Repairing an Electric Motor, Causes and Remedies by Fluke

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Repairing an Electric Motor, Causes and Remedies by Fluke




by Christophe » 15/03/19, 12:12

Fluke has just published an interesting article on maintenance (including preventive maintenance) of electrical or mechanical breakdowns electric motors (industrial types therefore heavy but some points are applicable to the general public of course): https://www.fluke.com/fr-fr/apprendre/b ... -de-moteur

Engines are present in all industrial environments and are increasingly complex and technical, which sometimes makes it difficult to perform optimally. It is important to understand that the causes of motor and training problems are not limited to a single skill area. Indeed, both mechanical and electrical problems can lead to engine failure. It is therefore important to equip yourself with good knowledge to avoid costly periods of unavailability and improve resource availability.

Breakdowns of winding insulation and bearing wear are the two main causes of motor failure, but these problems themselves result from very different reasons. This article explains how to identify the 13 most common causes of winding insulation failure and bearing malfunction.

1) Transient voltage

Transient voltages may have different sources, internal or external to the site. Enabled or disabled adjacent charges, power factor correction capacitor banks, or even remote weather conditions can generate transient voltages in the distribution systems. These transient voltages, the amplitude and frequency of which are variable, may degrade or even destroy the insulation of the motor windings. Identifying the source of these transient voltages can be difficult in that their occurrence is infrequent, and their symptoms can occur in different ways. For example, a transient voltage may appear on control cables that are not directly responsible for the damage, but may nevertheless disrupt operations.

Impact: Motor winding insulation degradation may result in anticipated failure and unplanned downtime

Measuring and diagnostic instrument: Fluke 435-II three-phase energy meter

Importance: high

2) Voltage imbalance

Three-phase distribution systems often serve single-phase loads. Any imbalance in the impedance or load distribution may cause an imbalance in all three phases. Potential malfunctions can come from motor wiring, terminations, and even windings themselves. This imbalance can lead to constraints on all the phase circuits of a three-phase system. At the simplest level, the three voltage phases should always be the same size.

Impact: the imbalance generates excessive current flow over one or more phases, which increases operating temperatures and degrades insulation.

Measuring and diagnostic instrument: Fluke 435-II three-phase energy meter

Importance: average

3) Harmonic distortion

In simple terms, harmonics refer to all additional sources of high frequency AC voltages or currents supplying the motor windings. This extra energy is not used to run the shaft motor but flows through the windings and ultimately contributes to the internal energy losses. These losses dissipate in the form of heat, which gradually deteriorates the insulation capacity of the windings. Some harmonic distortion of the current is normal on system components supplying electronic loads. To identify sources of harmonic distortion, use an energy meter to track current levels and transformer temperatures to make sure they do not get too much stress. Each harmonic has a permissible level of distortion of its own and is defined by standards such as IEEE 519-1992.

Impact: Reduced engine efficiency leads to additional costs and increases operating temperatures

Measuring and diagnostic instrument: Fluke 435-II three-phase energy meter

Importance: average
Frequency inverters

4) Reflections on PWM Training Output Signals

Variable speed drives use a pulse width modulation (PWM) technique to control the output voltage and frequency of a motor. Reflections are generated when the impedance is not adapted to the source and the load. Impedance discrepancies may occur due to improper installation, incorrect selection of a component, or gradual degradation of the equipment. In a motor drive circuit, the reflection peak may be equivalent to the voltage level of the DC bus.

Impact: Motor winding insulation degradation results in unplanned downtime

Measurement and Diagnostics Instrument: 4 Portable Oscilloscope for Fast Sampling ScopeMeter Fluke 190-204®.

Importance: high

5) Current sigma

Sigma currents refer to parasitic currents flowing through a system. Sigma currents depend on the frequency of the signal, the voltage level, the capacitance and the inductance of the conductors. These currents may pass through the protective earth conductors and cause spurious trips or even generate excessive heat at the windings. The sigma current is generally present in the wiring of the motors and corresponds to the sum of the current of the three phases at any time T. In an ideal situation, the sum of these three currents must be equal to zero. In other words, the return current of the drive must be equal to the current received by the latter. The sigma current can also be represented as asymmetric signals in a plurality of conductors capable of capacitively coupling currents in the earth conductor.

Impact: mysterious circuit tripping due to protective earth current

Measuring and diagnostic instrument: 4 portable oscilloscope ScopeMeter Fluke 190-204 ScopeMeter with current clamp (Fluke i400S or similar) and wide bandwidth (10 kHz).

Importance: low

6) Operational overloads

Motor overloads occur when an engine is overloaded. The first symptoms of an overload are excessive consumption, insufficient torque and overheating. Excessive engine heat is a major cause of failure. In case of overload, the various components of the motor, such as bearings, windings and other components, can operate normally, but the motor remains too hot. Therefore, it makes sense to start the troubleshooting procedure by making sure the motor is not overloaded. Since 30% of motor failures are due to overload, it is important to understand how to measure and identify motor overloads.

Impact: premature wear of electrical and mechanical components causing permanent failure

Measurement and Diagnostics Instrument: Fluke 289 Digital Multimeter

Importance: high
Mechanics

7) Incorrect alignment

Alignment becomes incorrect when the motor drive shaft is not properly aligned with the load or when the component connecting the motor to the load is misaligned. Many professionals believe that flexible coupling eliminates or compensates for misalignment. However, this only protects the coupling. Indeed, even with a flexible coupling, a misaligned shaft transmits damaging cyclic forces along the shaft and into the motor, which results in excessive wear of the motor and an increase in the apparent mechanical load. On the other hand, incorrect alignment may induce vibration in both the load and the motor drive shaft. Some types of incorrect alignment:

Angular: the axes of the trees intersect, but are not parallel
Parallel: the axes of the trees are parallel, but not concentric
Mixed: combination of angular and parallel defects (Note: most incorrect alignments are mixed alignments, in practice it is simpler to treat both forms separately.)

Impact: premature wear of mechanical drive components causing premature failure

Measurement and Diagnosis Instrument: Fluke 830 Tree Alignment Laser Tool

Importance: high

8) Imbalance of the tree

An imbalance denotes a state of a rotating part whose center of mass is located outside the axis of rotation. In other words, there is a "heavy point" somewhere on the rotor. Motor imbalances can not be completely eliminated, but it is quite possible to identify values ​​outside normal ranges and to apply measures accordingly. The imbalance may be due to many factors, including:

fouling
absence of counterweight
manufacturing variations
Unequal distribution of mass in the motor windings and other wear factors.

A tester or vibration analyzer can determine whether a rotating machine is balanced or not.

Impact: premature wear of mechanical drive components causing premature failure

Measuring and Diagnostic Instrument: Fluke 810 Vibration Tester

Importance: high

9) Loosening the tree

Looseness refers to excessive play between parts. There are different types of looseness:

the rotational loosening is caused by excessive play between the rotating and stationary parts of the machine, for example a bearing.
non-rotational loosening between two normally fixed parts, such as a foot and a foundation, or a bearing housing and a machine.

As with all other sources of vibration, it is important to know how to identify looseness and solve the problem before it becomes expensive. A vibration tester or analyzer can determine whether a rotating machine is loosened.

Impact: accelerated wear of rotating components causing mechanical failure

Measuring and Diagnostic Instrument: Fluke 810 Vibration Tester

Importance: high

10) Bearing Wear

Defective bearings increase friction, emit more heat and have lower energy efficiency due to mechanical problems, lubrication or wear. Bearing failures can have different causes:

load greater than rated load;
inadequate or incorrect lubrication;
inefficient bearing seals;
misalignment of the tree;
faulty assembly;
normal wear and tear;
induced shaft voltages.

When the bearing failure occurs, it has a cascading effect that significantly increases the risk of engine failure. 13% of engine failures are due to bearing failure and more than 60% of mechanical breakdowns at a site are attributable to bearing wear. Therefore, it is important to learn to identify the warning signs of this problem.

Impact: accelerated wear of rotating components resulting in rolling failure

Measuring and Diagnostic Instrument: Fluke 810 Vibration Tester

Importance: high
Factors related to improper installation

11) Wobbly foot support

A wobbly support means that the mounting feet of an engine or drive component are not level or that the mounting surface on which the feet are resting is not equal. This situation is the source of many frustrations, as the tightening of the foot mounting bolts induces more constraints and misalignment. Imbalances generally occur between two mounting bolts placed diagonally to one another, much like a chair or table that tends to swing on its diagonal. There are two types of wobbly foot support:

parallel: one of the mounting feet is higher than the other three;
Angular: One of the mounting feet is not parallel or "normal" to the mounting surface.

In both cases, the wobbly support is due to an irregularity in the mounting feet or floor on which they rest. Anyway, it is important to remedy the situation to get a good alignment of the tree. A quality alignment laser tool generally identifies any form of wobbly support on a rotating machine.

Impact: misalignment of mechanical drive components

Measuring and Diagnostic Instrument: Fluke 830 Tree Alignment Laser Tool

Importance: average

12) Pipeline constraint

Pipeline constraints refer to new efforts, new stresses and new forces that affect the rest of the equipment and infrastructure that are passed on to the engine and drive and cause misalignment. The most common example is a simple combination of motor / pump, where something exerts pressure on the pipes, such as:

a movement of foundations;
a new valve or other component;
a striking object, twisting or simply pressing a pipe;
a pipe support, or other wall support item, broken or missing.

These forces can be angularly or staggered on the pump, which in turn causes misalignment of the motor shaft or pump. Therefore, it is important to regularly check the alignment of the machine. Indeed, precision alignment is a temporary state that can degrade over time.

Impact: misalignment of the shaft and induced stresses on the rotating components, leading to premature failures.

Measurement and Diagnosis Instrument: Fluke 830 Tree Alignment Laser Tool

Importance: low

13) Tree tension

When motor shaft voltages exceed the insulation capacity of the bearing grease, currents directed to the outer bearing may occur and result in pits and grooves on the bearing races. This problem first results in the appearance of a noise accompanied by overheating due to the fact that the bearings begin to lose their original shape and that fragments of metal mix with the grease and aggravate the friction. This can cause the destruction of the bearing in just a few months. Rolling failures are costly for both engine repair and downtime. Therefore, it is important to take preventive measures such as measuring shaft tension and running current in order to anticipate problems. The shaft voltage is present only when the motor is energized and running. A carbon brush probe is used to measure the tension of the shaft during the rotation of an engine.

Impact: Arcs on bearing surfaces can lead to pinholes and splines causing excessive vibration and, ultimately, bearing malfunction

Measuring and diagnostic instrument: 4 portable oscilloscope, isolated ScopeMeter Scopes Fluke 190-204, with AEGIS carbon brush sensor for shaft tension measurement.

Importance: high
Four strategies for success

Engine control systems are present in essential processes at all levels of the production lines. Hardware failures have significant financial consequences for potential engine or part replacement as well as engine downtime. Providing engineers and maintenance technicians with the right knowledge, prioritizing the workload, and managing preventative engine maintenance and resolving intermittent and sometimes hard-to-identify problems can, in some cases, prevent stress-related failures. operating costs and reduce the overall cost of downtime.
Four key strategies can solve or even prevent premature failure of rotating motors and components:

Document operating conditions, machine specifications, and performance tolerance ranges.
Capture and document critical measurements during installation, before and after maintenance, and routinely.
Archive baseline measurements to facilitate trend analysis and identify changes in status.
Draw individual measurement graphs to establish a basic trend. Any change in the baseline trend of more than +/- 10% to 20% (or any other% depending on the performance and importance of your system) requires the identification of the root cause, in order to understand why this problem occurs.



13 good tips to check or follow (depending on engine technology installed) for any good engine installation!
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