This article was printed in CONTROL's May 2009 edition.
By Rich Merritt
The U.S. Department of Commerce estimates there are more than 12.4 million electric motors larger than 1 hp in service throughout industry in the United States, and that nearly 3 million of these workhorses will fail this year.
If a motor fails on a critical pump, damper, electric valve actuator or conveyor, it could shut your process down for days.
Motor and drive vendors and purveyors of diagnostic systems say diagnostics are the answer. Others aren’t too sure.
Larry Wells, principal at Atlanta-based systems integrator Confidential Control Systems Assessment (firstname.lastname@example.org), says anyone with relatively new motors already already has diagnostics. “Most new, low-voltage motor installations—480 VAC, three-phase—now use solid-state motor control centers that have diagnostics,” says Wells. “Even when locked out, the diagnostics still have power, since they use a separate safe 24-VDC bus. It actually costs more to buy them without built-in diagnostics, so why not?”
He says medium-voltage motors have had diagnostics as a standard feature for nearly 20 years, and so he questions the Commerce Department’s assessment. “The government’s statement is probably very flawed,” he says. “This would indicate that one in four will fail or have an average life of only four years. Motors last 20 to 30 years.”
It wouldn’t be the first time the government was wrong, of course, but a process control system can’t afford any unscheduled failures. “In general, we have way too many motors that fail,” says Jim Reizner, control engineer at Procter & Gamble.
Danny Vandeput, a sales engineer at Emerson Process Management in Europe, says to help lessen motor failures, a motor repair shop in Austria conducted an extensive investigation into motor failures in the Swiss cement industry. Figure 1 illustrates the data from the study and shows some types of motor failures you can expect.
Major sources of motor failures.
Even if a motor doesn’t fail completely, it can still run badly. “When issues are not immediately recognized and remedied, significant energy can be wasted, and motors and loads can unnecessarily degrade,” says Adam Krug, power protection product manager at Eaton Corp. “Energy consumption and downtime impact the top and bottom lines of every operation.”
Diagnostics to the Rescue
Many users haven’t taken advantage of the available diagnostics. Romel Bhullar, a control engineer at Irving, Texas-based Fluor, says, “In the old days, to bring out any diagnostic parameters to a monitoring and control system was extremely expensive. We would just bring two to four parameters out to control systems.”
But since then, things have changed, he adds. “Microprocessors in starters, variable-speed drives and motor control centers have given us tons of data that was previously unavailable. Now, with digital serial communications, you can bring several key parameters to various systems at little cost.”
Bhullar explains that more than 32 parameters are available, even from relatively small motors, and intelligent use of this information can improve your bottom line. “We’ve been using all the information we can get from intelligent motor control centers, starters and variable-speed drives,” he says. “Control, safety and monitoring parameters, such as variable speed, acceleration, torque, current, vibration, bearing and windings temperatures, and other motor health and diagnostic parameters, are picked up and fed to control, machinery monitoring and proactive maintenance management systems, sequence-of-event recorders and data historians for post-mortem analysis.”
Wells agrees. “All diagnostics for low- and medium-voltage applications are available via DeviceNet, Profibus and other buses,” he says. “On large, new installations, starting and stopping can be done over the bus, though some medium-voltage users still prefer the hardwired approach.”
What Motor Problems?
Renewable Energy Group (REG) in Ames, Iowa, installed Rockwell Automation’s control systems on its biodiesel plants three years ago and has rarely seen motor failures. That doesn’t mean REG hasn’t had a few motor problems; it’s just that its motors rarely fail, mainly because of the modern motors, drives, controls and diagnostics that were installed.
REG uses Rockwell’s Allen-Bradley ControlLogix programmable automation controllers (PACs) for a continuous-flow process control system that makes biodiesel from soybean oil, animal fats and other oils, monitors quality, and controls the discrete, process, safety and drive functions that make up the biodiesel process. The company chose PACs to combine all these functions into one controller and wanted it to simplify troubleshooting and maintenance—which brings us to the motors and drives.
The PACs connect to Allen-Bradley PowerFlex AC drives via DeviceNet. The plant has a few 100-hp motors, but most are in the 15-hp to 20-hp range and are mostly used to control pumps (Figure 2). The PowerFlex variable-speed drives that control motors are not located in the hazardous area of the facility, but in the motor control center with the rest of the starters.