Motors / Drives

Coordinated Motion, Concentrated Power

Drives Choreograph Motors for the Greatest Effect and Energy Savings. Here's How Experienced Users Gain Their Advantages

By Jim Montague

It's all about being in the right place at the right time—even if it looks like you're just spinning wheels. 

In the case of motors and rotating equipment in process control applications, drives are what help them meet those appointments and get to the exact right positions. Lately, variable-frequency drives (VFDs) and variable-speed drives (VSDs) enable more precise motion and efficiency, but the evolution of drives isn't stopping there.

For instance, Daqing Petrochemical Corp's refinery in Daqing, Heilongjiang, China, produces 600,000 tons per year of gasoline, diesel oil, kerosene, paraffin and other petroleum products as a subsidiary of PetroChina Co. Ltd.. As a result, the crude oil pump for Daqing's 3500-ton, pressure-reduction process needs to maintain a steady flow at 9000 to 10,600 tons per day, but it consumes huge amounts of electricity, and required two full days of downtime and maintenance per year, costing more than $16,000 in lost production per day. This was mainly because the pump and its motor were always running non-stop and at full speed, and used a valve-throttle control method that was only adjusted periodically, causing increased pipe pressure, leaks and seal replacements.

To adopt more efficient pump controls, Daqing's engineers sought a medium-voltage (MV) drive solution, and recently implemented a PowerFlex7000 MV drive from Rockwell Automation. This drive has a 6-KV rectifier and a 6-KV inverter that can be built with fewer 6.5-KV components without an output transformer. It also uses current source inverter-pulse width modulation (CSI-PWM) to reduce its power devices, and it achieves this efficiency with over-current protection capabilities that don't require fuses and electronic fusing circuits to protect the power devices, which simplifies the drive's complexity.

By adopting PowerFlex7000 for pressure reduction, Daqing cut its power consumption for 9000 tons of output by about 41% from 426 KWh to 250 KWh, which saves 1.5 KWh per year. Likewise, for 10,500 tons of output, the new drive cut consumption by 32% from 585 KWh to 400 KWh, which saves 1.6 KWh per year. Also, the new drive safeguards the refinery's pump motors against over-currents, and saves about $15,000 per year in maintenance and downtime.

Grace Under Compression

Similarly, the Dez Gas natural gas collection and distribution facility, located in the Deir Es Zor desert region in northeastern Syria, takes previously flared natural gas from nearby oilfields, uses reciprocating compressors to boost pressure, and pushes it through a 250-km pipeline to the national grid. The facility includes five plants, each of which can process 20 million standard cubic feet (scf) per day.

However, because Dez Gas distributes directly without silo storage, its volume must be regulated based on actual consumer demand, which can vary considerably. And, since reciprocating compressors displace a constant volume of gas regardless of operating conditions, they must be configured to displace slightly more gas than required, so some excess gas still has to be flared. In gas plants, fuel-driven engines usually adjust compressor speed, but their disadvantages include limited process control and vibrations, which are a major problem on skids (Figure 1). Also, it's very difficult to cool this equipment in the desert's 52 °C ambient temperatures.

Consequently, Daz Gas evaluated and implemented ACS 1000 medium-voltage VSDs from ABB with help from Singapore-based integrator Gas Services International. These drives include three-level, voltage source inverters (VSIs), water and air cooling, and induction motors. By using VSDs to soft start the compressors, high starting currents, voltage dips and trips on other electrical devices on the same bus are eliminated. This smooth ramp-up protects the skids' mechanics, which reduce maintenance and prolong their lifecycles.

In addition, the VSDs eliminate vibration on the skids, while configuring them with 12- and 24-pulse rectifiers minimizes harmonics and allows gas pressure to be regulated according to demand without having to flare excess gas. Because there's no local water for cooling in the Deir Es Zor area, ABB also supplied chillers, which consist of a closed-loop, water-cooling system, enabling the VSDs to run in the desert heat.

Cows and Power

Of course, one of the most instructive proving grounds for drive and motor innovations is in water and wastewater applications, which must continuously and reliably move large volumes over long distances.

For example, Joseph Gallo Farms in Atwater, Calif., has 5000 cows at its dairy in nearby Cottonwood, where it's added a 44.2-million-gallon lagoon digester with seven acres of surface area to convert manure into electricity. The herd produces 200 tons of manure daily, and the digester produces 3000 ft3 of methane daily. This powers two reciprocating engine generators that produce 740 kW, or about 9.4 GWh per year, which is about half the dairy's on-site power requirement.

The lagoon's methane production is accelerated with warm water from plant clean-up, which was previously controlled by a vertical, hollow-shaft AC motor that drove a pump with a check valve and pressure regulator limiting the line to 70 psi. However, when plant engineers sought to reduce the flowrate during manure flushing to prevent over-watering the digester, Control Techniques suggested a design using its 200-hp Commander SK AC VSD custom-configured AC drive package and 300-psi pressure transducer. Pump software is pre-loaded on a Control Techniques LogicStick, and all parameters are accessed via a door-mounted CT-Vue HMI.

Previously, to reach a 1000 gpm flowrate, power consumption was 154A at 60Hzm, but the VSD was able to achieve the same rate with 25% less speed, which halved the current to just 75 Amps at 45Hz. This allowed the optimal 70-psi pressure to be maintained without a regulator, but it's still retained for when bypass mode is used.

"This VSD pump control is in line with our environmental policy at Joseph Gallo," says Mark Tovar, Joseph Gallo's project engineer. "We're constantly looking for ways to cut greenhouse gas emissions and save energy. This small project produced significant savings, while giving us improved control over this part of our digester system."

Smarter Mill 'Spotting'

Beyond granting improved operations and efficiency, variable-speed capabilities and the data their components need to exchange are drawing drives and motors into new diagnostic and intelligence capabilities. For instance, 100-year-old Ash Grove Cement in Chanute, Kan., relies on nearly 1000 motors with a combined 45,000 hp to continuously produce about five tons of cement per minute or up to 4500 tons daily.

Similar to most cement plants, Ash Grove mixes limestone with other ingredients, heats it to 2000° F in its 150-foot-long kiln, and then grinds the resulting clinker in mills filled with steel balls. However, the plant's workers had trouble using an old, 60-hp generator motor to slowly rotate and position these ball mills for monthly servicing. This manual positioning process, called "spotting," was difficult because Ash Grove's technicians had no effective way to accurately apply torque to the medium-voltage motor directly from their power system.

"Along with the problems we had moving the bulky mill to a precise position, cogging, or abrupt starting and stopping of the motor, can cause mechanical and electrical damage to equipment," says Bob Wright, Ash Grove's electrical operations manager. "And each hour we shut down operations to perform routine maintenance or resolve a fault translates to 300 tons of cement we could've produced."

Consequently, Ash Grove replaced the generators powering the ball mills' spotting functions with Rockwell Automation's preconfigured Allen-Bradley 480-V, 450-hp AC VFDs (Figure 2). These drives run three existing 4000-V, 2300-hp AC motors exclusively during spotting, and have the torque control required to operate at the 6 Hz needed to efficiently rotate the mill and bring it to controlled starts and stops. For instance, the drive can separate and independently control motor flux and torque-producing current, allowing it to deliver full torque down to zero speed.

As a result, Ash Grove saved an initial $250,000, increased production, and achieved 90% uptime. "Besides costing less than half as much as a medium-voltage drive, using a low-voltage AC drive helped save money on maintenance and energy," adds Wright.

Shortcuts to Motor Control

Logically, just as VFDs can lead to better drive-related data and intelligence, the same phenomenon is emerging in other motion technologies, too. Sergio Gama, power control market development director at Rockwell Automation, reports that motor control centers (MCCs) are beginning to combine fixed and variable-speed capabilities, and find more uses in process equipment.

"A 10-V output card for a drive can behave just like any other I/O card in a process control system, which is allowing drives to become actuators in control strategies, and letting drives in MCCs become integrated into process controls and applications," says Gama.  "Previously, everything was hardwired with discrete I/O, but then we took the intermediate step to VFDs that saved wire and commissioning time. Now, we're moving further ahead on EtherNet/IP networking, and can map data structures in devices directly to process controls. So, instead of building tables in PCs and then associating variables from controls to field devices, we can now directly command variables in the field.

"These new data structures are also providing more than on/off and slow/fast functions, and enabling real-time intelligence and diagnostics to be sent to PCs and historians. In fact, our motor controls have internal databases that send all operating and diagnostic conditions—such as status, current, voltage, hours running, bearing temperatures and other performance and asset data—up to larger distributed control systems (DCSs) and asset monitoring systems. Also, when you're replacing a VFD, the control system will recognize it and replace its IP address, automatically download the right configuration, and get it back to run mode."