Repair or replace? Make the right decision on motors

Motor failure is never a good thing, and it always occurs at the worst possible time. If there's a bright side, it's the chance to evaluate your repair and replacement options and "get it right" for the long pull. To make the best of this opportunity, you need to be aware of the available repair options and follow a logical process for making motor repair vs. replacement decision.

By Thomas H. Bishop, PE April 1, 2006

Motor failure is never a good thing, and it always occurs at the worst possible time. If there’s a bright side, it’s the chance to evaluate your repair and replacement options and "get it right" for the long pull. To make the best of this opportunity, you need to be aware of the available repair options and follow a logical process for making motor repair vs. replace decisions.

The flow chart in Figure 1 shows the paths you might take in deciding whether to repair or replace a failed motor. It doesn’t cover every possibility, of course, because each application has unique circumstances.

Application review

The first step is to determine if the failed motor suits the application. A motor with open enclosure, for instance, may not be practical for a sawmill application with lots of airborne dust and debris. A better choice might be a totally enclosed, fan-cooled (TEFC) replacement. Processes and duty cycles often change over time, so it always pays to reexamine the application when deciding whether to repair or replace a failed motor.

If the failed motor is a "good fit" for the application, assess the condition of the stator core. Has it sustained significant damage? Prior to failure did the motor exceed its rated temperature rise (i.e., high core losses)? Absent special features that might affect price or availability, it may cost less to buy a new motor than to repair a badly damaged stator core.

If the stator core is in satisfactory condition, consider the following decision points simultaneously:

  • Has catastrophic failure occurred?
  • Is there evidence of a prior catastrophic failure?
  • Is the rotor damaged?
  • Are other mechanical parts severely damaged?
  • Is it an EPAct or NEMA Premium motor?

Catastrophic failure

If a catastrophic failure has occurred, weigh the cost of repairing the motor against that of replacing it. Such failures typically cause significant damage to the stator core and windings, as well as to the rotor, shaft, bearings and end brackets. In such cases, replacement may be the most economical option-especially if you question the suitability of the motor for the application.

Rotor damage varies widely-from surface smearing due to contact with the stator, to melted bars and end rings on die-cast designs, to lifted bars or broken end rings on fabricated designs. Surface smearing can often be repaired economically. Other kinds of rotor repair may not be feasible unless the motor is very large or has special features.

The shaft, frame or other mechanical parts may also be damaged so badly that they must be replaced. Here again, the cost of buying or making a new shaft, or of purchasing a new frame, may make repair a less attractive choice than replacing the motor-unless the motor is very large or has special features.

Prior catastrophic failure

Sometimes evidence of a prior catastrophic failure is discovered only after disassembly, when the components are inspected and tested.

Examples include a bent shaft that has bent again; a damaged rotor core or damaged rotor bars or end rings; and damaged or missing stator core iron. If previous repairs were effective and show no sign of renewed degradation, consider repair again.

On the other hand, replacement may be warranted if the present failure stems from a previous catastrophic failure that degraded the motor. The rare exceptions are cases where the damage from both failures can be repaired successfully and economically.

Whether you choose to repair or replace the motor, be sure to identify the contributing causes of failure to prevent a recurrence.

Energy-efficient motors

The points discussed so far have shaped motor repair-replace decisions for more than 50 years. The advent of energy efficient motors during the past decades introduced a new consideration-whether to replace the failed motor with a more energy-efficient model.

Broadly speaking, energy efficient motors are those covered by federal regulation (EPAct), as well as newer, premium-efficient (e.g., NEMA Premium) models. Repair considerations for these motors are the same as for standard efficiency models.

Following industry best practices, qualified service centers can repair either type without affecting the efficiency rating.

Before repairing a standard efficiency motor, consider the return on investment for a more energy-efficient replacement, based on the expected life of the motor or process.

To do so, compare repair and replacement costs (including the cost of any modifications needed for the new motor), and estimate the energy savings for the expected hours of operation. Note that energy savings will be more significant for motors that run 24/7 than for those that operate for 8 hours a day, 5 days a week, or only intermittently. Larger motors (250 hp and up) also tend to be fairly efficient already, so for these sizes the differences in efficiency between standard and premium efficiency models are relatively small.

If the return-on-investment analysis shows that replacement is preferable to repair, your next consideration is whether you have the money in your budget. If not, you may still opt for repair as long as it costs less than a new motor.

Assuming you have the funds for a new motor, the next decision point is availability. Standard motors, such as those that fall under EPAct rules, are normally stock items.

Delivery times for larger motors, or for those with special features, often range from a few weeks to several months. If the delivery time is longer than you require, a qualified service center usually can repair the original motor in far less time. It also may be able to add the special features you need to a stock motor- for example, by converting it to a D-flange mounting.

Repair options

To make repair-replace decisions effectively, it also helps to be familiar with the various repair options for squirrel cage induction motors. Repair Levels 1 – 4 in Table 1 illustrate the expanding scope of work performed-from basic reconditioning through stator rewinding to major repair of the stator core, rotor, shaft or frame. Level 5 repairs apply to motors that normally would be replaced as a result of a straightforward repair-replace decision process, but for which other factors must be considered.

Level 1 repair is a basic overhaul or reconditioning. It covers cleaning the components and minor repairs like replacing bearings and replenishing the lubricant. It also includes initial inspection and testing (before, during and after repair).

Level 2 repairs include everything in Level 1, plus varnish/resin treatment of stator windings, repair of worn bearing fits, and straightening of shafts. Due mainly to the extra labor required, Level 2 represents a significant expansion in the scope of repairs. These repairs may cost several times more than Level 1 repairs, and take quite a bit longer to complete.

Level 3 repairs add stator rewinding (replacement of the windings and insulation system) to Level 2 repairs. Smaller, single-speed motors are relatively easy to rewind. Special windings (e.g., two-speed or very low-speed windings) often require more labor, material and expertise to repair. In either case, the extra step of rewinding the stator expands the scope and increases the cost of repair considerably.

Level 4 repairs are the most comprehensive. Besides Level 1-3 procedures, they encompass major repairs of the stator core and/or replacement of rotor bars and end rings. They also may include replacement of the stator core laminations or the shaft.

Never undertake Level 4 repairs without first considering the option of replacement.

Level 5 repairs, as mentioned earlier, apply to motors that normally would be replaced, except in special circumstances-e.g., lack of a spare or replacement unit. Depending upon the standard or special features of a particular motor, Level 5 could apply to any of the other four levels of repair.

As these five levels imply, the damage resulting from motor failures varies widely, as do the associated repair costs. While repair costs generally increase with the scope of the work, there is no "rule of thumb" for how much. What is clear, however, is that an evaluation process that fails to consider the various levels of repair is far too simplistic to yield sound repair-replace decisions.

The Bottom Line

Evaluate motors not just for repair or replace, but whether a replacement motor would suit the needs of the application today.

Part of the repair/replace decision should include an evaluation of energy efficiency.

If a motor is not in stock, or if acquiring it may take weeks or months, repair may be a viable option.

Fig. 1. Determining whether to repair or replace a failed motor should be a systematic process that looks at the impact of each decision.

Thomas Bishop is a technical support specialist at the Electrical Apparatus Service Association, St. Louis.

Table 1: Levels of repair

Level 1:: Basic reconditioning: This includes replacing bearings, cleaning all parts and replacing lubricant. Also adds seals and other accessories as agreed with customer.

Level 2: Includes Level 1 with the addition of varnish treatment of stator windings, repair of worn bearing fits and straightening of bent shafts.

Level 3: Includes Level 1 as well as rewinding the stator (i.e., replacing windings and insulation).

Level 4: Includes rewinding of the stator plus major lamination repair or rotor rebar. It may include replacement of the stator laminations or restacking of laminations. Shaft replacement normally falls into this category. In short, Level 4 involves major repairs that are costly enough to justify examining the option of replacement.

Level 5: Motors that would normally be replaced except for special circumstances faced by the customer (e.g., no spare or unacceptable lead time for a replacement). Level 5 includes misapplied motors, inadequate enclosures and pre U-frame motors. A motor that should be replaced, if not for the owner’s inability to operate without it.

Experts weigh in at NMW Power Quality series

Experts in Power Quality issues offered their views on a variety of topics at a series of National Manufacturing Week conference sessions.

Plant Engineering magazine senior editor Jack Smith moderated the Power Quality Series presented on April 21, which included topics on harmonics, sags and swells, and transients and noise.

Rudy Wodrich, manager of the Power Quality Correction Group at Schneider Electric, Mississauga, Ontario presented "Identifying and Minimizing Harmonics in Your Plant’s Electrical System."

"Harmonics don’t really exist," Wodrich said. "Instead of being just one signal, what we call harmonic current is actually a resultant waveform. The ‘choppy,’ non-linear current drawn by electronic loads is actually a fundamental (60 Hz) component plus many integer multiples of that fundamental frequency."

Harmonics can cause cable insulation breakdown, random breaker thermal trips; transformer failures; limits on capacity of generators; random logic faults on CNCs, PLCs, drives, UPS and computers; resonating power factor capacitors; ac motor winding and bearing destruction; and generator faults.

To minimize the effects of harmonics in plant electrical systems, Wodrich recommended several options. Low and medium voltage detuned automatic capacitor banks, filtering automatic capacitor banks and active harmonic filtering.

Daniel J. Carnovale, Power Quality solutions manager at Eaton Electrical, PA presented "Sags and Swells: Sources and Solutions."

"Sags are the most common and costly PQ problem," Carnovale said. "According to the Electric Power Research Institute, (EPRI), sags account for 92% of voltage variations at a facility."

Voltage sags can cause computers to reboot or lock up, drive drop outs, contactors to chatter or open, PLCs to lock up or stop processes, corrupted data, HID lamp shutoff and flickering of incandescent lamps, according to Carnovale. Voltage swells can cause reduced equipment life, TVSS failure, transformer saturation, drive damage and reduced lamp life.

Sources of voltage variations include the switching in and out of power factor correction capacitors, which changes RMS voltage; load switching; regulator malfunction; motor starting; neutral-to-ground bond open on the utility transformer; and faults on a system with poor grounding or an ungrounded source.

According to Carnovale, one way to protect from voltage variances is to use a UPS. The four most common UPS types are standby, line-interactive, ferroresonant and online or double-conversion UPS. Adjustable speed drives can also protect power systems in some cases.

Carey Mossop, product line manager, Power Protection and Conditioning at Eaton Electrical, Pittsburgh, Pa., presented "Eliminating Transients and Noise from you’re Plant’s Electrical System."

"A transient is a high rising voltage condition on one or more phases lasting 2 msecs or less," said Mossop. "Frequencies can range from 20 Hz to 20 MHz, and voltages can reach 20 kV."

"Power disturbances create physical damage and affect logic signals in electronic equipment," Mossop said. "Noise disturbances can be interpreted as legitimate ON/OFF signals, resulting in operating errors and equipment downtime."

According to Mossop, 20% of voltage transients come from sources that are external to the facility such as lightning, capacitor switching and short circuits. "But 80% of voltage transients actually come from sources inside the plant: internal load switching, short circuits, capacitor switching, imaging equipment, adjustable speed drives, arc welders and light dimmers."