It is important to consider mechanical and electrical issues when troubleshooting motor failure. Here are four steps to preventing failures in motor drives and rotating components.
CAPTURE INITIAL CONDITIONS
The first step is to capture critical information on the asset – operating condition, machine specifications and performance tolerance ranges – at installation. Having data that confirms how the asset is supposed to run makes it easier to detect any variations during regular preventive maintenance checks.
Before getting the asset up and running, check for issues such as soft foot (when the mounting feet of a motor are uneven), pipe strain (forces acting on other parts transfer backward into the motor), shaft voltage (exceeding the insulating capacity of the bearing grease causes flashover currents to the outer bearing), motor winding insulation resistance, overall vibration level and power consumption, harmonics and electrical unbalance.
ESTABLISH A PM SCHEDULE
A regular preventive maintenance (PM) schedule tracks the operating conditions of the motors in a facility. On each inspection round, a comparison of the new measurements to the originals can identify any anomalies. Add thermal imaging to routine testing to capture the heat output of motors and assets (see also feature pp18-19).
Mechanical issues will occur over time. Common mechanical issues include misalignment, when the motor drive shaft is not in alignment with the load; shaft imbalance, when the centre of a rotating part does not lie on the rotational axis; shaft looseness, excessive clearance between rotating and stationary elements inside a motor; and bearing wear, when surfaces slide against each other without enough lubrication to keep them apart.
Many breakdowns that arise from mechanical issues show up first as vibration. Incorporating a vibration sensor system into a preventive maintenance schedule can help to catch many problems before motor failure.
RECORD MEASUREMENTS
Create a baseline of each asset’s performance by saving the measurements and thermal images taken during a preventive maintenance routine. Any significant change in the trend line should be investigated to identify the underlying factors. The percent change in the trend line that warrants further investigation should be based on system’s required performance or the asset’s criticality.
Since variable frequency drives (VFDs) take one wave shape and convert it to another, establishing a baseline for running motors will allow maintenance technicians to see when the output changes. Using a true-RMS multimeter and a portable oscilloscope with a bandwidth of at least 200MHz, which Fluke Corp supplies, can diagnose problems related to VFDs. These can help detect problems such as reflections on drive output PWM signals (impedance mismatch between the source and load), sigma current(stray currents circulating in a system), and operational overloads. Portable oscilloscopes operating on a lower frequency range (and which are less expensive) can validate wave shapes and help technicians decide how to repair a VFD.
RUN A TREND ANALYSIS
Once a baseline has been established, it’s important to continue tracking and recording measurements regularly. Storing data and creating a trend analysis can help to diagnose many power-quality-based problems such as transient voltage (high voltage peaks that are very short in time that come with switching loads), harmonic distortion (unwanted additional sources of high-frequency AC voltage or current supplying energy to the motor windings) and voltage and current imbalance. These issues should be validated with power quality analysers that are capable of capturing the transients or high frequent harmonics.
Equipping maintenance engineers and technicians with the proper tools, knowledge and data will enable them to spot and rectify motor problems before they become serious.