Voltage Sag Protection & Motor Restart

Voltage Sag Protection and Motor Restart for Low-Voltage Motor Groups

A voltage sag is a short-duration reduction in RMS supply voltage, typically lasting from a few cycles to a few seconds. For a plant running a large population of low-voltage contactor-controlled motors, the critical effect is not the voltage reduction itself but the contactor drop-out that follows: once coil voltage falls below the hold-in threshold, the contactor opens, the motor trips, and production stops even though the supply may recover within a fraction of a second. LOGGON TECH supplies voltage sag protection and motor restart equipment for this specific problem — a batch restart device for low-voltage motor groups and a family of restart controllers that supervise contactor coils, distinguish a recoverable voltage sag from a genuine supply failure, and re-close motors in a controlled sequence rather than all at once.

What it does

Voltage sag protection works on two distinct principles, and the equipment on this page covers both. The first is ride-through: the controller holds the contactor closed through a voltage dip that is shorter than the allowable sag time, so the motor continues running and the process is not interrupted. The second is controlled restart: where the dip exceeds the allowable time, or the contactor has already dropped out, the device re-energises the motors after the supply has recovered, following a settable restart time and, on the group device, a settable batch sequence.

Simultaneous restart of a motor group is the main hazard in either case. If every contactor in a section re-closes at the same instant, the combined inrush current can pull the busbar voltage down again, cause a second sag, trip upstream protection, or overload the transformer. It also applies a mechanical shock to every driven machine at once. The FZQD-V therefore separates motors into restart batches (1–16 settable), so that inrush is spread over time instead of arriving as one block.

Models and key data

The FZQD-V batch restart device is the group-level unit, intended for plants where many motors share one or more busbar sections. It recognises 1–4 busbar sections, handles single-phase and multi-phase voltage fluctuation, and can restart up to 144 motors. Restart batches are settable from 1 to 16; the sag time window is 0–5 s; the restart time range is 0.1–300 s with 0.1 s setting steps. Control is by touchscreen or industrial PC.

The LOGGON-3B-1/2 restart controller is a single-controller unit for individual contactor coils. It is available for 220 VAC or 380 VAC coil voltage, with an allowable sag time of 0.1–999.9 s and a minimum recognisable voltage fluctuation time of 20 ms. Restart time is 0.1–25.5 (unit as published), and communication is RS-485 with Modbus.

The LOGGON-3B-3 anti-sag restart controller adds a dedicated anti-sag time setting. Coil voltage is 220/380 VAC, allowable time 0.1–999.9 s, minimum fluctuation 20 ms, anti-sag time 0.1–5 s, restart time 0.1–25.5 s, and communication RS-485 Modbus.

How it fits together / architecture

Voltage dip protection is applied at two levels, and the two levels are complementary. At the busbar level, the FZQD-V monitors the incoming sections, identifies which section dipped and for how long, and coordinates the restart of the motors on each section in batches. At the individual motor level, the LOGGON-3B-1/2 and LOGGON-3B-3 sit on the contactor coil circuit of a single starter; the 3B-3 adds a dedicated anti-sag (ride-through) time setting, and the controllers release the coil for a controlled restart when the dip is longer. The single controllers provide RS-485 with Modbus; the available registers and remote functions follow the selected controller's communication manual. The FZQD-V is operated from its touchscreen or an industrial PC. The typical arrangement is a group device per busbar section, single controllers on the motors whose process timing is most sensitive, and the batch settings on the group device set to match the transformer and upstream protection capacity.

Applications

• Continuous-production lines where a short supply dip interrupts a long process and the loss of one motor group stops the whole line.
• Plants with several busbar sections fed from a common transformer, where staggered restart is required to keep inrush within capacity.
• Motor groups of up to 144 units on one section, where individual restart timers would be impractical to coordinate by hand.
• Single critical starters, such as pumps and fans, fitted with a coil-level anti-sag restart controller and monitored over RS-485 Modbus.
• Retrofit of existing contactor-controlled starters, where the device is wired into the coil circuit rather than replacing the starter.

FAQ

What is the difference between ride-through and restart?

Ride-through keeps the contactor closed while the voltage is depressed, so the motor never stops; this is bounded by the anti-sag time or allowable sag time setting. Restart applies when the contactor has opened: the device waits for the restart time after the supply recovers, then re-closes the coil in its assigned batch. On the LOGGON-3B-3 controller, the anti-sag (ride-through) time and the restart time are both set on the same controller.

How many motors can one unit handle?

The FZQD-V batch restart device restarts up to 144 motors across 1–4 busbar sections, with restart batches settable from 1 to 16. The LOGGON-3B-1/2 and LOGGON-3B-3 are single-controller units, one per contactor coil.

Why must a motor group not restart simultaneously?

If all contactors re-close together, the summed inrush current depresses the busbar voltage again and can cause a second sag, trip upstream protection or exceed transformer capacity. Batch restart spreads the inrush over settable groups and is the reason a motor restart device is specified at group level rather than as individual timers.

Can the controllers be integrated into a plant control system?

Yes. The LOGGON-3B-1/2 and LOGGON-3B-3 provide RS-485 with Modbus. The FZQD-V is operated from a touchscreen or an industrial PC. Settings such as allowable sag time, anti-sag time and restart time are made on the device; the minimum recognisable voltage fluctuation time on the single controllers is 20 ms.

Selection depends on the number of motors per section, the transformer and protection capacity that limits total inrush, and the longest dip the process can tolerate without stopping. The LOGGON TECH contact page lists the inquiry route for matching a device configuration to a specific busbar arrangement.