Fault Diagnosis Manual for Electric Motors: Identification, Causes, and Solutions
When a critical electric motor stops, the clock is ticking against productivity. Identifying whether the problem is a simple tripped protection or a burned winding is vital for making quick decisions: to repair or replace.
This technical manual is designed to guide you through a logical troubleshooting protocol, allowing you to accurately and safely isolate the root cause of the failure.
Types of Failures in Electric Motors: Mechanical or Electrical?
Before grabbing the tools, we must classify the "patient." Statistically, it is estimated that more than 50-60% of failures in motors are of mechanical origin, primarily related to bearings. However, electrical failures tend to be more catastrophic and immediate.
Differentiating the origin is the first step in diagnosis:
Electrical Failures (Short Circuits, Overloads, Phase Imbalance)
Affect the internal circuit (stator/rotor) or the power supply.
Typical Symptoms: burnt varnish smell, immediate tripping of protections (thermal or differential), excessive heating without load, electrical humming without rotation.
Common Causes: short circuit between turns, loss of insulation to ground, phase imbalance in the grid, or failure in the stator winding.
Mechanical Failures (Bearings, Misalignment, Shaft)
Affect the kinematics of the motor.
Typical Symptoms: metallic or cyclical noise, excessive vibration, progressive temperature increase, shaft locking.
Common Causes: wear in the motor bearings, lack of lubrication, misalignment with the load, or issues in the squirrel cage rotor (broken bars).
Essential Tools for Diagnosis (Multimeter, Megohmmeter, Thermal Camera)
An accurate diagnosis requires proper instrumentation. Do not rely solely on intuition.
Digital Multimeter: essential for verifying the supply voltage (that it is balanced and correct) and conducting basic continuity and ohmic resistance tests on the windings.
Megohmmeter (or Megger test equipment): the multimeter is not sufficient to see current leaks. The megohmmeter injects high voltage (500V / 1000V) to measure the insulation resistance between the coils and ground. It is the ultimate tool for detecting moisture or degraded insulation.
Clamp Meter: vital for measuring current consumption in operation and detecting imbalances between phases under load.
Thermal Camera: allows you to see "hot spots" invisible to the human eye, indicating poor connections on the terminal block or cooling issues.
Step-by-step Diagnostic Process: Inspection Protocol
Follow this workflow to safely isolate the failure.
Step 1: Visual and Auditory Inspection (Smell, Noise, Vibration)
Before touching anything, disconnect and lock out the energy (LOTO).
Smell: Does it smell "burned"? The acrid smell of melted insulation is unmistakable and indicates severe damage to the winding.
Connection Box: open the terminal block. Look for charred wires, loose connections, or oxidized contact bridges. Poor contact here generates arcs and heat.
Exterior: look for grease leaking from seals (bearing failure) or areas of the housing darkened by heat.
Step 2: Continuity and Resistance Tests of Windings
With the motor disconnected from the grid and the links of the terminal block removed:
Set the multimeter to ohms (Ω).
Measure the resistance between the phases (U-V, V-W, W-U).
Interpretation: the three values should be practically identical. A significant difference indicates a short circuit between turns or an open coil.
Step 3: Insulation Test (Megger Test)
This test determines if the motor is "grounded".
Connect the negative lead of the megohmmeter to the motor housing (ground) and the positive to one of the terminal block terminals.
Apply the test voltage (generally 500V for low voltage motors).
Interpretation:
> 100 MΩ: excellent insulation.
10 - 100 MΩ: acceptable insulation (monitor).
< 1 MΩ: critical insulation failure. The motor is grounded or has significant internal moisture. Do not start.
Step 4: Verification of Consumption and Amperage
If the static tests (resistance and insulation) are correct, we can attempt a no-load start (if it is safe).
Start the motor and use the clamp meter.
Measure the amperage in each of the three phases.
Interpretation: the consumption should be balanced. A phase imbalance greater than 5-10% in current suggests a problem in the stator or in the supply voltage.
Quick Guide: Symptom → Probable Cause → Suggested Solution
Use this table for quick identification of issues in the plant.
Symptom | Probable Cause | Suggested Technical Solution |
The motor hums and does not start | One phase is missing (single phasing) or the capacitor is broken (single-phase). | Check fuses, contactor, and replace the capacitor if applicable. |
Immediate tripping of the differential | Ground leakage. | Perform a Megger test and rewind if the insulation is < 1 MΩ. |
Excessive heating | Thermal overload, obstructed ventilation, or low voltage. | Clean grills, check mechanical load, and measure network voltage. |
Metallic noise / squeaking | Failure in motor bearings. | Replace bearings and check lubrication. |
Excessive vibration | Misalignment, loose anchorage, or rotor imbalance. | Align with laser, tighten bolts, and check fan/shaft. |
Imbalanced consumption | Short circuit between turns or unbalanced grid voltage. | Measure resistance of coils and input voltage. |
Predictive Maintenance: How to Avoid Failure Before It Happens
The diagnosis described above is reactive (when failure has already occurred). Modern industry moves from corrective vs. predictive maintenance.
Implementing predictive techniques allows detecting the "disease" months before a "heart attack":
Vibration Analysis: detects spikes in bearing tracks or emerging misalignments through the frequency spectrum.
Periodic Thermography: reveals hot spots in connections and electrical panels before they fuse.
Frequently Asked Questions (FAQ)
Why is an electric motor overheating?
Heat is the number one enemy of insulation. The main causes are: dirt blocking ventilation, operating above rated current (thermal overload), low or unbalanced supply voltage, and excessively frequent starts.
What does a humming noise indicate if the motor does not start?
It generally indicates that the motor is missing a phase (in three-phase motors) or that the starting capacitor has failed (in single-phase motors). The magnetic field attempts to generate but lacks rotational force. Attention: disconnect immediately, as the current spikes and will burn out the winding in seconds.
Conclusion
Diagnosing failures in electric motors is not magic; it is a systematic elimination process. Ignoring early signs such as excessive vibration or a slight temperature increase often leads to costly production stoppages and complicated repairs.
Having the right equipment and following this inspection protocol will allow you to maintain the reliability of your plant and make informed decisions about when to repair and when to replace.
