Subscribe to our 

Pumps & Systems, June 2007

When a motor fails, users can (1) rewind, possibly for high efficiency; (2) replace the failed motor with a new motor; or (3) invest in a premium efficiency product. Here are the advantages and disadvantages of each approach and the precautions that must be taken to assure the best investment.

When a motor fails, the user must decide whether to repair or replace it. To make a proper decision, one must consider the cost of the repair, the availability of a replacement, the age of the motor, the electrical design required for the application, any special mechanical features, and the urgency of returning the failed motor to service. Placing the driven equipment back into service is frequently the highest priority, and users often make their decision based on this criterion alone. Plant managers tend to be less concerned if the rewound motor is less efficient when their operation's downtime is costing thousands of dollars a minute.

Increased Profits by Energy Saving

U.S. industry will continue to feel the squeeze on profits, and manufacturers who fail to implement energy reduction programs will find themselves at a competitive disadvantage. In some industries, motor operation costs may even exceed those of labor cost.

It is no longer practical to view the power bill as a fixed base cost, not worth the effort it would take to reduce. The need to minimize power usage should be as important to the CFO as it is to the plant manager. Due to the improvements that have occurred in motor technology, even companies that already had energy programs ten years ago should now reevaluate their criteria.

Future Power Costs

In 1970, electricity cost the average industrial user about one cent per kilowatt-hour. By 1980 that had jumped to four cents: a 300 percent increase. Power costs in some areas are already over seven cents. The cost for electricity may fluctuate with economic cycles, but long-term it will continue to increase.

The nature of today's power bill has also changed. The contract rate of the past now only covers 60 percent to 75 percent of the actual amount paid by users. In addition to taxes, today's bill can include such additions as "fuel adjustment" and "demand charges." The bottom line is that electric power can be a major cost element in your product.

First Cost vs. Operating Cost

Manufacturers who understand their business look at the total cost of a motor, rather than simply making a decision based on initial purchase price.

Let's consider the example of a pre-EPAct 75-hp, 1800-rpm TEFC motor that originally sold for approximately $2700, with an efficiency of 91.7 percent. This motor, operating continuously and using power costing $.07/kW-hr, will in just one year cost $37,414 to operate - or 1,386 percent of the original purchase price! In fact, operating costs will overtake the purchase price after the first 26 days of operation. Even if the motor is only used for two shifts (assuming 4,160 hours per year), this will still occur after 55 days.

For this reason, any expenditure related to repair or replacement of a failed motor should be evaluated based on a total cost of operation calculation.

An Incorrect Decision Costs Money

Building on the example above, consider the impact on the operating cost of the same 75-hp motor using a more efficient motor. Let's say the motor efficiency improves to that of an EPAct design with a nominal efficiency of 94.1 percent. The new EPAct motor would do the same amount of useful work, but use $979 less energy.

In mid-2001 the motor industry, in cooperation with conservation groups and the DOE, introduced products with even higher efficiencies: NEMA Premium. With this new specification, the same motor rating would have a nominal efficiency of 95.4 percent and save $1,510 in the first year. These savings continue to accrue as the cost of power goes up. Without increasing the cost of a kilowatt-hour, a NEMA Premium motor would generate $10,563 in savings over seven years, compared to $6,854 for an EPAct level efficiency motor.

Factors in the Rewind Decision

Hundreds of thousands of older T-frame motors were manufactured well before there were any government standards. Many of these still operate in U.S. industry. Each time one fails, an opportunity is created to improve the user's bottom line.

Power costs will certainly continue to rise and further escalate motor operating expense. So the question of how repair affects motor efficiency is an important one. Some claim a rewound motor is never as efficient as the original; others say a well-executed rewind can be better than the original design. These differences in perception suggest there may be several factors involved.

Armed with the right information, understanding the factors that affect rewind performance does not need to be complicated. Let's examine the various types of motor losses and how they are influenced by engineering decisions.

Keep in mind that actual motor losses may differ between two motors of the same design, depending on how the motor is used. Figure 1 shows how motor losses vary with load.

Figure 1, left. How motor losses vary with load.

As a motor approaches 100 percent of rated load, losses increase dramatically, with most of the increase found in the form of rotor and stator losses. The age of the motor is also a factor. Figure 2 shows the progression of motor efficiencies through the years, driven by improvements in engineering design and material technologies. (Note that these ratings are for typical GE motors from 1944 up to EPAct; actual efficiencies will vary from manufacturer to manufacturer.)

Figure 2, left. The history of motor efficiency, TEFC, 1800-rpm.

The distribution of losses will also be different for various motor designs. Variations in speed, design and enclosure will all affect loss distribution, as shown in Figure 3.

Figure 3, left. Representative losses as a percentage of total losses.

The ability of the repair shop to analyze and replace those parts which most influence losses, such as the stator core, the windings and the rotor, will affect the outcome of a rewind.

With all that in mind, let's take a look at losses in a typical 50-hp, 1800-rpm, TEFC standard efficiency design. The distribution of losses is shown in Figure 4.

Figure 4, left. Losses in a 50-hp, 1800-rpm, TEFC Pre-EPAct motor.