By Darrell T. Mears
As summer approaches, the "spring cleaning" is nearly done, the golf scores are improving, the days are getting longer, the electric loads are increasing and the higher summer electric rates are about to go into effect. As your electric bill approaches its annual peak, perhaps it's time to contemplate which electric motors in the plant contribute most to the bill and do some "spring cleaning" of your own.
Electric motors in continuous or near-continuous service have high energy costs, even at the relatively low electric rates common in much of the chemical industry (Table 1). Unfortunately, the savings from motor efficiency improvement will rarely justify motor replacement on its own.
Example: An aging 100 hp, 1,800 rpm, totally enclosed, fan cooled (TEFC), standard-efficiency AC motor runs at 92.1% efficiency at 75% load and at an electric rate of 4 cents/kWh. In continuous service, this motor contributes more than $22,000 to the annual electric bill. A new energy-efficient motor in the same service will offer about 3% higher efficiency. That's worth about $700 per year in electric bill reduction, but replacing the original motor costs more than 10 times that amount.
These economics can halt efficiency upgrade projects so that original motors are replaced only after they have failed. A delay in planning efficiency upgrades, however, is risky because it may squander related savings opportunities or lead to potential perils of specification in haste. Here are eight prerequisites for replacing existing motors with more efficient models.
Consider adjustable speed
- If motor load is highly variable, you may obtain substantial energy savings by installing an adjustable speed drive (ASD), such as a variable frequency drive (VFD). Prematurely installing a higher-efficiency motor will adversely affect ASD project payback, and a new motor chosen for efficiency upgrade may not be the optimum choice for operation with an ASD. Review all ASD applications before evaluating motor upgrades.
Unlike immediate motor replacement, immediate ASD installation may have a good payback. For example, one published study of nine cases of pump and fan ASD applications demonstrated measured energy savings in the range of 23% to 79% in the 7.5 hp to 125 hp motor size range .
Adapt to process evolution
- Process design sometimes requires a crystal ball. Pump specification and selection on new projects, for example, often has to be well under way long before the process loads and requirements are final. The result can be an over-design of the pump, where excessive capacity and excessive total head capability are built into the design. This over-design "margin" often escapes the commissioning team, so pump motors often begin life well away from their best efficiency points.
Process evolution can also lead to having a piece of process equipment operate far off its original design point, and any correction of "margin" and process evolution should be considered prior to re-specifying the motor. Look for indications of process evolution such as throttled discharge valves, simultaneously running parallel standby pumps, excessive bypassing and severe vibration. As with ASD installation, correcting these inefficient operations can be more lucrative than motor efficiency upgrades and should take a higher priority.
For example, a centrifugal pump may benefit from impeller trimming to adjust the operating point. Many impeller trimmings pay for themselves in just a few months. Sometimes the choice is between an ASD and an impeller trimming, and the impeller trimming wins on the basis of lowest cost, lowest risk and sheer simplicity .
Process evolution may result in low motor load, but don't let low motor load hijack the pump, fan or blower analysis. The common notion that motor efficiency plummets at part-load conditions is largely myth; electric motor part-load efficiency can sometimes be slightly higher than the full-load efficiency, and large electric motors maintain relatively high efficiency even down to the region of roughly 50% load. You should correct the problems on the process end before addressing any perceived problems on the electrical end.
It's all in the timing
- Once you address variable loads and process evolution, you can proceed with the specification of a new motor. The best time to select an energy-efficient or premium-efficiency replacement motor is before the original motor fails in the middle of a production run and the production manager demands everyone's attention to avert a crisis. In crisis mode, some possible, but dubious, motor selection procedures include:
- Using any motor in the warehouse that will suffice.
- Sending the failed motor to the local rewind shop for repair to original or unknown efficiency.
- Giving the old nameplate data to someone who doesn't know much about motor efficiency, but does know how to buy a motor in a hurry.
Under such circumstances, even maintaining efficiency may be difficult. Your maintenance plan should be more proactive than that. Many aren't, but even if yours is, there are still a few additional considerations that you should address well in advance of motor failure.
Avoid the full-load speed trapThe "affinity laws" indicate that the power requirement (i.e., brake horsepower (bhp)) of a centrifugal pump or fan varies as the cube of rotational speed (rpm).
(bhp2/bhp1) = (rpm2/rpm1)3
This approximation is good news if you decrease the speed after installing an ASD, but it's bad news if you increase the speed through a motor replacement. If an existing 1,800 rpm motor runs at 1,745 rpm at full load and you change it to a new, energy-efficient motor that runs at 1,775 rpm at full load, there may be enough additional load to offset any gain from improved efficiency.