Industrial Motors

How PdM can keep your motors running

How to spot warning signs for your electric motors and catch problems before they turn into catastrophes.

By Paul Barna, SKF USA

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Across industries, plant operations routinely depend on electric motors to drive processes, productivity, and profitability. But when motors fail for one reason or another, the resulting unplanned downtime and maintenance costs can exact a heavy toll. The good news is that proactive initiatives consistent with enterprise-wide predictive maintenance strategies can help detect motor abnormalities before failure occurs and keep equipment humming as intended.

The road to failure for electric motors can begin as early as installation, which all too often is incorrectly or improperly performed, especially with small motors – even if they integrate high-quality components. Once a motor is up and running, “turn-to-turn” insulation faults can cause trouble, while adverse operating conditions can affect a motor’s bearings and cause them to fail prematurely. 

Ultimately, optimized electric motor performance will be reinforced with a mix of best practices, motor testing protocols, and supporting technologies. From the perspective of predictive maintenance, such proactive measures will share common goals: averting unplanned downtime and making timely fixes.

Proper setup from the start

Unless an electric motor is installed and set up properly, its expected service life will likely be in jeopardy.

Poor alignment often is an issue. If the shaft of an electric motor isn’t aligned carefully with the driven component’s shaft, the bearings in both applications will be subjected to added forces that could significantly reduce bearing service life in both pieces of equipment. The best practice is to use precision alignment tools to confirm alignment is on track.

Another issue involves unbalance. Substantial unbalance in the driven unit can be transferred to a motor, and the vibrations then can shorten bearing service life. The best practice here: Check the vibration level of the driven unit as a clue toward the root cause of the problem.

Excessive belt tension also can become problematic. In most cases, excessive loads from a belt can cause unnecessarily high loads on a motor’s bearings, significantly reducing the service life of the bearings and the belt. Higher loads also mean higher operating temperatures, which can reduce lubricant effectiveness and consequently bearing service life. Best practice: Check that the belts have the correct tension using appropriate tools for the job.

Testing to uncover ‘hidden’ problems

Various testing techniques can be employed to identify hidden problems in electric motors – with an eye toward allowing for repair or replacement in a timely manner. Testing can be conducted when a motor is offline or in service. 

Static testing (performed when a motor is offline) can help you ascertain the condition of a motor’s insulation and circuit, while dynamic motor analysis (performed while a motor is operating) can pinpoint issues throughout the motor’s system relating to power quality, motor performance, and load. Together, these tests will provide a picture of motor health and deliver information to accurately diagnose and predict imminent failures.

The majority of motor failures electrically develop from “turn-to-turn” end-turn insulation system faults. In general, insulation in a motor typically begins to wear down as turns rub together from movement generated during motor startup. Insulation can further degrade from the introduction of chemical deposits (usually found when a motor is overgreased). In the end, a compromised insulation system in an electric motor will escalate the chances for failure from an electrical perspective.

Equally troublesome is normal aging of insulation that can be expected from thermal, chemical, and/or mechanical causes. Dielectric strength will diminish to a point where the in-rush voltage causes electrical arcing – and every start and stop of a motor increases this fault in severity until failure is inevitable. In short, insulation deterioration gets worse; dielectric strength drops below operating voltage; arcing action causes high levels of induced current and high heat; and the outcome is rapid failure (sometimes within minutes).

Static testing, supported by enabling equipment, can help anticipate whether a motor is running toward electrical failure. Static testing domains include the following:

  • a winding resistance test (confirming that windings are balanced with no connection issues)
  • a meg-ohm test (verifying ground wall integrity and presence of moisture and/or contamination)
  • a polarization test (determining winding cleanliness, potential thermal degradation, and contamination issues indicating embrittlement and insulation deterioration)
  • ramp voltage/step voltage tests (highlighting ground wall integrity and contamination issues and useful in determining severity of insulation breakdown)
  • a surge test (identifying turn-to-turn insulation integrity, coil shorts, and inductance).

Resulting test values can be trended over time with the application of fully automated route-based testers able to track the various domains throughout a motor’s service life. When a motor is operating, dynamic analysis will open a wide window into the health of a motor. Technologies for on-line testing range from portable dynamic motor analyzers to network-connected monitoring systems. They can measure power quality (including voltage levels, voltage unbalance, and any distortion of incoming power), motor performance (how hard a motor can work through speed, torque, and operating temperature), overcurrent or current imbalances, torque characteristics (whether a motor is over- or underworked and related levels of energy consumption), connections (verifying all phases are operating symmetrically without unbalance), and variable-frequency drives (usually installed to improve overall plant energy consumption and efficiency, but liable to create bad power feeds leading to premature wear and possible failure).

Among recent innovations in online testing technologies, a network-connected and permanently installed electrical processor analysis system can automatically monitor motors from anywhere in the world and continuously collect data on their health and performance. Such a system – with the capability to monitor up to 30 motors per unit – can measure more than 40 electrical parameters, compare results with standard limits, and display alerts if limits have been exceeded. 

Early warning signs from bearings

Bearings are critical components for electric motors: They support and locate the rotor, keep the air gap small and consistent, and transfer loads from motor shaft to motor frame. Proper selection, installation, and maintenance of bearings can help promote extended service life in electric motors.

But bearings can fail prematurely thanks to root causes such as electrical erosion, inadequate or unsuitable lubrication, and/or heavier-than-expected or too-light loads.

Electrical erosion, also known as electric arcing, develops when a stray current uses a bearing as its path to ground. The most frequent causes of electric arc bearing damage include asymmetry in the motor’s magnetic circuit, unshielded power cables, and/or fast-switching variable frequency drives.

Once electric arc bearing damage has occurred, excessive vibrations, increased heat, increased noise levels, and reduced effectiveness of the motor bearing’s lubricant will together shorten a bearing’s service life. The extent of the damage will depend on the amount of energy and its duration, but the result is usually the same: pitting damage to the bearing’s rollers and raceways, rapid degradation of lubricant, and premature bearing failure.

Recommended fixes: One solution for resolving the problems associated with electric arcing is to insulate the bearings from the shaft currents. Specialized ceramic coatings can be applied to the outside or inside of a bearing to provide the insulation properties and prevent currents from flowing through the bearings. Another solution is offered by hybrid bearing designs, which substitute ceramic balls or rollers for the metal rolling elements within a bearing. This effectively insulates bearings from the inside. Hybrid bearings also possess a higher speed capability and can sustain longer service life than all-steel bearings in most motor applications.

Another problem will develop from inadequate lubrication, which can cause surface distress or abrasive wear in a bearing and substantially reduce bearing service life. If the lubricant film between a bearing’s rolling elements and raceways is too thin because of inadequate viscosity or contamination, the surfaces will no longer be fully separated, and there will be metal-to-metal contact with potentially dire consequences.

Recommended fixes: Check first whether the proper lubricant is being used for the bearing and that regreasing intervals are adequate for the application. If the lubricant contains contaminants, check the seals to determine whether these should be replaced or upgraded. Depending on the application, a lubricant with a higher viscosity may be needed to increase the oil film.

Yet another potential spoiler for electric motors: insufficient bearing load. When a motor runs without a load, risks increase that the bearings will become damaged, because they always require a minimum load to function well. The damage will appear as “smearing” on the rolling elements and raceways occurring over a relatively long period of time.

Recommended fix: Unless preloaded bearings are used, always apply external loads to the bearings.

When electric motors are installed properly and tested appropriately and their precision bearings are monitored regularly, the risks of potential unplanned downtime can be significantly reduced. Furthermore, when fixes can be made according to schedule and before any catastrophic failures can occur, the objectives of predictive maintenance will be advanced with dividends all along the line.