Vibration Analysis for Electric Motors
Electric motors are essential electromechanical devices that convert electrical energy into mechanical energy. These motors come in two primary categories: direct current (DC) motors and alternating current (AC) motors. AC motors further branch into single-phase and polyphase motors, with polyphase motors being divided into induction and synchronous types. Among these, squirrel cage rotor induction motors stand out as the most commonly used in modern industry due to their simplicity, reliability, and low maintenance requirements.
Induction motors operate on the principle of electromagnetic induction, earning them the name "asynchronous motors" because their rotation speed (RPM) doesn't align with the synchronous frequency of the electromotive field. These motors are designed for various applications, taking into consideration factors like operating speed, power, and efficiency, with the number of poles determining the motor's operating speed.
Key Components of an Electric Motor
An electric induction motor with a squirrel cage rotor comprises several vital components, including:
Stator
Rotor
Motor Frame
End Shield
Bearings
Terminal Box
Support Base
Cooling Fan
Vibration Measurement Points
When measuring vibrations in an electric motor, it's crucial to place the sensors on the bearing housing's centerline, ensuring secure mounting on a stable surface. Measurements should ideally cover the horizontal (H), vertical (V), and axial (A) directions for each bearing.
On the non-drive end, sensor placement may be obstructed by the fan's protective cover. In such cases, positioning the sensor as close to the bearing as possible is recommended. Marking monitoring points and consistently collecting data from the same locations is essential. Some motor housings are made of non-magnetic materials, so a sensor can be manually held in place if necessary. It's advisable to avoid using extensions, as they can dampen high-frequency vibrations. Safety precautions are paramount to prevent contact with hot or rotating parts during data collection.
Common Failures Detectable by Vibration Analysis
Imbalance
In electric motors, an imbalance is usually caused by one of the following causes:
Wrong balancing procedure in the workshop.
Wrong selection of balancing quality grade.
Use of keyway out of specification.
Do not consider keyway during workshop balancing.
Deformation of the rotor due to excessive temperature.
Wear or breakage of the cooling fan.
Wear or breakage of the coupling.
Breaking or improperly mounting the cooling fan.
When an imbalance is diagnosed in an electric motor, the following actions may be recommended:
Review the operation and maintenance history to verify when the problem arose: in operation, after maintenance, new engine, etc. This will help with root cause analysis.
Verify the procedures and the quality grade of balancing for the application.
Inspect the coupling and cooling fan, and verify its integrity and conditions.
Perform a runout check to detect deformations in the rotor.
Check the calibration of the balancing machine.
Do a precision balancing according to the application, criticality, and characteristics of the motor.
Check the cooling fan.
Misalignment
In electric motors, misalignment is usually caused by one of the following causes:
An inappropriate alignment procedure.
Inappropriate calculation of alignment standards or tolerances.
Thermal expansion.
Weakness or inappropriate support base.
Soft foot or wraped motor base.
Failures in the coupling, excessive runout or deterioration.
When misalignment is detected in an electric motor, could recommend the following:
Evaluate staff procedures and training.
Do precision alignment by applying the relevant standards.
Measure and correct soft foot.
Evaluate the condition of the base and the coupling.
Evaluate the influence of thermal expansion and consider this in the alignment procedure.
Bearing issues
In electric motors, bearing problems are usually caused by one of the following causes:
Bad assembly, excessive preload, or wear of bearing housing.
Failures in the lubrication procedure, excess or deficiency of lubricant.
The lubricant is of poor quality or incompatible with the application.
Contaminated lubricant.
Excessive vibration during operation.
Grounding wiring failures.
When bearing issues are detected in an electric motor, the following actions may be recommended:
Analyze the characteristics of the damage: corrosion, erosion, wear ...
Evaluate the bearing assembly procedure.
Verify the balancing and alignment.
Verify the right selection of the bearing.
Check the dimensions and tolerances of mounting in the housing and rotor.
Optimize the lubrication process.
Check the earthing wire.
Eccentricity
In electric motors, eccentricity is usually caused by one of the following causes:
Excessive wear of bearing or bearing housing.
Wear in the rotor or bearing.
Misalignment between housings.
Deformation of the rotor.
Eccentricity of coupling holes or pulleys.
When eccentricity is detected in an electric motor, the following actions may be recommended:
Check alignment between bearing housing or motor end shields.
Check wear on bearing housings.
Measure the runout in coupling, pulleys, and rotor.
Rotating looseness
In electric motors, rotating looseness or clearances are usually caused by one of the following causes:
Wear of bearings or housing.
Wear of babbit bearings.
Bad adjustment of parts.
When gaps are detected in an electric motor, the following actions may be recommended:
Dimensional checks on bearing housing, bearings, and rotor.
Structural issues
In electric motors, structural issues are usually caused by one of the following causes:
Weak or damaged bases due to corrosion.
Defective anchor bolts or bolts.
Loose screws or anchor bolts.
When structural issues are detected in an electric motor, the following actions could be recommended:
Evaluate the condition of the bases, both of the machine and the foundation.
Evaluate the condition of the anchor bolts.
Adjust bolts to the right torque.
Electrical issues
In electric motors, electrical issues tend to originate from any of the following causes:
An uneven air gap between the rotor and stator.
Loose or broken rotor bar.
Poor quality of electric power: voltage imbalance, harmonics ...
Failures from frequency inverters.
Excess load.
Excess of motor starts and stops.
Insulation problems.
When electrical problems are detected in an electric motor, the following actions may be recommended depending on the symptoms:
Perform a power quality analysis.
Make a study of the air gap and evaluate the origin (rotor/stator).
Perform electrical tests on the motor (insulation quality).
Evaluate the integrity of the connections in the rotor bars.
Analyze the number of successive startups.
Resonance
In electric motors, resonance is usually caused by any of the following causes:
Operate near a critical speed or natural frequency of the system.
Changes in structural stiffness.
Speed changes that bring the motor closer to a critical speed (variable speed motors).
When resonance is detected in an electric motor, the following actions may be recommended:
Perform a test to calculate critical speeds.
Make an impact test to verify if any natural frequency is being excited.
Evaluate if the stiffness of the system has changed.
Engine Nameplate
The identification plate on the motor provides crucial information for reliable and efficient operation, inspection, and maintenance. Operating a motor beyond its design limits can significantly reduce its lifespan and efficiency. Operators, inspectors, and maintainers must understand and consider these parameters in relation to the machine's operational condition. Protecting and preserving the nameplate from environmental factors like solar radiation, pollution, and humidity is essential to maintain its readability and accuracy.
In conclusion, understanding the inner workings of electric motors and being able to detect common failures through vibration analysis are vital aspects of ensuring their efficient and reliable operation. By recognizing the components of an electric motor and the key points for vibration measurement, we can proactively identify issues and take appropriate actions.
Remember that proper maintenance and attention to detail, from balancing to alignment and bearing care, can extend the lifespan of these critical electromechanical devices. Furthermore, always pay heed to the information provided on the motor's identification plate, as it holds the key to optimizing performance and preventing premature wear.
By implementing these practices and prioritizing motor health, we can contribute to the smooth operation of various industries that rely on electric motors while minimizing downtime and maintenance costs.
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