Image courtesy of Marathon Electric Motors

Motor-driven equipment account for 64% of the electricity consumed in the U.S. industrial and commercial sectors. All industrial or commercial operations have some type of motor in their facilities. Motors are used to drive fans, pumps, compressors, saws, crushers, or conveyors, they are prone to failure if not properly sized, installed, and maintained.

Even the best motors will fail if they are installed in applications or conditions they are not rated for, or if preventative maintenance guidelines are not followed. The most common causes of motor failures are summarized below.

Overloads

Motor overloads cause the motor to draw more current and this, in turn, produces more heat than the motor was designed for, which reduces the life of the motor. Motor overloads usually result from improperly sizing the motor or some change in the driven load including plugging of the system, a broken gear, or frozen bearings. It is not nuance tripping if the motor-overload protective device trips and takes the motor off-line repeatedly, the cause should be located rather than simply installing larger protective devices.

Short Cycling

Short cycling is the process of repeatedly stopping a motor that is already heated to operating temperature and then starting it again before it has a chance to cool. The result of underestimating the starting frequency of an application or an application operating improperly and starting more frequently than it is supposed to. Most motor manufacturers specify the maximum number or frequency of starts for a given motor type.

Electrical Supply

Low or high voltage is not the cause of failure as frequently as suspected. Motors do not give satisfactory service if the voltage consistently varies by more than +/- 10% from the nameplate rating.

Phase voltage unbalance in a three-phase supply can cause motor failure because of an excess temperature rise. A 3.5% voltage unbalance causes a 25% temperature rise in the motor and a current increase of six to ten times the voltage unbalance. Phase voltage imbalance should be less than 1% for proper motor operation. If a motor must be operated with a phase unbalance greater than 1%, the motor should be derated. A motor should not be operated if the phase unbalance is greater than 5%.

Single phasing refers to conditions where one phase of a three phase electrical system is de-energized. Single phasing creates a problematic situation for a three-phase motor that can continue to operate by drawing more current (thus more heat) on the remaining two phases. Many standard motor starters contain overload relays which will not respond to this situation, however, three-phase “phase protection relays” are available to protect motors.

Physical and Environmental Conditions

Images courtesy of Marathon Electric Motors

Motors must be rated and installed to withstand the physical and environmental conditions their operation subjects them to.

Restricted ventilation or covering the motor’s enclosure with materials will cause a motor to operate at a higher than desired temperature.

Improper lubrication will not only damage bearings, but can also throw grease into the windings.

Moisture in the form of condensation can cause rust within a totally enclosed motor.

Vibration, belt tension, and misalignment are among the major causes of motor failures that started as bearing failures. Where vibration is a problem, electrical connections, if not tight and secure, will be an endless cause of trouble.

High ambient temperatures (over 104º F for a 40º C rated motor) must be taken into account by derating to a lower horsepower, or installing a motor designed with proper insulation for operation at higher ambient temperatures.

The first step to solving the question why has my motor failed is to inspect everything. A simple inspection of the bearing and motor windings can often point in the right direction: Do the bearings show obvious wear, or lack of lubrication? Now look at the winding of the motor, if overloaded all windings will be appear to be evenly burned, or is there lubricant in the windings? What does your motor tell you?

If you have experienced or are experiencing unexplained motor failures and want a solution or explanation, contact your IBT Sales Rep to assist with analysis.