Maintenance Reliability Reliability & Maintenance Center

These Five Factors Kill Your Motors

EP Editorial Staff | November 20, 2020

If a motor fails, it’s most likely caused by one of these five reasons.

Motors are the lifeblood of most operations. When they’re functioning at peak efficiency, systems operate smoothly and energy consumption is minimized.

A well-managed maintenance program will keep them operating properly but that isn’t always sufficient. We all know that, sooner or later, mechanical devices fail. When they do, the first question is always, Why? According to the experts at ABB, Cary, NC (, there are five primary reasons motors fail. For considerably more detail about each of these failure factors, explore ABB’s extensive ebook about motor failures at the URL at the end of this article.

51% of failures are bearing issues

Bearing failures are caused by:

• Insufficient, excessive, or wrong lubrication — follow manufacturer recommendations or condition-monitoring protocols

• Misalignment — alignment should be checked after motor and driven machine have reached maximum temperature and load. Re-check every 2,000 hr.

• Shaft overload — rapid belt wear is a simple visual sign of an overloaded shaft

• Vibration — causes can be electromagnetic, mechanical imbalance, rubbing parts, bearing failure, resonance

• Overheating — make sure the motor is designed to handle the heat to which it’s subjected.

15% of failures involve stator windings

Stator winding issues result from:

• Overheating — excessive starts are a major cause

• Overloading — overloading at the motor shaft causes heat build up.

16% of failures are caused by external factors

Monitor these and other external factors:

• Motor operating temperature — ensure that motor cooling systems are properly maintained

• Humidity and the environment — if the ambient air is humid, open drain-hole plugs, use anti-condensation heaters, consider anti-corrosion protection

• Contamination — use correct IP ratings to protect motors from ingress of foreign particles

• Ambient temperatures — ensure motors are rated for ambient temperatures in all seasons.

5% are due to rotor-bar failures

Causes of rotor-bar failures include:

• Excessive starting frequency — this puts high electro-mechanical stress on the rotor, resulting in thermal stress and inertia fractures

• Overloads — in locked-rotor or stall situations, the rotor can experience sudden, excessive temperature rise, causing rotor-cage failure

• Under voltage — increases running current, causing overheating and reduced efficiency.

2% of motor failures involve the shaft coupling

Shaft-coupling issues include:

• Misalignment — check for mechanical, parallel, and angular alignment

• Bad installation — incorrect mounting is the biggest cause of failure, along with proper selection of rigid or flexible couplings.

Considerable technical information about each of these factors is available in the ABB ebook here. Included in each factor are technical information, useful illustrations, tips, and frequently asked questions. It’s an excellent resource for inexperienced maintenance professionals and veterans who need a refresher.  EP

For additional information about motor technology, click here.


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