Today’s innovations in drive technology and design offer powerful benefits even beyond energy savings.
by Jerry S. Wang

Finding ways to effectively reduce equipment, the space it occupies and associated costs is a goal in nearly any industrial application. Today’s pipeline, pumping, gas compression systems and other medium-voltage control applications can tap synchronous transfer drive systems to improve overall profitability.

Adjustable frequency drives (AFDs) have been mainly associated with saving energy. Today’s innovations in drive technology and design offer powerful benefits beyond energy savings. Advanced drives allow users to do more with less equipment—even improve safety and power reliability—all while continuing to reduce energy requirements.

Image 1. An adjustable frequency drive that synchronizes power density and reliabilityImage 1. An adjustable frequency drive that synchronizes power density and reliability (Images courtesy of Eaton)

Synchronous transfer systems control multiple motors with a single AFD. In multi-motor or soft-start applications, a synchronous transfer system is designed to ramp up multiple motors in a series, transfer the load to adjacent bypass contactors and operate the motors at full speed. The system can also ramp the motors down in a series in the same way.

By increasing pressure in stages, the system reduces mechanical shock to pipelines. Pumping stations can regulate the required pumping capacity by bringing pumps on- and offline. Using a single AFD can help reduce overall equipment requirements and costs.

Synchronous Transfer Systems

Synchronous transfer control systems incorporate an AFD and programmable logic controller (PLC) per system, as well as a bypass contactor and motor select contactor per motor. The basic idea is to adjust AFD output voltage, frequency and phase to match the utility. Matching these parameters allows the system to transfer the motor to the utility in a “bumpless,” or smooth, manner.

With an integrated control gear double-bus design, the drive output and motor select contactors are close-coupled under a common power bus. This design reduces space and cabling requirements. Synchronous transfer systems that incorporate stacked contactor designs can reduce space requirements by nearly 50 percent. A stacked enclosure for contactors up to 800 amperes (A) has a similar footprint to a 400A contactor enclosure.

Image 2. An adjustable frequency drive with stacked 800A contactorImage 2. An adjustable frequency drive with stacked 800A contactor

How It Works

The AFD and PLC system is programmed to follow “start/stop” and “sync-up/sync-down” general commands. Once the system bus is energized, the AFD and feeder bus are also energized. The start command will begin the synchronous transfer process.

First, the PLC closes the appropriate motor select contactor and then starts the AFD, which will operate at the preset or reference speed. When the motor is required to transfer to the utility line, the PLC sends the sync-up command to the AFD. The AFD will adjust the output to match the line voltage, frequency and phase angle.

Once synchronization is locked, the bypass and motor select contactor are closed and momentarily paralleling both sources. The output reactor provides impedance to the utility. Then the AFD is shut down and waits for another “start” command for the next motor, while the first motor runs on bypass.

If the PLC receives a command to sync down, the PLC will start the AFD and close the motor select contactor. The AFD output will adjust to match the utility voltage, frequency and phase. Once these parameters are locked in, the AFD output contactor will close and momentarily paralleling both sources. The AFD will operate at a set speed point or follow a reference speed.

These sequences can be commanded to start any number of motors, depending on equipment size and layout.

Image 3. An arc-resistant synchronous transfer systemImage 3. An arc-resistant synchronous transfer system

Closed-Transition Synchronous Transfer

Synchronizing the drive’s output to the utility line’s power and providing a seamless transition are critical to the performance of synchronous transfer systems. With some transitioning methods, the motors are free-coasting momentarily before the drive or contactor picks it up. This method does not provide seamless transition and can put unnecessary stress on the mechanical system.

New closed-transition technology can provide smooth transitions during both up-sync and down-sync transitions. The drive technology is designed to prevent motors from free-coasting. During the process of transfer to the contactor, the drive matches 60 hertz (Hz) line power and closes the contactors before powering down the drive. The process is reversed during re-transfer.

Arc-Resistant Drives

Recent innovations in drive technology have yielded arc-resistant medium-voltage drive systems. These are designed to protect personnel in danger of arcing faults by containing and redirecting the arc energy up and away from the user. While arc-resistant switchgear and motor control centers have been around for years, drives have only met arc-resistant standards since 2015.

Arc-resistant drive designs can incorporate a double-bus technology that enables synchronous transfer while meeting arc-resistant standards to help enhance safety, minimize unplanned downtime and prevent equipment damage.