Repair or Replace?

The cost of repairing an existing motor is often much less than replacing. However, the number of running hours the motor sees per year will play heavily in the decision to repair or replace. The following example (Table 10 1) illustrates a simple example of an economic analysis.

Table 10-1 : Economics  of Repair or Replacing a Motor
Motor Size HP 40
Existing Motor Efficiency 87%
Premium Motor Efficiency 94.5%
Annual Energy Cost Savings ($0.05/kWh) $1073
Repair Price – Rewind Only $1000
New Premium Eff. Motor $1700
Price Differential $700
Load Factor 0.75
Hours/Year 8000
Simple Payback Months on Differential ~8.3
Simple Payback Months on Replacement Cost ~25

The motor in this example has a both a high load factor and annual running hours which contribute significantly to the case for replacement with an energy efficient motor. Even replacement of a healthy standard efficiency motor is somewhat attractive; however, the decision to repair is less clear for motors that have few running hours per year. If the motor is a critical piece of equipment, replacement may be a good option in terms of reliability.

If the decision is to repair, exercise caution in selecting a repair facility. For larger motors, it is prudent to request core loss testing of the motor before and after the rewind is performed to ensure that the core has not been damaged. A damaged core will not only have higher losses, but if the damage is localized it may lead to hot spots in the stator core, resulting in a potentially shortened life expectancy. Disassembly and repair of the core would be needed to correct the problem (Ref. 14)

Quality motor repair requires care to be taken at each step. Since modern insulation systems may include epoxies, stripping insulation from the stator core is often a difficult task to perform. Many repair shops bake the core to effectively burn out the old winding so it can be stripped. If the oven temperature is too high, the varnishes used to insulate the laminations could break down. Laminations accidentally peened over could short together resulting in hot spots and increased losses. Using either the wrong gauge of wire or not enough wire, will turns in the slots. For these reasons, it is important to get to know your repair facility and to insist on loss tests being performed. A low cost poor quality rewind may cost more in the long run due to higher run losses and a possible shorter life; however, done properly, even a high efficiency motor can be rewound without incurring any loss in efficiency.

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