by Thomas H. Bishop, PE
Many decisions need to be made between the time a motor fails and when the final repair or replace option is chosen.
Use a flowchart similar to the one shown below, to avoid overlooking any of the key steps in the decision-making process.
Your first step in considering the condition of a failed motor is to determine whether it’s actually suited for the application, regardless of whether the motor is to be repaired or replaced.
If the motor suits the application, then the remaining decisions are related to repair choices. The first task is to determine the stator core’s condition. If the stator core is in satisfactory condition, the decision process moves to assess other repair decisions. But if there is core damage, the cost to repair the damage must be weighed against the replacement cost.
Keep in mind that a motor with special features may be expensive to replace, and time-consuming to obtain.
Once you’ve assessed the condition of the core, ask the following questions:
- has catastrophic failure occurred?
- is there evidence of a prior catastrophic failure?
- is the rotor damaged?
- is there severe damage to other mechanical parts?
- is it an EPAct or premium efficient motor?
If a catastrophic failure has occurred, or if there’s evidence of a prior catastrophic failure, the repair cost should be weighed against the cost of a replacement. Now is a good time to consider the motor’s suitability for the application. The failure may suggest application issues that were not recognized earlier, such as excessive radial load on the shaft and bearings. Catastrophic failures usually involve extreme damage to the stator core and windings, rotor, shaft, bearings, and end bracket. This damage may be so severe that the root cause is not apparent. Even so, possible causes that led to the failure should be identified to prevent a recurrence.
|Melted rotor bars or bars lifted from their slots will, at a minimum, require rebarring.|
|Stator rub is caused by rotor contact with the stator.|
| Melted aluminum rotor. |
Rotor damage can vary, from surface smearing by contact with the stator, to bars and end rings melted on a die cast design, or bars lifted out of the slots and endrings broken on a fabricated design. While smearing might be repaired economically, melted bars or bars lifted from their slots will, at a minimum, require rebarring. The downside is that unless the motor is very large or special, rebarring is seldom an economic alternative.
If major core iron damage has occurred, new laminations add to the repair cost. In such a case, the damage can be considered catastrophic, shifting the economics of repair versus replacement heavily toward replacement.
Other mechanical parts, such as the shaft or frame, can be damaged so badly they must be replaced. The economics of buying or making a new shaft, or buying a new frame, may make repairing the motor an unattractive choice.
EPAct/Premium Efficiency Factor
Over the last decade, a new factor has emerged which can affect motor repair decisions: the energy-efficient motor. These are included in the Energy Policy Act of 1992 (EPAct), and premium efficient motors. Regardless of the motor’s label, a motor repair should ultimately maintain the motor’s original efficiency. Energy-efficient motors have a greater volume of material than most earlier motor products. In particular, an energyefficient motor may have a longer rotor and stator core, and more conductive material than a similarly-sized motor that doesn’t meet the definition of EPAct, or of a premium motor as defined by National Electrical Manufacturers Association (NEMA).
Energy efficiency also introduces return on investment considerations to the repair-versus-replacement equation. This means considering the economic impact of all costs, including energy, and then determining which option provides the best overall benefits. Therefore, a new, energy-efficient motor may be selected because of cost projections, based on the expected useful life of the motor or process.
Another element in decision making is availability of funds for a replacement motor. This often comes down to deciding which group within the company or plant is going to pay for the new motor. If the responsible group lacks funds for a replacement, it may opt for a motor repair, so long as the cost is less than that of a new motor. If funds for the new motor are available, the next decision is based on availability of the motor. Standard feature motors, such as those that fall under EPAct rules, are normally stock items. Larger motors, or motors with special features, may not be widely available locally, and may have long delivery times.
Five levels of motor repair
Five levels of repair can be identified for squirrel cage induction motors. The first four represent an expanding scope of repairs.The fifth applies to motors that normally would be replaced, but present other factors to consider.
Level 1 repair is a basic overhaul or reconditioning. It includes cleaning components, replacing bearings, and replenishing lubricants. It also includes inspection and testing during the incoming stage, during the repair process, and during final testing.
Level 2 repairs include everything in Level 1, but add stator winding, varnish-resin treatment, worn bearing fit repair, and shaft straightening.
Level 3 adds stator rewinding; that is, replacing the windings and insulation system. Smaller motors with two or four poles are relatively simple to rewind. Special windings such as two-speed, or very low-speed windings, can add considerable time and material to a repair.
Level 4 is the most comprehensive repair level; it encompasses all of the previous levels, and adds major lamination repair and/or rotor rebar. It also may include restacking or replacing the stator laminations, and shaft replacement. Before taking on major repairs at this level, you may first want to consider the motor replacement option. Not only will the cost of repair be high at Level 4, but the uncertainty associated with the size of the repair compounds the risk of successfully completing the job.
Level 5 repairs apply to motors that normally would be replaced except for special circumstances, such as the unavailability of a spare or replacement unit. Level 5 could easily apply to any of the circumstances at the other four levels of repair. This level also includes misapplied motors and vintage pre U-frame motors. This latter group generally is not rated at voltages consistent with modern power systems, and has only Class A original windings. —TB
Thomas Bishop, PE, is a technical support specialist for the Electrical Apparatus Service Association, Golden, CO. This article originally appeared in the Summer 2003 issue of the U.S. Department of Energy’s Energy Matters newsletter.