Electric Motors — Repair Vs Replace

The high cost of plant downtime combined with the varying price of motors and a host of new methods for detecting and fixing faults has made the decision of whether to repair or replace an electric motor far more complex. Mike Brook, business development director for WYKO Industrial Services, provides a basic guide for plant and maintenance engineers, outlining how shutdowns can be kept to a minimum and costs can be reduced over a longer period of time by making the right decision

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 the various options available beforehand, 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 will be immediately to hand.

Why do motors fail?

Both the condition-monitoring and plant surveys should give clues as 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 and 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 plant/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 an insufficient power rating or due to increases in the driven load or gearing, as more power or torque output is then required.

Environmental factors should also be taken into account, such as increased moisture levels or ambient operating temperatures. The repair history of the motor 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 this 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 for the provider or motor tasks are more specialized 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, however, 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 48h, so can be an option.

Exceptions in this 11–250kW range include servomotors used for high-precision automated 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, as well as more costly repairs. Additionally, standard motors that only require an electrical or mechanical overhaul can also be quicker and more cost-effective to repair than to replace.

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 the motor re-installed within 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 back-up. Some companies can repair motors more quickly depending on the urgency of the job and the 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, allowing 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 because it has the wrong specifications for its job (ie if more output is required or 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, while high-speed bearings, lubricants and balancing can improve rev/min performance or even extend operating life beyond that expected from a new motor.

It is also worth noting that during the repair process, the cause of motor failure can be ascertained and that information fed back to the customer to help 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.

How to choose a supplier

Unless a plant already has an established survey of all its motors and a sound working relationship with both motor suppliers and repairers, choosing the right company to work with can be a daunting 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 specialize 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 and maintaining the units and providing impartial advice in all these areas.

Wyko Industrial Services, Amber Way, Halesowen, W. Mids, B62 8WG; tel: (0121) 508 6341; fax: (0121) 508 6333; email: [email protected]