Motor repair and replace by the numbers
The simplest way to make a preliminary repair/replace decision is to use the calculator shown in Figure 2. Here is how it works. First determine the following:
- Annual running hours for the motor application (HA)
- Local energy cost, in $/kWh (E)
- Efficiency change, in % (FNEW – FREPAIR)
- Cost of repair, in $ (CR)
- Cost of a new motor, in $ (CN)
- and insert the values in the following equation:
- Decision coefficient = HA • E • (FNEW – FREPAIR) • CR/CN
For example, assume that a 100 hp, 3,000 hr energy-efficient motor is showing signs of failure. Energy cost is $0.09 from the local utility. Also, conservatively estimate that if the motor is rewound, it will lose 1% efficiency, from 95.5% for the new one to 94.5%. Cost of rewinding is $2,600, and a new replacement costs $8,800. Should it be rewound or replaced?
Decision coefficient = 3,000 • 0.09 • 1 • 2,600 /8,800 = 79.77
On the calculator, this figure falls well within the “repair” region. On the other hand, had the motor run another 1,500 hours and been located in an area where energy costs are $0.13, then the answer would have been right at the breakpoint.
This equation shows the overriding importance of efficiency. By simply changing the efficiency shift from 1 to 2, the coefficient doubles. Also, the equation is conservative in that it does not include an escalator factor for increases in power costs—which, like death and taxes, are a certainty. Thus, if the calculator indicates that the repair/replace decision is near the breakpoint, the wise choice would be to replace the motor to take advantage of the new motor’s greater efficiency as power costs increase.
Decisions on dc motors
The dc machine is, as the saying goes, a different animal. Except for bearings, its wear points are different than those of ac machines and it’s much more expensive to build—by a factor of 2 to 4, depending on its design. Likewise, dc machines are somewhat more expensive to repair than ac units, but not by a factor of 4 unless the commutator and armature are completely destroyed.
Large dc machines cling to those specialized applications where extremely precise control of speed is paramount, or where it is possible to use a regenerative drive to recapture a portion of the energy already invested in the rotating mechanism. Other than that, dc technology has been shouldered aside by ac, and large dc motors have become relegated to those applications where nothing else can be controlled as precisely.
Repair/replace decisions for dc are usually simpler than those for ac because there is no premium efficiency line manufactured. Also, because the motors are more expensive to manufacture, users tend to hold onto them longer. For dc machines, the repair/replace breakpoint tends to be higher than that for ac, about 65% the price of a new unit.
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