Prevention is always better than cure. The most effective way to deal with a motor failure is to have as much information about the plant and 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 is immediately to hand.
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. They usually fail because of poor maintenance practices such as overloading, using incorrect or excessive amounts of grease for the application or lack of cleaning.
Rules of thumb
It is a general rule that standard motors of 11kW (15hp) 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.
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 (15hp-335hp) range are usually available in less than 48 hours, so can be an option. Exceptions to this rule can be when failure is catastrophic. Servomotors, explosive atmosphere and high-efficency motors may be exceptions to this rule.
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. There are exceptions to this rule when failure is catastrophic, though over the long term it is often worth repairing the unit and keeping it as a backup.
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.
If a motor has failed due to having the wrong specifications for its job (i.e. 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 rpm 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 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, 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.
