Routine care of motors is a familiar-sounding and important part of a maintenance routine, for sure. But did you know that rogue electrical currents can blast bearings, that dust can knock drives offline or that a special screwdriver can keep...
February 1, 2013 | By By Carroll McCormick
Routine care of motors is a familiar-sounding and important part of a maintenance routine, for sure. But did you know that rogue electrical currents can blast bearings, that dust can knock drives offline or that a special screwdriver can keep muscle-bound electricians from causing motors to behave unreliably?
Read on to pick up some tips from on-the-job experts.
Perform regular health checks on motors. There are lots of parts in motors that need regular care. “Not unlike having a check up at your doctor’s office, periodic checks and measurements of your motors assist in early detection of any possible issues,” says Mark Gwinnett, head of service division, ABB.
“Tests such as PdMA (which can refer both to predictive maintenance and the PdMA Corporation, which specializes in predictive maintenance and condition monitoring), vibration and oil analysis will provide a complete health check on your motor.
“Critical applications in Canada’s oil sands in Fort McMurray – those that are in harsh conditions, for example – rely on these tests to ensure the motors are in top operating condition. These tests are also done on critical spares that could be required during emergency replacement.”
Tighten motor electrical connectors to the manufacturers’ recommended torque settings. For three months, a part of the domestic and international baggage handling system (BHS) at the Montreal Pierre Elliot Trudeau International Airport had logged faults, saw baggage totes delayed and go missing, and one time, the bag handling system stopped working.
The big trouble was that troubleshooting revealed nothing. That was until maintenance noticed one day that one of the system’s 75 motors was running very slowly. “We opened the motor junction box, where six bolts hold electrical connectors. They were misshapen and cut, giving a bad electrical connection. They had been tightened down so hard that they broke,” recalls Ghislain Reindeau, interim site manager, Cofely Services, which operates the BHS.
Maintenance discovered that 10% of the motors had bad connections. Cofely bought a set of high-end Wera torque screwdrivers and set them to the torque settings recommended by the motor manufacturer. “With the torque screwdrivers, I am 100% certain that the bolts will not be overtightened,” Reindeau declares.
Protect motor bearings from transient voltage damage. “The biggest issue we see with variable speed drives (VSD) and motors is the failure of the purchaser to buy some sort of protection for the bearings. VSDs can send harmful transient voltages down the shaft and wreck the bearings,” says Rob Van Bavel, district manager, Baldor Motors and Drives Ltd. He’s based in Edmonton, AB.
An unprotected motor may last only a few weeks. “You can hear the noise from metres away. The user thinks it is a bad bearing. Users in heavy industry understand the problem, but a lot of users in lighter industries do not,” Van Bavel says.
One solution is to order new motors with grounding bushings or retrofit them into existing motors – a task that maintenance teams should be able to do in-house. The bushings may cost between $200 and $500. For Class I, Division 2 hazard areas, however, which have a potentially explosive atmosphere, ceramic bearings are the safe solution, he advises.
Remove dust to prevent drives from tripping off. Nuisance trips that down drives may be caused by a dust and static discharge problem. “They may appear as ground faults, temperature faults, over-current faults or communications faults,” says Bill (who only goes by his first name). He works at BJ Electric Motor & Control Ltd. in Stewiacke, NS. “Dust will stick to the boards and cause a trip because of a static charge. Everything on your drives works on very low voltage. It does not take much to cause a nuisance trip.”
Cleaning dust out of drives should be part of the preventive maintenance program. Bill has two tidbits of advice for what to do come cleaning time: “One, make sure the power of is off for 30 minutes to let the capacitors bleed off before you blow them out. Some people say five minutes is enough, but it is not. Two, use clean, dry air to blow the dust out of the drives. It cannot be compressor air.”
Ventilate and clean motors and drives to prevent overheating. Motors and drives are designed to operate at certain temperatures. Bill offers this rule of thumb. “If a drive or motor is meant to operate at a 30-degree Celsius rise, it should be running at 60 degrees in a room with an ambient temperature of 30 degrees.”
What happens if the operating temperature is higher? “Every 10-degree increase in temperature above normal running temperature shortens the life of the drive or motor by half,” Bill says.
“For your PM, open up your drive and look for dust buildup and contaminates. Check to ensure that drive fans are not only freewheeling, but also are operating. Blow out the drives and motors.”
Regularly rotate the shafts in spare motors. There are stories out there of a maintenance crew pulling a brand new spare off the shelf, installing it and having it quickly fail or not work at all. A possible reason, especially for heavier motors, is that it sat for a long time without the shaft being rotated, so the weight on the bearings damaged the race.
“The weight of the armature will, over time, cause a bearing to put an indentation in the race. You put the motor in service and it doesn’t last very long,” says Peter Phillips, owner of the maintenance and training consultancy Trailwalk Holdings, in Windsor, NS. Consider incorporating a twice-yearly, 180-degree rotation of the shafts in your CMMS PM routine.
Double-check the amperage setting in your drives before starting up a new system. Phillips tells the story of a Canadian company that installed a packing line that had been built in the United States. The variable-frequency drives in the packaging equipment had been set in the US for 480 V and 3 A, but the Canadian plant was running on 575 V. The installers forgot to change the amperage from 3 amps to 0.3 amps and all 26 motors fried within days of starting up the line.
The moral of the story is that if you have a voltage change, double-check that the amperage is appropriate to the plant voltage; e.g., by removing the motor cover and verifying with a meter that the motor is drawing the right amperage.
How to halve the work of recording bearing temperatures and make the data more accessible. When it comes to recording motor temperature readings manually, Phillips says, “People drop the ball all the time. And there is no way to trend the temperatures if they are only on paper.” It can also be time consuming. “One company I know was taking down about 800 readings every day. It took a whole day to key them into the computer,” Phillips says.
Phillips suggests the following setup: First, put a barcode sticker on each motor. Read a sticker with a handheld reader (he recommends the Intermec CS40) and refer to screen shots you have taken and stored in the reader for where to take the readings for the motor that belongs to that bar code.
Use an infrared scanner to take the bearing temperature readings. Key the temperature readings into the reader. Move on to the next motor and repeat. Either by wireless or back in the office with the device in a cradle, transmit the data to your CMMS using software that lets the reader and computer communicate (Phillips uses EAM Mobile software).
This setup not only cuts by half the number of data entries, but the data is now in the computer, where it can be crunched to do temperature trending.
Consider purchasing drive PM kits. Kits with replacement parts can extend drive life. “Having a pro-active PM program is essential to maximizing the life expectancy of your drives,” ABB’s Gwinnet says. “Many companies offer PM kits that help keep your drives in top operating condition. They are assembled based on the age of the drive and the life expectancy of key components.
“Customers with a pro-active PM program have seen improved reliability and extended life expectancy of their drives.”
If a fuse blows in a drive, don’t just replace it. Look for the cause. “I’ve opened up cabinets and found 20 blown fuses sitting in there,” says Bill from BJ Electric. “My biggest beef with electricians is that they just replace the fuses and go home. I don’t like that.
“Don’t just reset the breaker or replace the fuse to keep the motor running. If a fuse blows, there is a reason. That is very dangerous. There is a reason why the protection devices are there.” MRO
Carroll McCormick is MRO Magazine’s award-winning senior contributing editor.