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 ([email protected]), 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.
(Source: Emerson Process Management)
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.
Full diagnostics for the drives were designed by system integrator Interstates Control Systems, Sioux Center, Iowa. The enhanced diagnostic software reduces manpower and physical diagnostic checks. This reduces REG’s overall repairs.
Scott Kingery, process engineer at REG, says the PACs primarily monitor the each motor’s current draw in amperes and the run time of all motors. “Every few minutes, motor readings are logged into the data historian in case any problems come up,” says Kingery. “If a motor fails, we can look at the historical data to get an idea of why it failed.”
Figure 2: Renewable Energy Group (REG) uses motor-driven pumps to make biodiesel. These pumps are located on the bottom of the glycerin methanol stripper. REG uses built-in diagnostics and reports few motor problems.
(Photo courtesy REG)
So far, very few have. Kingery says this is because of the newer technology. “In the old days, motors would overload, heat up, pull too much current and ‘kick out’ or burn up,” he explains. “With the new technology the drive controller spots an overload fault condition almost instantly and reacts accordingly. Today’s systems have shut-down parameters that protect the motor from stress and wear. The PAC identifies the fault, and the plant operators can reset it using an HMI, thus avoiding going out into the plant and physically hitting the reset button. We’ve rarely lost a motor in two and half years.”
Not everybody is in love with modern motor diagnostics.
Eaton’s Krug is aware of the overload problem in motors. “Some users don’t want their motor or load coming offline for anything but a motor overload,” he explains. “When the revenue of throughput is more valuable than motors and loads, one can understand this logic.” So Krug suggests using modern overload relays.
“Many advanced overload relays can measure current and voltage,” he explains. “The combination of information gathered from current and voltage and the processing power available in today’s overload relays can provide protection. For example, Eaton’s Motor Insight relay uses line voltage and motor current to provide 11 protective settings, such as volts, power consumption, current, voltage imbalance, power factor, frequency, ground fault current and thermal capacity, and provide protection from overloads and poor line conditions. Unlike more complex diagnostic solutions, these overload relays install just like bi-metallic overloads, and arrive with default protective settings,” he says.
Jim Reizner of Procter & Gamble also does not use built-in diagnostics. “We do various things in the area of motor preventive maintenance,” he says. “We do thermography of connections and sometimes of motor bearings. We megger [apply a meg ohm meter] motors—rarely as a diagnostic tool—but more generally just before installation to ensure the motor is OK.”
“Motor bearing vibration monitoring is our most common method of predicting motor failure,” he says. “In most of our plants, this is done as a rounds-based task, and then only for specified larger and critical motors. Some larger motors have sensors installed to monitor vibration, and we have experience using some newer wireless vibration sensors, most usually from VenTek.”
Vibration analyzers are widely available from several vendors. As Reizner says, checking motors is typically done manually, but some systems can be installed permanently. Like the built-in drive diagnostics noted above, they can connect to a process control system via the plant network or wireless.
“Some of our plants use Baker motor testers,” Reizner continues. “These are excellent, but they require an amount of knowledge, training and time that many plants do not seem able to afford.”
What Reizner wants is affordable and simple diagnostics. “Diagnostics would need to be broader than only in drives, as the vast majority of motors are operated off of motor contactors/starters, not drives,” he says.
No Magic Bullet?
Dr. Howard Penrose, general manager of All-Test Pro Division of BJM Corp., says, “There’s been a persistent misconception that there is a ‘magic bullet,’ in the form of a condition-based monitoring (CBM) instrument that will provide all the information you need to evaluate the health of your electric motor system.”
This misconception, he says, mostly comes from the vendors of those CBM systems. In reality, he says, there is no one CBM device that does it all. Instead, you need a multiple-technology approach to keep your motors humming. His laundry list of motor diagnostic tools include those that can test for:
- DC high potential
- Surge comparison
- Polarization index
- Resistance between windings
- Motor circuit analysis (MCA)
- Voltage and current
- Motor current signature analysis (MCSA).
"Infrared and vibration are normally used with each other, with great success,” Penrose says. “However, they miss a few common problems or will only detect them in the late stages of failure. Surge and high-potential testing will only detect some winding and insulation to ground faults.
“The newest and most effective approach has been vibration, infrared, MCA and/or MCSA,” he advises. “As found in a recent study, 38% of motor system tests involving only vibration and/or infrared see a significant return on investment. This number jumped to 100% in systems that used a combination of MCA/MCSA along with vibration and/or infrared.”
Dave Polka, an instructor at ABB, says simple maintenance techniques can keep variable-frequency drives (VFDs) running reliably. Keep it clean, keep it dry and keep the connections tight, he says. “Dust on VFD hardware can cut airflow, while dust on electronics can cause malfunctions. Moisture can corrode circuit boards. Bad connections eventually lead to arcing, nuisance overvoltage faults, clearing of input fuses or damage to power components.”
While REG’s engineers rely on their Rockwell Automation diagnostics to keep their motors humming, they still manually test all new motors before installation. And Scott Kingery says they still have a full set of meters, meggers, gauges and equipment to diagnose a motor if one ever does fail.
When it comes to keeping motors humming, it pays to be as careful as possible.
Rich Merritt is a Control contributing editor.