Efficient Plant October 2017 - 36
feature | equipment reliability
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.
2. Voltage imbalance. Three-phase distribution systems often serve
single-phase loads. An imbalance in impedance or load distribu-
tion 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.
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
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.