Fluke Motor Maintenance Solutions

Motors are used everywhere in industrial environments and they are becoming increasingly complex and technical, sometimes making it a challenge to keep them running at peak performance. It is important to remember that the causes of motor and drive issues are not confined to a single domain of expertise – both mechanical and electrical issues can lead to motor failure – and being armed with the right knowledge can mean the difference between costly downtime and improved asset uptime.

Combining thermal, vibration, insulation, and electrical testing are each a part of the entire operational picture. Technicians and teams would do well to keep electrical measurements, thermal images, insulation resistance readings, and vibration test results stored and accessible, as a baseline for future maintenance or troubleshooting - having a place to start from shortens the time on task, significantly.

1. Thermal

When dealing with operational equipment, technician should start with a thermal inspection. It is less dangerous to be several feet away from a motor and take a thermal picture to view what kind of heat signature the system is giving off.  Differences in temperature will also help techs and teams build a quick list of what to measure first with other measurement tools.

While an infrared camera is ideal for inspection work, since it provides an infrared view of the entire electromechanical system, an infrared thermometer works just fine for quick and easy spot temperature comparisons, and is much easier to carry around. Simply press the trigger to take the first measurement, save it, refocus, and take a second measurement - then compare.

Fluke offers a wide variety of imagers and thermometers to choose from, making it easy to beging building a motor maintenance routine.

2. Vibration

Common wisdom in the maintenance world dictates that the majority of mechanical failures with rotating machines come from four faults: unbalance, misalignment, roller bearing problems, and looseness.

• Roller bearings account for up to 60 percent of machine failures
• Unbalance accounts for up to 50 percent of machine failures
• Misalignment accounts for up to 45 percent of machine failures

What not everyone knows is that vibration is one of the earliest indicators of machine health. Vibration testing can detect an issue much earlier than thermal inspection, long before components are damaged. These days, people are also thinking about machine efficiency. When machinery is misaligned, friction starts to eat up not just the hardware but also the electrical supply, creating more than one reason to take a reading every so often.

Fluke has a suite of handheld vibration meters and testers make all of this testing much easier and quicker than it used to be.

3. Insulation

Insation problems on motors and drives are usually caused by excessive heat, improper installation, environmental contamination, mechanical stress, or age. Insulation testing can easily be combined with regular motor maintenance (to identify degradation before failure), and with installation procedures (to verify system safety and performance).

Fluke's insulation resistance testers can be the missing link in any maintenance program, enabling technicians to get a motor back into operation quickly and easily.

4. Electrical

Current unbalance is a common root cause of motor overheating. Current unbalance can be caused by several different things, including power delivery problems, low voltage on one leg, or insulation resistance breakdown inside the motor windings. The worst-case scenario of current unbalance, single phasing, is when one entire phase is lost and the fuse is blown.

The best measurement tool to detect single phasing is the clamp meter: a zero current measurement in one phase is a clear indication of power loss. Fluke makes many clamp meters, as well as DMMs that can test motors.

Infrared cameras, also called thermal cameras, are handy for inspecting motors because you can see what’s going on while the motor is running. Seeing a motor’s heat signature under normal operating conditions—with at least 40 % of design load—can tell you a lot about its condition. If you have to inspect a motor in a low load situation, keep an eye on even minor problems, because as the load increases, the temperature will increase, and if there’s a problem, those small temperature differences will be magnified.

The upper bearing on the far motor is failing, causing the entire motor to overheat. (Photo courtesy Greg McIntosh, Snell Infrared Canada)

• Scan the outside of the motor housing: Technicians should look for hot spots, or if the overall housing is much hotter than normal.
• Check the convection cooling fan (if present): Techs can use thermal imaging to to make sure convection cooling fans are running.
• Inspect windings: A localized hot spot could mean that the windings are burning out due excessive heat from transients or harmonics on the line.
• Pop off the termination box cover and scan the connections: Too loose or too tight connections, or broken strands on the lines can cause high resistance that will dissipate some of the electrical energy as heat.
• Check bearing temperature: Thermal imagers can detect if the bearing temperature is measuring 5°C to 10°C hotter than the motor housing, which might indicate a misalignment problem, that a bearing is worn out, or that a bearing is over-/ under-lubricated.

Maximizing the utility of thermal imaging for motor maintenance means starting with a newly commissioned and freshly lubricated motor. The technician should take a "snap shot" of the motor bearing housing while the motor is running, and use this image as a baseline. Take additional thermal images at regular intervals, comparing them to the baseline to analyze the motor's condition. If a technician or team builds a thermal image archive of critical motors over time, it will be much easier to tell whether a hot spot is an actual problem, or not.

As a motor and its lubrication ages, the bearings become worn. Heat-producing friction develops and builds up over time, which will cause the outside of the bearing housing to heat up as well.  When thermal images indicate an overheated bearing, technicians will want to generate a maintenance order to replace or lubricate the bearing housing, in order to reduce (or hopefully, to eliminate) the possibility of costly engine failure.

After the motor is repaired, the technician/team should use the thermal imager to verify that the repairs are successful, thereby creating a new baseline to work from.

Vibration analysis helps diagnose the most common faults that rotating machines are susceptible to: imbalance, looseness, misalignment, and wear. The compatible trio of owning the right tools, knowing what to test, and sustaining a functional vibration testing program can potentially reduce repair costs and limit unproductive maintenance hours.

Excess vibration can frequently be detected prior to other signs of trouble, and is often a precursor to machine failure. Signs of trouble that a technician might detect include changes in heat, sound, or energy consumption. Fortunately, data gathered from vibration screening makes it possible to accurately determine how much time a machine has left before it fails, enabling techs to take the appropriate steps to prevent a motor breakdown.

One benefit of vibration screening is predictable. The early warning offered by vibration screening means that maintenance staff has adequate time to acquire all parts required for repairs. It also means that the repairs can be scheduled in advance for a convenient time.

Safety is another major benefit of vibration testing. If a machine is on the path to failure, it can be taken offline before it deteriorates to the point of becoming a hazard or liability.

Preventive maintenance is almost always cheaper in the long run, as it allows for the avoidance of overhead, overtime and downtime. Regular maintenance and testing allows for better, more accurate budgeting, and decreases the likelihood of a failure that would lead to a hault in production. Using vibration testing helps keep the costs associated with repairs in check, with the potential to save money in a number of ways.

1. Test common faults and failures

Unchecked machine vibration can accelerate the rate of wear, damage to equipment, create noise, cause safety problems, and degrade plant working conditions. There are three main causes of excess vibration:

• Imbalance is caused by manufacturing defects or maintenance issues. As machine speeds increase, the effects of imbalance become greater and cause more damage. Most noteably, it can substantially reduce bearing life.
• Looseness can lead to vibration-derived damage, most likely to manifest in bearing wear and mount fatigue.
• Misalignment happens when (for example) the axis of a motor and pump are not parallel. This fault can occur during assembly, develop over time due to shifting components, or  when a motor is not reassembled correctly during maintenance.
• Abnormal or excessing vibration can—and often will—cause wear on a motor's drive belts, gears, and roller bearings.

Many teams get stuck repeating repairs to the same machines. The common stop-gap is to replace motor and pump bearings. This often leads to repeated failures, and more and more rounds of replaced bearings. Opting to replace worn bearings instead of regularly aligning machines is expensive, because of the cost of new componets, not to mention wasted resource and man-hours.

It goes without saying that any facility manager prefers operations to run as smoothly as possible. With this is mind, it makes perfect sense to incorporate vibration detection as a part of any facility's existing preventive maintenance program.

2. Start and sustain a vibration program

The following four tips are key to starting a new vibration maintenance program:

1. Start small, prove its success, and earn more budget to grow the program.
2. Begin to monitor vibration in simple machines, paying special attention to the onese that have a known history of failure.
3. Focus on the common machine faults. The four most common faults account for 80 to 90% of all machine faults.
4. Use automation and proven measurement methodology to get a complete picture of a machine's entire power train.

3. Buy the right vibration tools

In a vibration program, the vibration meter is used to screen machines, determining whether they are healthy or show signs of impending problems. Handheld vibration meters help technicians track bearing health, impacting, surface temperature, and measure overall vibration level. Multiple-function screening tools employ algorithms that can spot flaws early, providing methods to share digital results with othe members of the team members.

Periodic motor maintenance tests provide valuable information about the state of insulation deterioration and will help predict a possible failure of the system. Preventing problems will result in a more trouble-free system and extend the operating life of system equipment. Insulation resistance testers can be used to determine the integrity of windings or cables in motors, transformers, switchgear, and electrical installations.

This is what we know about motor insulation: it will deteriorate over time and with use. Deteriorated or damaged insulation leads to current leakage. Current leakage causes undo wear on the motor.

Preventive maintenance that includes insulation resistance testing gives maintenance managers more control over electric motor systems. Motor insulation will deteriorate and need service at some point in time. With preventive maintenance, it is up to the maintenance manager to determine just when that is.

Wire insulation coating inside motors deteriorates over time and with typical wear-and-tear. One way to identify insulation deterioration is by analyzing current data collected over time. When insulation deteriorates or has been damaged, current will leak into parts of the motor it shouldn’t, causing undo wear. The insulation keeps the current flowing along the wire, exactly as intended.

Insulation resistance testing of motors focuses on capacitance and current leakage. Timed ratio tests (spot tests) are used to detect insulation resistance and include the polarization index (PI) and dielectric absorption rate (DAR). Fluke’s insulation resistance testers automatically calculate the PI and DAR with no additional setup. The test data will identify changes in current over the specified period, then produce a comparison in terms of a ratio.

Drive input step-by-step guided measurement connections

Ripple on a DC Bus

Voltage and current on the drive output

Motor drives are a ubiquitous technology for transforming the constant voltage from the main ac power supply into a voltage that varies to control motor torque and speed, ideal for motors that are driving mechanical equipment loads. Motor drives provide higher efficiency than simple online motors and a degree of control not available on simple directly driven motors. These factors result in energy cost saving, higher production performance and extend the life of the motor.

Not surprisingly, motor drive and electric motors are commonly used in many industries and facilities - residential, commercial, industrial, entertainment, government, and anywhere else requiring controlled rotating output. To ensure uptime in these motor systems, maintenance and troubleshooting is a priority.

Electric motors are one of the most reliable pieces of electrical equipment around, provided that they are used and installed correctly, with the proper size and power source connection taken into account. In such conditions, they could be expected to last for a decade or more with minimal maintenance required. Motor drive analyzers are designed to quickly and easily test and troubleshoot typical problems on three-phase and single-phase inverter type motor drive systems.

1. Drive input

Analyze the power going into the motor drive is an excellent first step to determining if a feeder circuit to the drive has distortion, disturbance, or noise that may be affecting power ground.

2. DC bus

The conversion of AC to DC inside the drive is critical. Having the correct voltage and adequate smoothing with low ripple is required for the best drive performance. High ripple voltage may be an indicator of failed capacitors or incorrect sizing of the connected motor. The record function of a Fluke MDA-500 Series Motor Drive Analyzer can be used to check DC bus performance dynamically in the operating mode while a load is applied. Alternately, a Fluke ScopeMeter® Test Tool or advanced multimeter can be used for this test.

3. Drive output

Testing the drive output is critical to proper motor operation and can offer clues to problems within the drive circuits.

4. Motor input

Voltage supplied at the motor input terminals is key, and cable selection from drive to motor is critical. Incorrect cabling selection can result in both drive and motor damage due to excessive reflected voltage peaks. These tests are mostly identical to those for drive output above.

5. Motor shaft voltage

Voltage pulses from a motor drive can couple from a motor's stator to its rotor, causing a voltage to appear on the rotor shaft. When this rotor shaft voltage exceeds the insulating capacity of the bearing grease, flashover currents (sparking) can occur, causing pitting and fluting of the motor bearing race, and damage that can cause a motor to fail prematurely.

Starting at the inputs, technicians should make sure that a troubled motor and drive system is getting the high quality power it needs, and that it is not adversely affecting power quality upstream.

How to troubleshoot motors and drives, starting at the inputs - Application note (pdf)