Major guidelines will help you understand, evaluate, select, apply, and properly load premium efficiency motors.
Modern premium efficiency (PE) motors are improved versions of standard AC squirrel-cage induction motors that, when properly applied and installed, will save energy and reduce your electric bill. In recent years, manufacturers have designed PE motors that are better than ever. These modern motors have lower losses, are built to closer tolerances, and have better steel laminations, insulations, and slot designs. Also, they have more copper and more efficient cooling.
Because of their reduced losses, PE motors run at lower temperatures than equivalent standard motors, which results in longer insulation and lubricant life and less downtime. Inherent in their design is the ability to tolerate wider voltage variations and, when necessary, higher ambient temperatures. An additional benefit is that by generating less waste heat in the space around the motor building, ventilation and/or air conditioning requirements are reduced. This can result in additional savings.
When selecting a PE motor, you must consider numerous key characteristics such as horsepower, speed, type and effect of bad, service factor, and efficiency.
Workhorse of industry
The AC induction motor is the workhorse of industry. It's estimated that about 60 to 65% of all motors used by industry are integral horsepower AC induction motors. They are used extensively wherever heating, cooling, refrigeration, pumping, conveyors, and similar applications are needed.
In a typical commercial or institutional facility, squirrel-cage induction motors drive condensate pumps, chilled water pumps, air-handling blowers, drive combustion air fans, compressors: the list is endless.
Yet, it's these very motors that use well over haft of America's electric power; thus, they are the devices most responsible for major portions of electric bills. As such, it's vital that you take a serious look at the PE motors for your various motor applications.
Operating costs and savings
There are a great many ways to approach capital investment and to determine rates of return, payback periods, etc. Most of these are good for large capital investments where there may be risk involved if the project doesn't work out or if a product changes.
Electric motors and other conservation measures tend to be a simpler problem and usually don't need the rigorous mathematical treatment found in a complicated investment analysis. Here is a rule of thumb that will help you make intelligent decisions in regard to PE motors: At 5 cents per kWh, it costs $1 per hp per day to operate a motor continuously at full load. (At 10 cents per kWh, the cost doubles to $2 per day.) This value can be ratioed to reflect less than full load or less than continuous operation, etc.
Consider a 100-hp motor operating continuously in a 10-cents-per-kWh area. The annual cost of operation comes out to be approximately $70,000 ($2 per day times 350 days times 100-hp). This can represent about 25 times the first cost of the motor. By spending an extra 30% ($750) to get a PE motor (2.4% more efficient), the annual operating cost could be reduced by approximately $1800.
In the case of a small 3-hp motor at 10 cents per kWh, the annual operating cost would be over $2300 per year. An extra 40% spent on this motor could reduce the operating cost by $140 per year.
In both cases mentioned, the extra cost of the PE motor would be paid off by energy savings in a few months. When motors are running continuously at or near full load, the first cost of the motor is usually of little consequence compared with the annual operating cost.
Additional savings are also available through utility incentive programs. There are many customer incentive programs based on different concepts, including some where the utility invests in the conservation project and the resulting savings are shared by the utility and the customer over a period of time. Utility rebates, in whatever form they take, are a great incentive.
Ideal motor loading
While in the process of upgrading efficiency, you may ask what the ideal load conditions should be for replacement motors. A motor that's overloaded will have short life. In the opposite situation, a motor that is grossly oversized for the job is inefficient.
In many situations, the load is not changing as the motor is operating. This is especially true of heating, ventilating, and air conditioning applications such as circulator pumps and air handling equipment. On other types of machinery, such as air compressors and machine tools, the load may cycle on and off, heavily loaded for some periods and lightly loaded at other times.
Obviously on cycling loads, it's important to size the motor so that it can handle the worst case condition. However, on continuously loaded motors, it's desirable to load them at somewhere between 50 and 100%, and more ideally in the range of 75 to 80%. By doing so, high efficiency is available and motor life will be long. Also, by loading at somewhat less than 100%, motors can more easily tolerate such things as low voltage and high ambient temperatures, which can occur simultaneously in summer. This approach will result in optimum efficiency while preserving motor life.
With varying load levels and intermittent loading, projected savings based on full load efficiencies will be less or may not materialize.
PE motors, with their enhanced designs, result in lower operating costs at any level of loading, including no load. For example, the no-load losses of a 5-hp PE motor might be 215W. The no-load losses of a standard motor of the same type might be 330W. Fig. 1 shows a plot of watts loss for various load levels on a conventional 25 hp motor versus that of a PE motor of the same type. Curves of this type change dramatically with motor size, but trends are the same.
Efficiency curves can also be plotted for standard and PE motors, as shown in Fig. 2. These curves are so shaped for many reasons. First, energy is being used by the motor to excite its magnetic field. Second, energy is also being used to overcome the friction of the motor's bearings and the so-called windage of its rotating portion, when the motor is running idle (no load on the output shaft). Thus the efficiency at no load for both standard and PE motors is 0%.
Efficiency climbs as load is applied to the motor, up to the point where efficiency levels out and starts to drop from its highest level. In most motors, the peak efficiency will occur somewhere between 50 and 100 % of rated load. The point at which it peaks is determined by the specific motor design.
Fig. 3 gives a general outline of the flow of power through a standard motor and shows where losses occur. The flow is shown as 100% electrical power going to the motor (on the left side of the diagram), with the various losses involved shown in converting the power until it ends up as mechanical power at the motor's output shaft. In this case, the major losses are stator resistance loss (stator [I.sup.2]R). This is the largest single loss in the motor. It is followed by rotor resistance loss (rotor [I.sup.2]R).
Next come losses that are described as cog losses, which are losses resulting from the cycling magnetic forces within the motor. More specific terms used for these losses are hysteresis and eddy current losses.
Hysteresis loss is the result of the constant re-orientation of the magnetic field within the motor steel laminations.
Eddy-current loss results from the same phenomenon, which produces small electrical currents in the steel. These electric currents circulate on themselves and produce heat without contributing to the output of the motor. Hysteresis and eddy current losses occur in both the stationary and rotating portions of the motor; however, the largest share occurs in the stationary portion.
In addition there are friction and windage losses. Friction loss is the result of friction in the rolling of ball bearings. While bearings are extremely efficient, some losses still are generated.
Windage loss is actually a combination of things. First, the rotor spinning in air creates some drag. The faster it spins, the more drag it creates with the surrounding air. In addition, there has to be air flow through or over the motor to carry away heat being generated by the various losses. In most cases, a fan is either incorporated on the shaft of the motor or designed into the ends of the motor's rotor to provide air for cooling. This requires energy and uses input without developing output.
Finally, there is a category called stray load losses. These are losses that cannot be accounted for in the previous categories. Generally, stray load losses are dependent on motor loading and increase as load is applied.
Know efficiency terms
There are different terms used to compare efficiencies of motors. The two most often heard are nominal and guaranteed minimum efficiency. It's easy to get confused as to what basis you should use to determine potential savings from efficiency upgrades.
Nominal efficiency ratings can be explained in the following manner. If a large batch of identical motors are made and tested, the nominal efficiency is the average efficiency of these motors. Due to manufacturing tolerances, some units might be less efficient while others might be more. However, the nominal is the average of the lot.
Guaranteed minimum efficiency ratings recognize the variations from one motor to the next and set an arbitrary low limit. Such a rating says, in essence, that none of the motors in the batch previously described will be less efficient than this.
With these two efficiency ratings, what should be the basis of comparison? If you had to stake your life on the result and it involved a single motor, then guaranteed minimum efficiency would be the one to use. However, if you're considering a number of motors in a range of sizes, and you're not held precisely to what the final result must be, then nominal efficiency is the proper basis of comparison. Nominal efficiency also makes comparison easier because nominal efficiency is on the name-plate of all new 3-phase motors. In addition, nominal and minimum guaranteed efficiencies are related to each other by an equation established by the National Electrical Manufacturers Association (NEMA). Thus, comparing different motors on the basis of nominal efficiency is really equivalent to comparing on the basis of minimum guaranteed.
The efficiency of standard industrial 3-phase motors usually runs from a level of approximately 75% at I hp up to 94% at 200 hp. The curves shown in Fig. 4 illustrate the general trend of motor efficiency versus motor size for standard and PE motors.
Efficiency testing standards
It's important that you know the standard by which the efficiency of a motor is determined. This should always be IEEE 112-1991, IEEE Standard Test Procedure for Polyphase Induction Motors and Generators, specifically Method B. Of all standards developed for determining efficiency of motors, this is one of the most rigorous. This method also measures the efficiency in a hot running condition, making it more accurate because a motor's efficiency falls slightly as operating temperature rises.
Other standards that are used, particularly some international standards, do not demand such rigorous testing. In some cases, efficiency is merely calculated rather than measured. In virtually all cases, these standards will give efficiencies higher than the tougher IEEE 112 standard. The right basis of comparison should be that all motors considered should be compared on the same standard.
A few precautions
The use of PE motors is not necessarily without some pitfalls. For example, PE motors run somewhat faster (have less slip) than their less efficient counterparts. APE motor might run at a full load speed of 1760 rpm while the motor it is intended to replace might be running at 1740 rpm. This can help or hurt conservation efforts, depending on the type of load the motor is driving. For example, if it's driving a conveyor handling bulk materials, the higher speed will result in getting the job done faster. Also, if the conveyor has periods of light load, the reduced losses of the motor will save energy even during that period of time. The same situation exists on many pumping applications where a specific amount of fluid is going to be transferred. If the motor runs faster, the work is completed sooner and the motor is shut down earlier. In these cases, the consequences of the increased speed does limit energy use.
But there are applications such as chilled water circulating pumps where the extra speed can reduce expected savings. The reason this happens is that centrifugal pumps, along with other types of variable torque loads such as blowers and fans, require horsepower that is proportional to speed cubed. Thus, a slight increase in speed can result in a sharp increase in horsepower and energy used. A typical example might be where the original motor is directly connected to a centrifugal pump. Suppose its full load speed is 1740 rpm. The replacement PE motor, driving the same pump, has a higher speed of 1757 rpm. The resulting difference of 1% will increase the horsepower required by the pump by 1.01 x 1.01 x 1.01 = 1.03. Thus, the horsepower required by the load is increased by 3 % above what it would be if the pump speed had remained the same. Even with increased speed there remains, in most cases, some improvement in efficiency and resulting reduction in energy usage, although it may not be what you hoped to achieve.
The same thing would hold true on fans and blowers, if no changes take place to bring the equipment speed b ark to the original value.
In addition to the challenge of evaluating the different efficiencies of different makes of motors, there's also the matter of selecting properly sized equipment. For example, a pump oversized for a job may be much less efficient than a pump properly sized. Similarly, an air compressor oversized for the job may be much less efficient than one selected to more closely match actual requirements.
Existing motor efficiency upgrades
In a commercial or large industrial situation, one question frequently comes up: Should motors be replaced on a wholesale basis throughout a facility or selectively changed? There's probably no hard rule for this, but here are some ideas.
The wholesale change-out of all motors in a facility generally cannot be justified on a cost basis because some of these motors may be used only intermittently. Such applications as test equipment, trash compactors, and other similar situations support the case for not changing everything. There may be other complications such as specialized motors found on some types of pumps and machine tools as well as old motors (where direct interchanges are not readily available). These fall into a cloudy area where change-out may not be justified.
Motors having the greatest potential for savings are those that run on an extended basis with near full load conditions. These are the logical candidates for any change-out program.
When purchasing new equipment that will operate for substantial periods of time, ask for a PE motor option as part of your invitation to bidders. When asking for quotes on air compressors, pumps, HVAC equipment, process machinery, etc., a typical specification should read something like this:
The bidder should quote with its choice of standard induction motors and as an alternative, quote on the same machine equipped with premium efficiency motors. The bidder will separate the incremental cost for the addition of the premium efficiency motor(s) and provide the nominal efficiencies of both the standard and the premium efficiency motors offered.
By using a specification similar to this, the owner of the equipment will be in the position to make logical decisions on new motors being installed in the facility. In most cases, the incremental cost for a more energy efficient motor will be relatively small, especially when compared with the cost of the equipment it drives.
Edward H. Cowern, P.E. is New England District Manager, Baldor Electric Co., Wallingford, Conn.