Motor and drive system technology continues to advance throughout the industry. Oftentimes, this increased complexity presents a challenge to the teams who are making sure the equipment runs at peak performance. Even then, some motor failures result from mechanical failures or even problems introduced at installation. Thus, maintenance personnel need to take different categories of components into consideration when troubleshooting motor problems.
Understanding the common reasons for motor failure and knowing how to troubleshoot and repair these systems can prevent costly downtime and improve operational efficiency. The following are common reasons motors fail.
Transient voltage spikes. Transient voltages, like the name suggests, can come from several sources, either inside or outside a plant. They can strike distribution systems from loads for adjacent equipment turning on or off, power factor correction capacitor banks, and even weather events. These transients, which vary widely in amplitude and frequency, can erode or break down the insulation in motor windings.
Voltage imbalance. The 3-phase distribution systems that are common in industrial plants often serve single-phase loads. An imbalance in impedance or load distribution can, in turn, contribute to imbalance across all three of the phases. Potential faults may be found in the cabling to the motor, the terminals at the motor, and potentially in the motor windings themselves. This imbalance can lead to stresses in each of the phase circuits in a 3-phase power system.
Harmonic distortion. Harmonics are essentially any unwanted additional source of high-frequency AC voltages or currents supplying energy to the motor windings. This additional energy is not used to turn the motor shaft but instead circulates in the windings and ultimately contributes to internal energy losses. These losses dissipate as heat that, over time, will deteriorate the insulation properties of the windings.
Variable-frequency drive (VFD) issues
Reflections on drive output PWM signals. Variable-frequency drives use a pulse-width modulation (PWM) technique to control the output voltage and frequency to a motor. Reflections can be generated when there is an impedance mismatch between the source and load. Impedance mismatches can come from improper installation, improper component selection, or when equipment degrades over time. In a motor drive circuit, the peak of the reflection can be as high as the DC bus voltage level. Generally, reflections or transients >50% of nominal voltage are problematic.
Sigma current. Sigma currents are stray currents that circulate in a system. Sigma currents are created because of signal frequency, voltage level, capacitance, and inductance in conductors. These stray currents can circulate through protective earth systems and cause nuisance tripping or lead to excess heat in windings. Sigma current can be found in motor cabling. It is the sum of the current of the three phases at any one point in time. In a perfect situation, the sum of the three currents would equal zero. Thus, the return current from the drive would be equal to the current to the drive.
Motor overloading. As the name suggests, motor overload happens when a motor is under excessive load. Typical symptoms for motor overload are excessive current draw, insufficient torque, and overheating. Excessive motor heat is a major cause of motor failure. An overloaded motor’s individual components, such as bearings, motor windings, and other components, may appear to operate normally, but the motor will run hot. Because of this, checking for overload is a good place to start when troubleshooting motors. Some 30% of motor failures are caused by overloading, so it is important to know how to identify and measure motor overloading.
Strategies to prevent motor failures
Motors and their control systems are ubiquitous in critical processes throughout manufacturing and industrial facilities. If they fail, the cost of downtime as well as repairing or replacing motor can be substantial. That’s why your maintenance teams need the appropriate skills, training, and tools to properly prioritize equipment criticality, monitor equipment health, and develop a preventive maintenance program that keeps motors in good working order to help forestall failures. In addition, maintenance teams need the training and procedural knowledge to troubleshoot intermittent problems to avoid failures because of normal operating stresses.
Implement the following four strategies to prevent premature failures in motors, drives, and rotating components:
- Document normal operating conditions, machine specifications, and performance tolerance ranges.
- At installation, be sure to capture and document critical operating measurements. Also, track these measurements before and after maintenance and at other intervals, depending on their criticality.
- Develop an archive of reference measurements to facilitate trend analysis and identify changes in status and conditions. Modern computerized maintenance management systems (CMMSs) can help manage data and help with this type of trend analysis.
- Plot individual measurements to establish a baseline, and then capture measurements over time to determine trends. It is recommended that any change in the trend line of more than ±10 % to 20 % (or any other percentage determined, based on your system performance or criticality) should be investigated to the root cause to understand why the issue is occurring.