As motors and drives keep the process manufacturing world moving forward, technological innovation keeps motors and drives operating at efficient and optimal standards. From infrared camera technology that allows the user to sweep across running motors to find hot spots in real time to highly sensitive vibration sensors that alert you to potential issues — innovation is at the center.
The fact is that all equipment, including motors and drives, will wear down with use. The only way to prevent wear and tear on a motor is to simply not use it. Since this is unreasonable — and obviously counterproductive — the next best thing is to eliminate or substantially reduce the factors that cause undue wear and tear. Things like vibration, insulation thinning or tears, and overheating can shorten the life of a motor and reduce its capacity and performance.
With the right tools and knowledge, vibration, insulation and overheating in motors and drives can be discovered and repaired before irrecoverable damage occurs.
Vibration sensor technology
While all machinery vibrates, monitoring excess vibration in rotating equipment, such as motors, is key to early identification of asset issues. Vibration monitoring devices use accelerometers to measure changes in amplitude, frequency and intensity of forces that damage motors. Studying vibration measurements allows teams to discover imbalance, looseness, misalignment or bearing wear in equipment prior to failure.
Vibration screening allows teams to triage motor health issues while they are off doing more hands-on work elsewhere. The technology in sensors captures data that can be used to analyze and diagnose vibration issues in motors. These types of sensors are a scalable solution that can be installed anywhere, including in hard-to-reach or hazardous areas.
Data from vibration sensors is typically transmitted to a condition monitoring software accessible from smartphones and desktop computers, making the process of monitoring that much easier to do from anywhere. The combination of sensor and software makes it possible to wirelessly collect real-time information about the status of the motor and maintain historical records for that piece of equipment for future trending and analysis.
Insulation inspection technology
Insulation resistance testers identify current leaking from damaged or deteriorated insulation. Insulation keeps current moving along the wire, but if it is damaged in any way, current can leak out beyond the wire and travel into other parts of the motor, causing undue wear and tear and potentially dangerous working conditions. Insulation testers are ideal for testing capacitance and leakage of current in motors among other electrical equipment. Timed ratio tests are used to detect insulation resistance and include the polarization index (PI) and dielectric absorption rate (DAR). A good insulation resistance tester will automatically calculate the PI and DAR with no additional setup.
Insulation testers are engineered to check the integrity of conductor wires and check the insulation value of motor or compressor windings. What you are looking for is any breakdown that could potentially cause a fault. The insulation resistance tester allows you to perform streamline and baseline testing. The recommended procedure for using an insulation resistance tester is to double the value of the motor being tested (e.g., a 120 V motor should be tested at 250 V).
For facilities that use a run-to-fail maintenance approach, bringing systems back up quickly and efficiently is critical. Troubleshooting failed motors requires careful, step-by-step evaluation of several different motor elements. Insulation resistance testing provides useful information and data that can be used to determine the health of the motor. The two main tests to perform with an insulation resistance meter are line and load circuits to ground, and winding resistance to phase and phase to ground.
Infrared and thermal imaging technology
Thermal cameras collect infrared light and digitally translate it into visual temperature readings. Based on surface, or external, temperatures of objects, thermal cameras can make inferences about the internal temperature, depending on myriad other inputs such as density, material and environmental conditions.
Ideally, you should inspect a motor while it is running at a minimum of 40% of its typical load. That way, measurements can be properly evaluated compared to normal operating conditions. But is that dangerous? Not from a distance. The ability to see inside an enclosed motor while it is running can only be attributed to the technology found in infrared and thermal imaging cameras.
All motors have a listed normal operating temperature on the nameplate. The exterior surface temperature, detected by an infrared camera, is an indicator of the internal temperature. As the motor gets hotter inside, it also gets hotter outside. Hot motors can be a sign of inadequate airflow, impending bearing failure, shaft coupling problems and insulation degradation in the rotor or stator.
Experienced thermographers can also identify a misalignment in a shaft coupling associated with the motor. Even a novice can use a thermal imager to spot anomalies in a motor compared to other similar motors under similar loads and then flag that deviant motor for further investigation.
Motor drive analyzer
Voltage pulses from a variable speed 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, voltage arcs or electrical discharge machining can occur, causing pitting and fluting of the motor bearing race, damage that can cause a motor to fail prematurely. A motor drive analyzer and shaft voltage probe can be used to measure motor shaft voltage discharge events.
Motor drive analyzers are engineered to troubleshoot typical problems on three-phase and single-phase inverter type motor drive systems. The default motor shaft voltage measurement shows voltage waveforms as measured on the motor shaft itself. The voltage peak-to-peak value identifies the maximum level of the captured waveform, which is an indication that high voltage levels are present in the shaft.
Motor drive analyzer technology combines an oscilloscope with a power quality meter and recording capability for a complete range of electrical and electronic measurements.
- Measure key motor-drive parameters including voltage, current, DC Bus voltage level and AC ripple, voltage and current unbalance and harmonics, voltage modulation, and motor shaft voltage discharges.
- Perform extended harmonics measurements to identify the effects of low and high order harmonics on the electrical power system.
Check the voltage unbalance at the input terminals of the frequency speed drive to ensure the phase unbalance is not too high (> 6-8 %), and that the phase rotation is correct. Also check current unbalance, which may indicate a drive rectifier problem.
Sean Silvey has been a product application specialist at Fluke for the past five years. His focus is on application awareness and product education.