Protection selection
Start with what stops the process
Assess a DVR when critical loads require AC-side voltage restoration.
Assess flywheel DC bus support when compatible drives determine production continuity.
Include control power, auxiliaries, and the required ride-through duration in either assessment.
The lights flicker. A drive trips on undervoltage. The grid recovers almost immediately—but the production line stays down.
For manufacturers running continuous processes, a brief electrical disturbance can lead to scrapped materials, interrupted production, additional cleaning, and a lengthy restart.
A voltage sag, also called a voltage dip, is a temporary reduction in supply voltage. Power may still be available, but the voltage can fall below what sensitive equipment needs to operate. Utility faults and large motor starts are among the possible causes.1
Two approaches worth considering are a Dynamic Voltage Restorer (DVR) and DC bus support. Choosing between them starts with three questions: What trips first? Which loads must keep running together? How long must they ride through the disturbance?
How a DVR protects the AC supply
A DVR compensates voltage on the AC supply side. Installed in the supply path of a protected circuit, it detects voltage disturbances and injects a compensating voltage to keep the downstream supply within the required operating range.
Within its specified capacity, compensation range, and duration, a DVR can protect different types of equipment connected to that circuit. This makes it an option when critical AC loads need coordinated protection.
Explore WS-DVR AC-side protection →
How DC bus support protects a drive
A typical variable frequency drive (VFD) converts incoming AC power into DC, then uses an inverter to control the motor. During a voltage sag, the DC bus voltage can fall. If it reaches the drive’s undervoltage threshold, the drive may trip.
A DC bus support system supplies energy to that bus, helping a compatible drive continue operating through the disturbance under defined conditions. This approach deserves particular attention when one critical drive can interrupt an entire process. Servo applications require confirmation of the drive architecture and manufacturer’s interface requirements.2
| Selection factor | DVR | DC bus support |
|---|---|---|
| Protection point | AC supply to a protected circuit | DC bus of compatible drives |
| Protection scope | Loads supplied by that circuit | Drives connected to the support system |
| Application to assess | Critical AC loads requiring shared protection | Critical VFDs or compatible servo drives requiring continued operation |
| Integration | Circuit capacity, installation location, bypass, and protection coordination | DC voltage range, bus access, drive compatibility, and protection coordination |
| Control power | Confirm required controls are inside the protected circuit | Assess PLCs, contactors, and other controls separately |
| Cost comparison | Evaluate the complete protected circuit | Evaluate critical drives plus supporting control protection |
Some compensation systems draw energy from the remaining utility supply. Others use stored energy. Whether a system can support a complete loss of input power depends on its architecture, available energy, output power, and specified operating duration.
Where Wilson’s flywheel DC support fits
A flywheel stores energy in a rotating rotor. When support is required, a motor-generator and power conversion system convert that kinetic energy into electrical power. Flywheel technology is well suited to applications requiring high power for short periods and repeated charge-discharge operation.3
Wilson Technology’s flywheel product provides DC output for compatible drive DC bus support systems. Through a properly engineered interface, it supplies stored energy during an electrical disturbance, helping critical drives remain operational within the system’s specified power and duration limits.
For continuous processes dependent on critical drives, this allows the protection system to focus on the equipment that directly determines production continuity.
Selection should consider more than peak power. Usable energy, DC voltage range, recharge time, and readiness for successive disturbances all affect performance. Drive protection settings, braking or regenerative operation, auxiliary power, and control signals must also be checked.
Learn where flywheel energy storage fits →
Choose protection around the process
- A few critical drives cause the shutdown: Assess flywheel DC bus support, together with their control power and operating interlocks.
- Different critical AC loads must remain operational together: Assess DVR protection for the relevant supply circuit.
- Large drives and small control loads are both sensitive: Compare a combined solution—flywheel support for the drives and separate control power backup—with protection at the AC circuit level.
The number of sensitive devices alone does not determine the right solution. Their power requirements, electrical connections, and roles in the process matter more.
For example, maintaining a VFD’s DC bus will not keep production running if its PLC restarts, a contactor drops out, or the run command disappears. Effective ride-through protection must cover the complete chain of conditions required for continued operation.
Review Wilson’s load-layered protection architecture →
Define what successful ride-through means
Before final equipment selection, gather voltage disturbance records, drive fault logs, equipment models and ratings, electrical drawings, and the process limits for speed or torque variation. Define acceptance criteria around the actual load, remaining voltage, disturbance duration, and required process behavior.
The value of voltage sag protection is measured by the production it helps preserve.
Continue the engineering review
- WS-DVR Dynamic Voltage Restorer
- VFD DC Bus Support
- Why Use a Flywheel?
- Wilson Load-Layered Voltage Sag Protection Architecture
Technical background
- Fluke: voltage sags and power quality events.
- Bonitron: example of an engineered DC bus ride-through system.
- Active Power: flywheel energy storage principles.
Final ratings, interfaces, duration, protection settings, and acceptance criteria require project-specific verification.