Motors are obvious target in reducing downtime

Business Development Director Mike Brook explains how planning and making a decision on whether to repair or replace an electric motor should be central to any downtime reduction strategy.

Access to increasingly accurate data on the real cost of plant downtime is driving companies to look deeper into the causes of downtime and how it can be avoided.

The failure of electric motors is one of the most common causes of significant periods of downtime.

Since they are easy to isolate and are present in virtually all manufacturing, processing and handling systems, electric motors should be a prime target for action.

What can be done to minimise downtime? Prevention is always better than cure and regular condition monitoring surveys are already conducted on many plants to predict when and how a motor unit might fail.

Not only does this help plant managers allow for repairs to be carried out on a planned basis, but it can also help avoid being forced into taking the quickest and usually most costly option when an impending problem is identified.

As all plant/maintenance engineers are acutely aware, if an unexpected motor failure does occur, the losses in production, delivery problems and lost revenues can soon spiral out of control.

The most effective way to deal with such situations is to have as much information about the plant and various options available before hand, so that the quickest and most economic solution is immediately apparent.

By conducting a survey of all the motors used in a plant, noting their nameplate information, details of the application requirements and how important they are to production, spare units can be purchased, quickest/cheapest suppliers can be noted and all the information needed to ensure a reliable repair is immediately to hand.

Why do motors fail? Both the condition monitoring and plant survey should give clues to why a unit has failed or is failing.

Condition monitoring surveys can include electrical, thermal, noise, vibration and oil analysis, which can identify contaminated windings, failing insulation, bearings and lubrication to give engineers vital information as to the efficiency of the unit, as well as the cost and timescale of repairs.

The equipment survey on the other hand provides information that allows engineers to assess the suitability of the motor for its application.

For example, a motor maybe burning out because of insufficient power rating or due to changes in the driven load or gearing as more power or torque output is therefore required.

Environmental factors should also be taken into account, such as increased moisture levels or ambient operating temperatures, and the repair history of the motors can be used for working out the life expectancy of the unit after repair.

Bearings are responsible for over half of all motor failures and it is usually the result of poor maintenance practices such as overloading, using incorrect or excessive amounts of grease for the application or lack of cleaning.

Replacement options? To keep downtime to a minimum, standard 415V motors should ideally be kept in stock on site, but it is becoming more common for larger companies to have consignment stocks left on site by a contracted supplier to avoid the investment costs involved.

If holding stock is uneconomic to the provider or motor tasks are more specialised at a site, making an assessment of the potential suppliers with cost comparisons will also save time and money when the need arises.

When repairing a motor, gaining a good idea of what repair services are offered by a company and the respective costs of each level of repair can also be beneficial to avoid unexpected costs, ie the difference between basic reconditioning, re-insulating, stator rewinding, stator repair, major lamination repair or a new shaft.

What to repair or replace? It is a general rule that standard motors of 11kW or less should be replaced, as they are generally stock items available at short notice and are uneconomic to repair, however testing/dismantling to find the cause of failure is recommended.

Depending on the severity of the failure mode, motors of 11kW and above can be worth repairing.

If the unit proves economic to repair, that repair can be completed quickly to take advantage of a shutdown period or returned to site as replacement stock.

In an emergency breakdown situation though, the opportunity cost of waiting for an analysis nearly always outweighs the money saved through repair.

Standard replacement units in the 11-250kW range are usually available in less than 48 hours and are usually the preferred option.

Exceptions in this 11-250kW range include servomotors used for high precision automation positioning systems and motors that have been designed for specialist power transmission applications.

Eff1 high efficiency and ATEX explosive atmosphere motors can also prove to be exceptions as their complex designs make for expensive replacement.

Standard motors of 250kW and above are usually repaired.

Lead times for new replacements are quoted in months, whereas repairs to motors of this size can be completed relatively quickly; large medium voltage motors can be removed from premises, shafts re-machined, stators rewound, insulation replaced and re-installed in weeks if not days.

Exceptions to this rule can be when failure is catastrophic, ie where bearing mounts, frames, stator cores or shafts need replacing, but even if this is the case, over the long term it is often worth repairing the unit and keeping it as a backup.

Some companies can repair motors more quickly depending on the urgency of the job and nature of the failure, however extra costs may be incurred, putting further emphasis on the need to plan ahead.

A survey should take into account previous repairs enabling engineers to find out if the damage is isolated to the same area and whether the repair was to the correct standard or if the unit is again unsuitable for the application.

If the damage is limited to these areas, and the previous job proved economic, it is usually worth repairing the unit for a second time, but repairing additional damage may result in diminishing returns.

What are the benefits of a repair? If a motor has failed due to having the wrong specifications for its job (ie more output is required, it is subjected to dust, moisture or explosive gases), an upgrade repair can match the old motor to these changed conditions.

For example, replacing the insulation can allow the motor to operate at higher temperatures and high speed bearings, lubricants and balancing can improve speed performance or even extend operating life beyond that expected from a new motor.

It's also worth noting that during the repair process, the cause of motor failure can be ascertained and that information fed back to the customer helping to prevent a recurrence of the problem.

When considering whether to repair or replace a motor, it is also worth noting that service centres such as Wyko EMS can also increase the efficiency of motors during winding replacement etc, allowing the "whole life costs" of larger motors to be significantly reduced.

By keeping the survey up to date as cheaper/more efficient replacements come on the market, plant managers can also make sure that the most realistic payback period is always available.

Unless a plant already has an established survey of all the motors and a sound working relationship with both motor suppliers and repairers, choosing the right company to work with can be a demanding task.

Smaller sites may have all the knowledge required in the hands of a few experienced employees while larger plants may use consignment stocks and a complex repair schedule with all roles applied to a few key suppliers.

The majority of sites fall in between, with engineers always juggling a number of suppliers who offer the best option for one service, but not for others.

For example, an inexpensive motor supplier may not offer the required support in terms of problem solving for applications, manufacturers may offer custom units but not conduct repairs and large suppliers that leave consignment stocks may not repair units.

Wyko repair or replace over 10,000 motors each year and specialise in all aspects of motor replacement; from condition monitoring to conducting motor usage surveys, supplying motors off the shelf to leaving consignment stocks, repairing motors of any type or size to installing/maintaining the units and providing impartial advice in all these areas.

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