Electrical maintenance programs are designed to enhance equipment reliability and uptime while reducing operational costs. Electric Motor Testing is often one of the first areas to be scaled back when cutting expenses. However, savvy companies recognize that neglecting proper maintenance programs can result in billions of dollars in lost revenue due to increased motor repair costs, downtime, and inefficiency in industrial and commercial settings.
Why is Electric Motor Testing Performed?
After bearing failures, electrical faults are the next most common cause of motor failures. Therefore, a well-planned electrical testing regimen is crucial for ensuring plant reliability. According to a survey conducted by the Electric Power Research Institute (EPRI), 48% of motor failures are attributed to electrical issues. This 48% is further divided into rotor problems (12%) and winding problems (36%), with the remaining 52% due to mechanical faults.
Various diagnostic tools, such as clamp-on ammeters, temperature sensors, Meggers, or oscilloscopes, can help identify these issues.
Winding defects can arise due to contamination, insulation aging, thermal overload, power surges, damaged wires/materials, and other factors. These defects begin with energy crossing an insulation fault like moisture, which isolates at least one turn. This leads to additional stress and increased temperature across the fault, eventually causing the winding to fail.
Types of Winding Faults:
- Between turns in a coil
- Between coils in the same phase
- Between coils in different phases
- Between a coil or phase and ground
Identifying one of these faults early can save your facility countless hours of downtime and significant amounts of money.
What Happens During Electric Motor Testing?
There are several types of tests performed on motors:
Electric Motor Impulse Testing
Impulse testing is a vital part of predictive maintenance for electrical motors. Key questions to consider include:
- Can impulse testing damage healthy or deteriorated insulation?
- Can DC Resistance, Inductance, Megger, or HiPot tests detect weak turn-to-turn insulation?
- After failing an impulse test, can a motor with weak insulation still operate?
- Can motors with a turn-turn short continue to function?
These questions were investigated by subjecting a low voltage motor to extensive testing until failure was induced, by additional tests to explore the effects of impulse testing on turn-turn insulation beyond the motor's dielectric breakdown.
Electric Motor Rotation Testing
Check for rotation when testing offline with the Motor Circuit Evaluation (MCE). Fans or pumps may continue to rotate slowly due to drafting or shared headers, which can skew Standard Test results by creating higher than normal resistive and inductive imbalances.
Wound Rotor Electric Motor Testing
Wound rotor motors have a three-phase winding on the rotor connected to start-up resistors, which provide current and speed control during startup. Failed components in the resistor bank are common and often overlooked during troubleshooting, potentially affecting the motor's overall functionality.
Electric Motor Insulation Resistance Testing
Motor insulation has a negative temperature coefficient, meaning resistance decreases as temperature rises. Insulation resistance of a de-energized motor typically decreases after startup but may initially increase as moisture evaporates from the windings. The IEEE43 standard on insulation resistance testing requires temperature correction to 40 degrees Celsius, which can turn acceptable measured resistance into low corrected readings. Space heaters should be considered before sending a motor for refurbishment.
Meg-ohm Test
The meg-ohm test, commonly used by engineers, measures insulation resistance. While it plays a valid role, it cannot detect all potential winding faults.
PC Tests
Modern test equipment uses PC control for automatic testing and fault diagnosis, eliminating the need for operators to interpret results. The equipment can detect micro arcs and stop the test automatically, storing results in a database for future reference. Automated testing reduces operator error, inconsistency, and the risk of over-voltage.
Static or Insulation Testing
Performed with the motor disconnected from the power supply, typically from the motor control cabinet, and must follow a specific test sequence.
Winding Resistance Test
Highlights dead shorts, loose connections, and open circuits. Accurate equipment is essential, and resistance values should be corrected to a constant temperature, typically 20 degrees Celsius. The motor temperature should be measured precisely.
DC Step Voltage Test
Performed at twice the line voltage plus 1000 volts. Voltage is increased in steps, and leakage current is plotted. A linear plot indicates effective insulation, while a non-linear plot suggests insulation deterioration.
DC Hipot Test
Measures leakage current and calculates meg-ohms to determine insulation resistance. Passing values are higher than the minimum accepted, but this test alone may not detect all issues.
Surge Test
Used to verify insulation between turns, coils, and phases, typically performed at twice line voltage plus 1000 volts. It detects dead shorts, weak insulation, imbalances, and loose connections by injecting high voltage pulses into each phase.
Dynamic Motor Testing or Online Testing
A more recent technology, dynamic motor testing involves measuring voltage and current across the motor's three phases while it operates under normal conditions. This method can identify both electrical and mechanical issues, including power quality problems that may lead to motor overheating.
Recommended Tests
Off-line in-Service Motor Tests:
- Stator winding resistive imbalance
- Stator winding insulation resistance (Meg-Ohm checks)
- Polarization Index (PI)
- Step Voltage test
- Surge test
Spare Motor Tests:
- Similar to the off-line in-service tests.
New/Refurbished Motor Tests:
- Similar to the off-line in-service tests.
How is Motor Testing Conducted?
Three Phase:
- Check the power supply link and connection bar for the terminal (U, V, W).
- Confirm power supply voltage and continuity of winding between phases using a multimeter.
- Verify motor winding resistance and insulation resistance with appropriate testing devices.
- Measure running amps using a clamp meter.
Single Phase:
- Verify motor winding readings and identify electrical terminals (Common, Start, Run).
- Use an insulation tester meter to check insulation resistance.
- Measure running amps and compare them to the nameplate values.
All Types:
- Inspect motor appearance, body condition, and cooling fan blade.
- Manually rotate the shaft to check bearing condition.
- Record motor data from the nameplate.
- Ensure earth continuity with an ohmmeter, and verify power supply voltages.
Benefits of Motor Testing
- Increase uptime
- Save money
- Conserve energy
- Improve safety
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