Electric Motor Maintenance Strategy Combines Preventive and Predictive Tasks

The right mix of preventive and predictive maintenance to use in electric motor maintenance strategy

It’s vital to get electric motor maintenance strategy right because they are the most common sub-equipment type since ultimately they drive all other equipment

You need outstandingly reliable electric motors if you are to get world class reliable equipment and your choice of electric motor maintenance strategy greatly impacts electric motor reliability

Hi Mike,

While preparing Planned Maintenance (PM) strategy for electric motors on duty/standby pump sets, I noticed that my organization takes a time-based approach, although the operating service strategy of the pumps is 3 months running duty pump, and 1 month standby duty. Can you please advise on how we should maintain motors in this case?

We use predictive maintenance for MV motors (4.16 kV):

1- Vibration monitoring
2- Temperature monitoring
3- Bearing inspection every 5 years for duty and standby motors (sleeve bearing)

and preventive maintenance for LV motors (480 V):

1- Time-based every 5 years we overhaul motors (Bearing replacement) regardless of the criticality.

Should we be using either predictive or preventive maintenance for our electric motor maintenance strategy, or can we deploy both of them on the same equipment?

Dear Ahmad,

It is useful for your electric motor maintenance strategy decision making to be clear between preventive maintenance (PvM) and predictive maintenance (PdM).

Preventive maintenance is the replacement of aged parts before they fail. It involves doing physical work to renew components in time before they can fail during operation. In the past it was common to determine how long a component would last in service and then set a time-based frequency to replace it. These days, with process computerization, we instead measure the real working life of the equipment (such as production throughput; hours of operation; number of starts; or some other parameter that represents service life degradation) and do preventive maintenance renewal based on the amount of stressful work done by components.

Predictive maintenance is the observation of working parts to gauge how close they are to failure. It requires making condition monitoring observations at a frequency that allows you to trend the health of components and detect degrading condition in sufficient time to plan, schedule and do the corrective maintenance before failure.

The planned maintenance tasks for your electric motors depends whether you use preventive or predicative strategy, since it will affect your choice of work order period to set.

Chose Electric Motor Maintenance Strategy to Eliminate Risk of Component Failure During Service Life

Because “first parts fail and then equipment stops,” maintenance strategy must first maximize component reliability so that by default it maximizes equipment reliability. This logic also completely applies to the selection of electric motor maintenance strategy.

An electric motor is a series system of components operating in unison. To maximize electric motor reliability each component must itself be far more reliable than the entire motor’s reliability.

Doing maintenance is only worthwhile if it prevents unacceptable operating risks from equipment failure. If the business risk caused by an item’s failure is less than the cost of the maintenance due to failure you would let the item fail and then repair or replace it, since it would be the least expensive way to use the item. As an item’s reliability increases the operating risk from its use falls because the frequency of failure decreases. Maintenance that increases an item’s reliability or reduces its operating risk brings more operating profit to your business.

The decision to use preventive maintenance or predictive maintenance strategy depends on what will maximize a component’s reliability. You can use any mix of PvM and PdM tasks in order to get the maximum component reliability that delivers the most operating risk reduction. The deciding factor as to which strategy mix to use is what set of activities brings the greatest reliability and/or risk reduction. Your electric motor maintenance strategy becomes the sum total of each component’s maintenance strategy.

The maintenance task types you are now performing are all related to the mechanical rotating components of your electric motors. Take a look at the parts list of your electric motors’ bills-of-materials and you will also find many structural items and electrical items that suffer degradation when in service. The existing electric motor maintenance strategy does not identify potential electric motor failures caused by those components.

One aspect of your current strategy that you need to challenge is doing LV motor rebuilds after five years of service. A five year service life for electric motors seems to be extremely short. Ideally, you want to greatly increase the reliability of your motors by three or four times. How to do that is best answered after conducting a thorough reliability analysis of the range of electric motor assets in your operation. In order to make recommendations on changing your maintenance strategy, and for introducing new reliability growth strategy, you need to conduct extensive review and investigation of current operating and maintenance practices, existing operating and maintenance strategy justifications, motor design limitations, motor maintenance history, and motor reliability history.

I hope the above gives you useful guidance.

All the very best to you,

Mike Sondalini
Lifetime Reliability Solutions