruder in series with winders.

The features of safety VFDs are independent of the power source, allowing for use in modular applications.

A plastics company recently implemented a modular safety VFD solution to control a large extruder in series with several winders. Because modular VFDs allow the power generated by one motor’s deceleration to be returned to the common direct current (dc) bus to drive other motors, energy is used more efficiently.

For this application, a large 600 horsepower extruder drive was used to generate power on the common dc bus for use by the windup drives. Each VFD incorporated the STO feature, allowing for stopping flexibility. The system was designed so that an STO could be used to stop an individual drive or all drives simultaneously as required.

Unlocking the potential of safety VFDs

Safety VFDs hold tremendous potential within industrial manufacturing for remarkable system design and layout flexibility. The features of certified safety VFDs can be reliably used to design functionally safe operating states for production lines that would otherwise need to be powered down when personnel are nearby.

By integrating these advanced drives, manufacturers can more often maintain production while remaining in compliance with safety standards, reducing downtime and therefore increasing overall productivity. This makes them a valuable tool for advancing more innovative, less complex and efficient manufacturing processes.

Hargrove Controls & Automation is a certified member of the Controls System Integrators Association (CSIA). For more information, visit the company profile on the Industrial Automation Exchange.

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