Electric motors failures
Mechanical and electrical problems can lead to engine failures. It is therefore important to know the possible causes in order to consider preventive maintenance actions before arriving at the unavailability of machines and creating financial losses as well as production impossibilities which can lead to customer dissatisfaction.
Winding insulation breaks and bearing wear are the two main causes of engine failure, but these problems themselves result from very different reasons.
The causes of failures
Group 1: quality of the power supply
- Transient voltage.
- Voltage imbalance.
- Harmonic distortion.
Group 2: Frequency converters
- Drive output PWM signals.
- Stray current.
- Motor overloads.
Group 3: Mechanical part of the engine
- Incorrect alignment of the output shaft.
- Tree imbalance.
- Loosening of the shaft.
- Wear of bearings.
Group 4: Mechanical part linked to an installation
- Unstable or wobbly foot support.
- Pipe constraint.
- Shaft tension.
Transient voltage
Transient voltages can be from different sources, internal or external to the site. High power activated or deactivated loads, the commissioning of reactive energy compensation capacitor banks, meteorological disturbances due to lightning, possible operations of the energy distributor, can generate transient voltages.
These transient voltages, whose amplitude and frequency are variable, may degrade the insulation of the motor windings.
Identifying the source of these transient voltages can be difficult as their appearances can be random, and their symptoms can present in different ways.
For example, a transient voltage can appear on control cables which are not directly responsible for the damage, but can nevertheless disturb the functioning of the systems.
Impact: Degradation of the insulation of the motor windings may short-circuit them.
It is therefore important to monitor the supply voltages, and to choose drives with input supply surge suppression components to protect the drive from the risk of transient voltages.
It may also be necessary to install surge arresters at the head of the electrical installation.
Voltage imbalance
Three-phase distribution systems often supply single-phase loads.
Any imbalance in impedance or load distribution may cause an imbalance on all three phases.
Potential failures can come from the motor wiring, its terminations, or even the windings themselves.
This imbalance can cause stress on all phase circuits of a three-phase system.
At the simplest level, all three voltage phases should always have the same magnitude.
Impact: The imbalance generates excessive current flow on one or more phases, which increases operating temperatures and degrades insulation.
Separate the three-phase and single-phase load power supplies.
Harmonic distortion
Harmonics refer to all additional sources of high frequency AC voltages or currents feeding the motor windings. This extra energy is not used to run the shaft motor but flows through the windings and ultimately contributes to internal energy losses. These losses dissipate in the form of heat, which gradually deteriorates the insulation capacity of the windings. A certain harmonic distortion of the current is normal on the elements of the system supplying electronic loads.
See the article on harmonics.
PWM drive output signals
PWM: Pulse Width Modulation -> pulse width modulation (PWM)
Variable speed drives use a pulse width modulation (PWM) technique to control the output voltage and frequency of a motor: The rapid change in voltage at the output of the variable speed drive which occurs a few thousand times per second and that goes from zero to maximum voltage each time.
With this kind of characteristic, when the cable going to the motor is long, reflections in the cable can occur and make up to double the maximums of the signal sent by the variable speed drive, which gives important voltage peaks which can sometimes exceed what the insulation of some engine components can withstand.
Small electric arcs then occur that cannot be perceived, but which gradually create non-reversible damage, until failure occurs.
Filters can be used between the drive output and the motor. You should also try to limit cable lengths.
Parasitic current: Reflection waves
The parasitic currents flowing in a system 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 unwanted tripping, or even generate excessive heat in the windings.
The parasitic current corresponds to the sum of the current of the three phases at any time T, which in an ideal situation causes the sum of these three currents to be equal to zero.
The use of symmetrical motor cables helps to reduce the phenomenon. The earth conductor (protective earth, PE) of the motor cable must be symmetrically arranged to avoid level currents at the fundamental frequency. This symmetry is obtained with a PE conductor enveloping all the phase conductors or with a cable made up of three phase conductors and three perfectly symmetrical earth conductors.
A short, low impedance route must be defined for the common mode current return to the drive. The most efficient and easy way to do this is to use shielded motor cables. The shielding must be continuous and made of a good HF conductor material (copper or aluminum). The connections at both ends must be made with an earth connection. Also use of a low pass filter.
Impact: circuit tripping due to a protective earth current.
overloads
Motor overloads occur when a motor is subjected to excessive load. The first symptoms of an overload are excessive consumption, insufficient torque and overheating.
Excessive engine heat is a major cause of failure.
In the event of an overload, the various components of the motor, such as bearings, windings and other components, may operate normally, but the motor remains too hot. Therefore, it makes sense to begin the troubleshooting procedure making sure that 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.
Ensure the correct sizing of the motor and regular maintenance of the mechanical systems.
To give motors additional protection against overheating, it is possible to attach a PTC temperature sensor (thermistor) to the motor and connect it to the converter control terminals.
Incorrect alignment
Alignment becomes incorrect when the motor driveshaft is not properly aligned with the load or when the component that connects the motor to the load is misaligned.
Many professionals believe that flexible coupling eliminates or compensates for misalignment.
However, this in fact only protects the coupling.
This is because even with a flexible coupling, a misaligned shaft transmits damaging cyclic forces along the shaft and into the motor, resulting in excessive motor wear and increased apparent mechanical load.
On the other hand, improper alignment can cause vibrations both to the load and to the motor driveshaft. Some types of incorrect alignment:
- Angular: the axes of the shafts cross, but are not parallel.
- Parallel: the axes of the shafts are parallel, but not concentric.
- Mixed: combination of angular and parallel defects (Note: most incorrect alignments are mixed alignments.
In practice, it is easier to treat the two forms separately.)
Impact: premature wear of mechanical drive components.
Shaft imbalance
An imbalance refers to a state of a rotating part whose center of mass is located outside the axis of rotation. In other words, there is a “heavy spot” somewhere on the rotor. Motor imbalances cannot be completely eliminated, but it is quite possible to identify values outside normal ranges and to take action accordingly. The imbalance can be caused by many factors, including:
- Contamination.
- Lack of counterweight.
- Uneven distribution of mass in motor windings and other factors related to wear.
A vibration tester or analyzer can be used to determine whether a rotating machine is balanced or not.
Impact: premature wear of mechanical drive components.
Loosening of the shaft
Looseness refers to excessive play between parts. There are different types of looseness:
- Rotary loosening is caused by excessive play between rotating and stationary machine parts, for example, a bearing.
- Non-rotational loosening between two normally stationary parts, such as a foot and a foundation, or a bearing housing and a machine.
As with all other sources of vibration, a vibration tester or analyzer can be used to determine whether a rotating machine is experiencing looseness.
Impact: accelerated wear of rotating components.
Bearing wear
Defective bearings increase friction, emit more heat, and have lower fuel efficiency due to mechanical problems, lubrication, or wear. Bearing failures can have different causes:
- Load greater than the nominal load.
- Incorrect lubrication.
- Bearing seals damaged.
- Poor alignment of the shaft.
- Faulty assembly.
- Normal wear.
- Induced shaft tensions.
Impact: accelerated wear of rotating components.
Unstable or wobbly foot support
A wobbly bracket is when the mounting feet of an engine or drive component are not level or the mounting surface on which the feet rest is not level.
Imbalances usually occur between two mounting bolts placed diagonally to each other.
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 in “normal” position with respect to the mounting surface.
In both cases, the wobbly support is due to an irregularity in the mounting feet or the floor on which they rest.
A quality laser alignment tool can usually identify any form of wobbly media on a rotating machine.
Impact: misalignment of mechanical drive components.
Pipe constraint
Line stresses are the new stresses, stresses, and forces acting on other equipment and infrastructure that pass through the engine and drive and cause misalignment. The most common example is a simple motor / pump combination, where something is putting pressure on the pipes, such as a pipe bracket, or other wall bracket item that is broken or missing.
These forces can be exerted angularly or offset on the pump, which in turn causes misalignment of the motor or pump shaft. Therefore, it is important to regularly check the alignment of the machine. This is because precision alignment is a temporary condition that can deteriorate over time.
Impact: misalignment of the shaft and induced stresses on rotating components.
Shaft tension
When motor shaft voltages exceed the insulation capacity of the bearing grease, currents directed to the outer bearing can occur and cause pitting and grooving on the bearing races. These currents are caused by the build-up of electrostatic energy in the machine’s rotor, which abruptly discharges when the electric field is sufficient to “crack” the insulating grease of the bearings. This problem is manifested first by the appearance of noise accompanied by overheating as the bearings begin to lose their original shape and fragments of metal mix with the grease and increase the friction. This could destroy the bearing in just a few months.
A current that is not uniformly distributed in the windings can inductively produce a high frequency voltage between the ends of the motor shaft, causing a high frequency current to flow through the two bearings and the frame of the motor. engine.
The probability of occurrence of these currents is influenced by the size and power of the motor, the supply voltage and the cut-off frequency of the frequency converter. This current is the most dangerous for large motor bearings and it flows with the same amplitude in both bearings.
The impedance of the transient current return circuit has the effect of modifying the potential of the engine frame in reference to the neutral level. If the shaft is accidentally grounded through the driven machine, some of the feedback current may flow through the motor bearings, the shaft and the driven machine bearings to the converter.
The current will therefore flow through and damage the bearings as soon as the voltage exceeds the insulation value of the lubricant film. This depends on the bearing, the load on the bearings, the type of lubricant, the speed of rotation and other operating conditions, such as vibrations, temperature etc. The cut-off frequency of the converter also has a direct influence. , and if the value that causes the bearing damage is reached, any increase in this cut-off frequency accelerates the deterioration of the bearings.
Impact: Electric arcs on the bearing surfaces can cause pitting and splines to appear causing excessive vibration and ultimately bearing malfunction.
