Voltage Sags and Ride-Through How to Protect VFDs from Power Disturbances
Voltage sags, brownouts, and short power interruptions can cause VFDs to trip, stop production, or interrupt critical processes. Ride-through solutions help support the drive DC bus during power disturbances so equipment can remain operational or shut down in a controlled manner.
In This Article
Use this guide to understand what voltage sags are, how they affect VFDs, and which ride-through solution path may fit the application.
Bonitron Products
These solutions are commonly used to reduce nuisance undervoltage trips and protect critical drive applications from power disturbances.
What is a Voltage Sag?
A voltage sag is a short-duration reduction in incoming AC line voltage. In industrial facilities, a sag may last only a fraction of a second, but that can still be long enough to cause a variable frequency drive to fault, shut down, or interrupt the process it controls.
A VFD converts incoming AC power to DC power on the drive DC bus. The drive then uses that DC bus to create the controlled output needed by the motor. When incoming voltage drops, the DC bus can also fall. If the DC bus falls below the drive's operating threshold, the drive may trip on undervoltage or stop producing controlled motor output.
This is why a short voltage sag can cause a much larger production problem. The power disturbance may be brief, but restarting the machine, reestablishing process conditions, clearing faults, and returning to production can take much longer.
How Voltage Sags Affect VFDs
VFDs depend on stable DC bus voltage to keep motors operating as commanded. During a voltage sag, the incoming AC line may not provide enough energy to maintain the DC bus. If the disturbance is deep enough or long enough, the drive may detect an undervoltage condition and shut down to protect itself.
In some applications, a brief coast or restart may be acceptable. In others, even a momentary interruption can create major problems. A process may lose synchronization, material may be damaged, a batch may need to be scrapped, or equipment may require a controlled restart sequence before production can resume.
The severity of the problem depends on the process, the drive settings, the load, and how long the application needs to remain operational during the disturbance. Some systems only need enough support to ride through a short sag. Others need enough time to transition to backup power or perform a controlled shutdown.
Common Causes of Voltage Sags
Voltage sags can originate from the utility supply, from equipment inside the facility, or from a combination of both. Because the event may be short and intermittent, it is often difficult to diagnose without reviewing when the fault occurs and what else is happening in the facility at that time.
Utility Disturbances
Grid switching, nearby faults, storms, lightning events, and utility equipment operation can all create short voltage disturbances. These events may affect the whole facility or only certain parts of the electrical system.
Large Motor Starts
Starting large motors can create temporary voltage dips, especially in facilities with limited electrical capacity or long feeder runs. Drives located on the same distribution system may experience enough voltage drop to fault.
Transformer or Distribution Limitations
Undersized transformers, long conductors, overloaded panels, and weak distribution systems can make voltage sag problems worse. The drive may operate normally under light load but fault when other equipment starts or production demand increases.
Generator Transfer or Backup Power Transitions
When a facility transfers between utility power and generator power, there may be a momentary disturbance. If the drive cannot tolerate the transition, it may trip before backup power stabilizes.
Process Equipment Cycling
Welders, compressors, pumps, heaters, and other high-demand equipment can create localized voltage disturbances. These events may be repetitive and tied to a specific operating cycle.
Symptoms of a Voltage Sag Problem
Voltage sag issues often appear as intermittent drive faults or unexplained production interruptions. The timing of the event can help identify whether the issue is related to incoming power, plant equipment cycling, utility disturbances, or insufficient ride-through capacity.
Undervoltage Faults
The drive trips when the DC bus falls below the operating threshold.
Momentary Shutdowns
Equipment stops during brief power disturbances even though utility power returns quickly.
Failed Restarts
Machines may not automatically recover after a sag event, requiring operator intervention.
Process Interruptions
Critical operations can be disrupted when drives coast, reset, or lose control during a sag.
Intermittent Fault Logs
Faults may only appear during storms, utility switching, large motor starts, or plant load changes.
Unplanned Downtime
Repeated short disturbances can create costly production stops and maintenance calls.
How Ride-Through Works
Ride-through is the ability of a drive system to continue operating through a short power disturbance. Instead of allowing the DC bus to collapse when incoming voltage drops, a ride-through solution supports the drive long enough for the sag to pass, backup power to stabilize, or the process to shut down in a controlled way.
Different applications require different ride-through approaches. Some systems only need support for a brief sag. Others require enough stored energy to maintain drive operation through a longer interruption. The right solution depends on the load, horsepower, DC bus requirements, process criticality, and required ride-through time.
Short-Duration Sag Support
Some power disturbances are very brief but still long enough to trip a drive. In these applications, a sag solution may be used to support the DC bus during short undervoltage events and reduce nuisance trips.
Capacitor-Based Ride-Through
Capacitor-based systems use stored energy to support the drive during short power interruptions. This approach can be useful when the application needs ride-through for a limited duration without relying on batteries.
Battery-Based Ride-Through
Battery-based systems can provide longer ride-through time for critical processes. These systems are often used where the drive must continue operating through a longer disturbance or bridge the gap until backup power becomes available.
Critical Process Protection
In some applications, the goal is not only to avoid a fault but to protect the process itself. Ride-through may help prevent material loss, maintain airflow, preserve cleanroom conditions, support controlled motion, or allow a safe shutdown.
How to Diagnose a Voltage Sag or Undervoltage Fault
Diagnosing voltage sag problems starts with understanding when the drive faults and what the machine is doing at that moment. Instead of only resetting the drive, document the timing, incoming voltage, fault history, process conditions, and other equipment operating nearby.
Use the guide below to narrow down the likely cause and determine whether the application may require electrical system review, parameter adjustments, sag support, capacitor-based ride-through, battery-based ride-through, or a complete undervoltage protection system.
| When the Problem Happens | Likely Cause | What to Check | Possible Solution Path |
|---|---|---|---|
| During utility disturbances | Incoming line voltage drops briefly due to utility switching, storms, or upstream faults. | Review plant voltage logs, drive fault history, event timing, and whether multiple drives fault at once. | Evaluate sag support or ride-through systems to support the DC bus during short disturbances. |
| When large motors start | Facility equipment creates a temporary voltage dip on the shared electrical system. | Check motor starting events, feeder loading, transformer capacity, and whether drive faults align with equipment starts. | Review facility distribution, sequencing, and drive ride-through requirements. |
| During generator transfer | The drive loses voltage long enough to trip before backup power stabilizes. | Check transfer time, generator startup sequence, drive DC bus voltage, and process restart requirements. | Use ride-through support to bridge the transfer period or allow controlled shutdown. |
| During short outages | The interruption is longer than the drive can tolerate without additional stored energy. | Measure outage duration, load demand, horsepower, DC bus voltage, and required operating time. | Evaluate capacitor-based or battery-based ride-through depending on required duration. |
| Only during heavy production | The electrical system may be more vulnerable when the facility is heavily loaded. | Check production load, feeder voltage, transformer loading, and equipment cycling patterns. | Review the plant power system and consider drive-level undervoltage protection. |
| Faults repeat after reset | The root cause has not been corrected, so the drive continues to see undervoltage events. | Document fault codes, event timing, voltage data, affected drives, load conditions, and process impact. | Review the full application profile and size the proper ride-through or sag solution. |
Bonitron Engineering Note
A drive parameter change may help in some situations, but it does not create energy. If the DC bus falls too low during a power disturbance, the application may require a ride-through solution that provides stored energy or DC bus support during the event.
How to Prevent Voltage Sag Related Drive Trips
Preventing voltage sag related downtime starts with understanding the required outcome. Some applications only need to avoid nuisance trips during brief sags. Others need continued operation during a transfer to backup power. Critical processes may need enough ride-through time to maintain production, protect material, or shut down safely.
Document the Disturbance
Record when the fault occurs, how long the disturbance lasts, which drives are affected, and what other equipment is operating at the time. This information helps determine whether the problem is isolated to one drive or related to a broader facility power issue.
Review the DC Bus Requirement
Because the VFD depends on DC bus voltage, the ride-through solution should be evaluated based on the drive's DC bus requirements and the application load. The horsepower, operating current, load profile, and required ride-through time all matter.
Choose the Right Ride-Through Method
Short sag events may require a different solution than longer outages or generator transfer events. Capacitor-based systems, battery-based systems, voltage regulation, and complete undervoltage systems each serve different application needs.
Consider the Process, Not Just the Drive
The best solution is not always the one that only keeps the drive powered. The correct approach should support the process goal, whether that means maintaining operation, avoiding material loss, preventing unsafe motion, or allowing a controlled shutdown.
Applications Commonly Affected
Voltage sag and ride-through problems are common in applications where even a brief drive interruption can create downtime, process loss, safety concerns, or difficult restarts.
Cleanrooms
Airflow interruptions can affect room conditions, pressure control, and process stability.
Fiber Extrusion
Drive trips can disrupt production and create material loss during continuous processes.
Boiler Systems
Draft fans, feedwater pumps, and related equipment may require stable drive operation.
Data Centers
Cooling, airflow, and support equipment may need protection from momentary power disturbances.
Manufacturing Lines
Repeated nuisance trips can interrupt production and require manual restart procedures.
Material Handling
Conveyors, elevators, and automated systems may lose sequence or require controlled recovery.
Oil and Gas
Critical motion and process equipment may need support during power disturbances or generator transfer.
HVAC and Fan Systems
Fans and pumps may require ride-through to maintain process air, cooling, or ventilation.
Batch Processing
Short interruptions can affect product quality, timing, and process completion.
Frequently Asked Questions
A voltage sag is a short reduction in voltage, while an outage is a loss of power. A sag may only last briefly, but it can still cause a VFD to fault if the drive DC bus falls below the required operating level.
VFDs rely on DC bus voltage to control the motor. When incoming voltage drops, the DC bus may fall. If it falls below the drive's undervoltage threshold, the drive may shut down or fault.
Drive settings may help in some applications, but they cannot replace lost energy. If the application needs to keep operating during a sag, a ride-through or DC bus support solution may be required.
VFD ride-through is the ability of a drive system to continue operating through a power disturbance. Ride-through solutions help support the DC bus so the drive can remain operational or shut down in a controlled manner.
Capacitor-based ride-through is often used when the application needs support for short-duration disturbances without using batteries. The required energy storage depends on the drive load and desired ride-through time.
Battery-based ride-through may be a better fit when the application requires longer support time or must bridge a disturbance until backup power stabilizes.