Why do pump motors fail?

Dave Hawley, General Manager – Motors, ABB Limited, writes:

There is a lot of debate about whether it is better to repair or replace a failed pump motor.

A far better option is to ensure it doesn’t fail in the first place. In theory, a motor could run for as long as 50 years, substantially beyond the retirement date of the connected machine. This can be achieved with regular preventive maintenance targeting the most common points of failure.

Keep your eye on the ball

Some 51% of all motor failures are down to bearings with incorrect lubrication accounting for around 80 percent of these. Problems can arise from too much, too little or using the wrong type of lubricant.
On average, re-greasable bearings need servicing every 2,000 hours – that’s roughly every three months. But don’t over-grease, as this can overheat bearings and lead to failure.
When re-greasing, only use grease with the correct properties. These include a base oil viscosity of 100-160 cSt at 40°C and a temperature range -30°C to +120°C, continuously.
It is important to use the manufacturer’s specification to lubricate the bearings properly, taking into account how the motor is mounted, the speed of the motor and the type of bearings installed.

Don’t let your motor catch a fever

Overheating is another common cause of premature motor failure.
Heat comes from many sources – the loading on the motor, the ambient temperature, mechanical stress and even the thickness of the paint. Any of these can cause the motor’s insulation to break down, putting the motor under stress and causing it to fail.
Most industrial electric motors have an insulation class B or F, the latter of which can operate up to a maximum temperature of 155°C. Check the motor’s insulation class to ensure that the designed temperature rise is not being exceeded in the application’s environment.
The running time and load on the drive-end shaft also needs to be considered as these can cause a motor to overheat. Motors that are running continuously need to be fitted with bearings able to cope with high operating hours, and will have to be re-lubricated in-line with the motor manufacturer’s recommendations.
High load on the drive-end shaft can also put unnecessary stresses through the shaft bearings, and this can cause excessive friction, and eventual overheating to the point of failure.
Over-heating can also contribute to winding failures, another major cause of premature motor failure. Temperature has a great impact on winding life. Increasing the running temperature by just 15 °C can halve the lifetime of the winding.

Protect against external factors

External factors account for 16% of motor failures. If it is not feasible to move the motor then it is important to protect it.
High humidity can allow moisture to enter the motor and cause damage and corrosion. However,
there are a number of steps motor users can take to combat this problem including opening the drain
hole and using enhanced paint systems.
Where motors are operating in harsh outdoor conditions, consider the effects of cold as well as heat.
For example, condensation heaters should be fitted to motors used outdoor in cold winter months to
minimise condensation within the motor.
Finally, ingress of foreign particles into the motor enclosure can also cause damage – particularly to a
motor’s bearings or windings. It is therefore important to use the correct IP ratings to protect your
motor.

To find out more ways to prevent motor failure, email [email protected] and request ABB’s
motor tips e-book, or download your copy at http://bit.ly/2EUvDZ7