Variable Frequency Drives: Managing Network Outage Responses
Even with the most reliable networks, network outages are inevitable for a host of reasons, and managing these outages in VFD systems becomes critically important.
VFDs, or variable frequency drives, are used to control three-phase motors with ease. They are also called variable speed drives or adjustable frequency drives. They have the ability to contribute significantly to saving energy. This benefit shows up as cost savings for your organization. VFDs also contribute to extending equipment lifetime. They also help in converting single-phase power to three-phase output for driving relatively small motors, increasing their usefulness in various situations where a three-phase supply is unavailable.
The most significant benefit of using VFDs is the granularity of control possible by using them. This makes it favorable for use in process control applications. VFDs control the speed and operation of motors by varying the frequency of the alternating current supplied to each of the phase supply lines. The operation and role of VFDs in motor control is discussed in detail in this article: Variable Frequency Drives (VFDs) In Motor Control.
Figure 1. Industrial 3-phase motors with VFD controls. Image used courtesy of Canva
VFDs in connected plants
Today all sensors and actuators in industrial plants are connected over a network. The control inputs to the VFDs in a plant are coming from a central processing unit or from a cloud computing network. This setup helps the plant engineers to have granular control of every VFD and other equipment in the plant.
Networked devices are incredibly helpful in improving the automation capability of the plant. They can be managed by computer algorithms that adjust according to dynamic conditions in the plant. But this also poses a significant challenge concerning the network connection. For VFDs and other connected loads to work smoothly and properly, a stable and reliable network connection is necessary.
But even with the most reliable networks, outages are inevitable for a host of reasons. Unforeseen events that reliability engineers have not envisioned might happen. Cybercriminals also employ network disruption as a technique to destabilize operations of critical infrastructure. Managing network disruption has to be a contingency plan for all industrial plants.
Managing Network Outages in VFDs
Managing network outages in VFD becomes even more important. This is because VFDs control one of the most critical components in any plant operation: motors. Not only is operation affected, but safety is a major concern when motion is uncontrolled. OEMs, vendors, and plant engineers employ various techniques to manage network outages for VFDs. Three prominent methods are discussed in the following sections.
Figure 2. Variable frequency drives (VFDs) allow safe and controlled responses to network failures. Image used courtesy of Canva
Quick Stop Function
VFDs come with an emergency stop function. Some device manufacturers have a quick stop function, in addition to that. The emergency stop function is generally activated by operators during distress situations. Quick stop functionality is different, being instead a protocol to quickly stop drive rotation, in a predefined manner, that need not be an emergency.
When you design a VFD implementation in your plant, you can design it in such a manner that the quick stop function is activated when a network outage is sensed. Access to these network outage responses is usually through the parameter menus. Most OEM software also lets you vary the deceleration curve for quick stop functionality. Using this function allows you to automatically stop the drive connected to the VFD in case of a network outage.
The quick stop function can be implemented in operations where a network outage is least expected. It is a better solution when the drive controlled by the VFD is not critical to the operation. When the drive is not required to run without fail, the quick stop function can be used to handle network outages. The quick stop function is not always an optimal way to manage network outages, but works well in a pinch when an immediate equipment halt is the best choice.
The VFD used can have some preset operating conditions stored in it. It can be activated when a network outage occurs. When the network is available, the VFD functions according to the control instructions from the processing center. When network issues are detected, the preset conditions of the VFD takeover work according to the presets. When a stable network connection is back, control is handed back to the processing center.
Figure 3. Some VFDs will use backup manual or edge controls to run the system if network power is lost. Image used courtesy of Canva
Presets can be used when the available VFD does not have a quick stop function or when edge computing is not available. They are the way to go when drive operation is critical and should not be hampered due to network outages or other external factors. Extreme caution has to be exercised while designing presets for VFDs during network outages.
In the world of Industry 4.0, the sub-section of edge computing refers to processors located near the equipment that perform many data analysis operations that would be done in a remote or cloud environment, but exist on the shop floor to reduce delay and bulk of data that must be sent and received across a network.
In the context of a VFD, an edge computer is a computing unit directly coupled to the VFD that does not rely on any inputs from external networks. The edge module will be able to read information from the VFD, process the data, and send necessary commands. This can be a permanent running solution or may simply take over in the event of the VFD facing network failure. When the network is up and running again, the VFD can once again receive inputs from the external network. You will be able to adjust the configurations as you wish.
Edge computing can be used for VFDs during network outages when the connection between the edge device and the VFD is not compromised. There also need to be contingency plans in place when there are disruptions in the connection between the VFD and the edge module. Edge computing for VFD operations is the ideal solution for managing network outages in an IIoT-enabled plant.
Troubleshooting VFD network problems
When a network outage arises with the VFD, it has to be inspected as quickly as possible. Each specific device and network configuration will have unique variations in the troubleshooting process. The below list showcases some of the common problems and how you can fix them - some simpler than others.
- Check the integrity of network cable terminals.
- Check and verify the integrity of the complete run of network terminals.
- Ensure network terminals are not inverted.
- Check the network terminator.
- Check the grounding of network cables. They are generally grounded at the controller.
- Verify whether the correct communication protocol is selected at the network hub and VFD.
- Verify whether the VFD network address is reflected correctly at the network controller.
- Check whether the VFD and the network controller are connected to the same network.
- Check the network speed and latency.
- Identify if the network is overloaded with more devices and data than it is rated for.
- Ensure there are no high voltage lines in proximity to network cables.
- Ensure the motor ground is connected to the VFD.
- Disconnect the motor from the VFD and check whether the network connection returns.
In the end…
In a connected manufacturing plant, a stable and reliable network connection is important for smooth plant operations. This is especially true when there are VFDs and other critical equipment in the factory. You need to have contingency plans in place for any network outages. Most of the contingency plans are temporary in nature. Therefore, the prime focus has to be on building out a robust network, maintaining network uptime, and providing proper guidance and training to technical staff in order to efficiently solve downtime events.