Note These Causes of Motor Failure
EP Editorial Staff | October 8, 2017
Brushing up on important root causes now can prevent headaches later.
Winding insulation breakdown and bearing wear are the two most common causes of motor failure, but they arise for many different reasons. Based on information in a recently released “Application Note” from Fluke (fluke.com, Everett, WA), the following list highlights the 13 most common causes of failure in winding insulation and bearings.
1. Transient voltage. Transient voltages can come from a number of sources either inside or outside of the plant. Adjacent loads turning on or off, power-factor-correction capacitor banks, or even distant weather can generate transient voltages on distribution systems. These transients, which vary in amplitude and frequency, can erode or cause insulation breakdown in motor windings. Note: Finding the source of these transients can be difficult because of the infrequency of occurrences and the fact that the symptoms can present themselves in different ways. For example, a transient may appear on control cables that doesn’t necessarily cause equipment damage directly, but may disrupt operations. Criticality: High
2. Voltage imbalance. Three-phase distribution systems often serve single-phase loads. An imbalance in impedance or load distribution can contribute to imbalance across all three of the phases. Potential faults may be in the cabling to the motor, the terminations at the motor, and potentially the windings themselves. This imbalance can lead to stresses in each of the phase circuits in a three-phase power system. Note: At the simplest level, all three voltage phases should always have the same magnitude. Criticality: Medium
3. Harmonic distortion. Simply stated, harmonics are 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 circulates in the windings and ultimately contributes to internal energy losses. These losses dissipate in the form of heat, which, over time, will deteriorate the insulation capability of the windings. Note: Some harmonic distortion of the current is normal on any part of the system serving electronic loads. Criticality: Medium
4. Reflections on drive output with PWM signals. Variable-frequency drives employ a pulse-width modulation (PWM) technique to control the output voltage and frequency to a motor. Reflections are generated when there is an impedance mismatch between the source and load. Impedance mismatches can occur as a result of improper installation or component selection, or equipment degradation over time. Note: In a motor-drive circuit, the peak of the reflection could be as high as the DC bus voltage level. Criticality: High
5. Sigma current. Sigma currents are essentially stray currents that circulate in a system. The sigma currents are created as a result of the signal frequency, voltage level, capacitance, and inductance in conductors. These circulating currents can find their way through protective earth systems causing nuisance tripping or, in some cases, excess heat in windings. Sigma current can be found in the motor cabling and 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. In other words, the return current from the drive would be equal to the current to the drive. Note: Sigma current can also be understood as asymmetrical signals in multiple conductors that capacitively couple currents into the ground conductor. Criticality: Low
6. Operational overloads. Motor overload occurs when a motor is under excessive load. The primary symptoms are excessive current draw, insufficient torque, and overheating. Excessive motor heat is a major cause of motor failure. In the case of an overloaded motor, individual motor components, including bearings, motor windings, and other components, may be working fine, but the motor will continue to run hot. For this reason, it makes sense to begin your troubleshooting by checking for motor overload. Note: Overload is the cause of 30% of motor failures. Criticality: High
7. Misalignment. Misalignment, be it angular, parallel, or compound, occurs when the motor drive shaft is not in correct alignment with the load, or the component that couples the motor to the load is misaligned. Many professionals believe that a flexible coupling eliminates and compensates for misalignment, but a flexible coupling only protects the coupling from misalignment. Even with a flexible coupling, a misaligned shaft will transmit damaging cyclical forces along the shaft and into the motor, leading to excess wear and increasing the apparent mechanical load. Note: Misalignment may also feed vibration into the load and the motor drive shaft. Criticality: High
8. Shaft imbalance. Imbalance is a condition of a rotating part in which the mass center does not lie on the axis of rotation. In other words, there is a “heavy spot” somewhere on the rotor. This condition can be caused by numerous factors, i.e., dirt accumulation, missing balance weights, manufacturing variations, uneven mass in motor windings, and other wear-related issues. Note: Although motor imbalance can never be completely eliminated, personnel can identify when it is out of normal range and rectify the problem. Criticality: High
9. Shaft looseness. Looseness occurs when there is excessive clearance between parts, and it can show up in several places: Rotating looseness is caused by excessive clearance between rotating and stationary elements of the machine, such as in a bearing. Non-rotating looseness occurs between two normally stationary parts, such as a foot and a foundation, or between a bearing housing and a machine. Note: As with other sources of vibration, it’s important to know how to identify looseness and resolve the issue. Criticality: High
10. Bearing wear. A failed bearing has increased drag, emits more heat, and has lower efficiency. Regardless of the cause, once bearing failure begins, it creates a cascade effect that accelerates motor failure. Note: Bearing failure causes 13% of motor failures and bearing wear causes more than 60% of the mechanical failures in a facility. Criticality: High
Improper installation factors
11. Soft foot. The term “soft foot” refers to a condition in which the mounting feet of a motor or driven component are not even, or the mounting surface upon which the mounting feet sit is not even. This can create a frustrating situation in which the act of tightening mounting bolts on a machine’s feet introduces new strains and misalignment. Soft foot is often manifested between two diagonally positioned mounting bolts, similar to the way that an uneven chair or table tends to rock in a diagonal direction. Note: Be it parallel or angular in nature, soft foot must be remedied before proper shaft alignment can be achieved. Criticality: Medium
12. Pipe strain. The term pipe strain refers to the condition in which new stresses, strains, and forces, acting on the rest of the equipment and infrastructure, transfer backward onto the motor and drive to induce a misalignment condition. The most common example of this is in simple motor/pump combinations, where the pipework is subjected to some type of force, i.e., a foundation shift; a newly installed valve or other component; an object striking, bending, or pressing on a pipe; or broken, inadequate, or missing pipe hangers or wall-mounting hardware. These situations can cause the motor/pump shaft to be misaligned. Note: It’s important to check machine alignment more than just at the time of installation. Precision alignment is a temporary condition that can change over time. Criticality: Low
13. Shaft voltage. When motor shaft voltages exceed the insulating capability of the bearing grease, flashover currents to the outer bearing will occur. These currents will cause pitting and grooving in the bearing races. The first signs of this problem will be noise and overheating as the bearings begin to lose their original shape and shed metal fragments that mix with the grease and increase friction. This can lead to bearing destruction within a few months of motor operation. Measuring shaft voltage and bearing current is an important step in preventing this type of situation. Note: Shaft voltage is only present while a motor is energized and rotating. Criticality: High
For more details on each of these 13 causes and their impact, as well as information on tools and strategies for detecting such problems in advance, download Fluke’s entire “Application Note” and/or visit fluke.com.