The sigma currents are created as a result of the signal frequency, voltage level, capacitance and inductance in conductors. Sigma currents are essentially stray currents that circulate in a system. Instrument to measure and diagnose: Fluke 190-204 ScopeMeter® Test Tool, Fluke 1587 FC Insulation Multimeter Impact: Motor winding insulation breakdown leads to unplanned downtime ![]() In a motor drive circuit, the peak of the reflection could be as high as the DC bus voltage level. Impedance mismatches can occur as a result of improper installation, improper component selection or equipment degradation over time. Reflections are generated when there is an impedance mismatch between the source and load. Variable frequency drives employ a pulse width modulation (PWM) technique to control the output voltage and frequency to a motor. Impact: Decrease in motor efficiency results in added cost and an increase in operating temperaturesĬriticality: Medium Variable frequency drives 4 - Reflections on drive output PWM signals Each harmonic has a different acceptable level of distortion, which is defined by standards such as IEEE 519-1992. ![]() To start investigating harmonic distortion, use a power quality analyzer to monitor electrical current levels and temperatures at transformers to be sure that they are not overstressed. Some harmonic distortion of the current is normal on any part of the system serving electronic loads. These losses dissipate in the form of heat, which, over time, will deteriorate the insulation capability of the windings. This additional energy is not used to turn the motor shaft but circulates in the windings and ultimately contributes to internal energy losses. Simply stated, harmonics are any unwanted additional source of high frequency AC voltages or currents supplying energy to the motor windings. Impact: Imbalance creates excessive current flow in one or more phases that then increases operating temperatures–leading to insulation breakdownĬriticality: Medium 3 - Harmonic distortion At the simplest level, all three phases of voltage should always have the same magnitude. This imbalance can lead to stresses in each of the phase circuits in a three-phase power system. Potential faults may be in the cabling to the motor, the terminations at the motor, and potentially the windings themselves. An imbalance in impedance or load distribution can contribute to imbalance across all three of the phases. Three-phase distribution systems often serve single-phase loads. Instrument to measure and diagnose: Fluke 438-II Power Quality and Motor Analyzer Impact: Motor winding insulation breakdown leads to early motor failure and unplanned downtime For example, a transient may appear on control cables that don’t necessarily cause equipment damage directly but may disrupt operations. Finding the source of these transients can be difficult because of the infrequency of the occurrences and the fact that the symptoms can present themselves in different ways. ![]() These transients, which vary in amplitude and frequency, can erode or cause insulation breakdown in motor windings. ![]() Adjacent loads turning on or off, power factor correction capacitor banks or even distant weather can generate transient voltages on distribution systems. Transient voltages can come from several sources either inside or outside of the plant. This article demonstrates how to detect the 13 most common causes of winding insulation and bearing failure in advance. Winding insulation breakdown and bearing wear are the two most common causes of motor failure, but those conditions arise for many different reasons. It’s important to remember that the causes of motor and drive issues are not confined to a single domain of expertise–both mechanical and electrical issues can lead to motor failure–and being armed with the right knowledge can mean the difference between costly downtime and improved asset uptime. Motors are used everywhere in industrial environments and they are becoming increasingly complex and technical, sometimes making it a challenge to keep them running at peak performance.
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