Motion System Design
Getting the most from high-efficiency motors

Getting the most from high-efficiency motors

As the cost of energy rises, you may be looking at premium-efficiency motors to reduce this cost. But, in which applications should you use them? What will it cost you? What will you save? Plus, how will the National Energy Act affect you

When someone says, “I’m here from the government and I’m here to help you,” skepticism is probably one of your reactions. With the passage of the Energy Act of 1992, however, the government may really help you. This law defines the efficiencies of several electrical devices including electric motors. It states that after October 1997, general purpose motors must meet specific efficiencies, see box “New motor efficiency requirements.

Moreover, you may reduce your electric bills by specifying premium-efficiency motors before 1997 and by replacing existing motors with the newer watt savers.

Let’s take a look at these motors and put things in perspective. Motors account for approximately 64% of the electricity consumed in the U.S., at a yearly cost of $112 billion. Every 1% reduction in motor demand cuts 0.64% — or $716,800,000 — off the industry-wide bill. As energy demand decreases, peakdemand may also drop, leading to lower surcharges and lower rates.

Savings also accrue from lower maintenance costs. Premium-efficiency motors are built better and provide longer service life backed by longer warranties.

But because these motors are built better, they typically cost up to 25% more than a comparably rated standard motor. Buying higher-priced equipment always needs to be cost-justified.

Evaluating payback

Yearly operating costs of standard-efficiency motors can be 10 to 20 times the purchase price, compared to 8 to 12 times the purchase price for premium-efficiency motors. A 100-hp standard-efficiency motor with an efficiency of 91.0% and a list price of $3,785, running a 100- hp load continuously (8,760 hr/yr) at $0.08 /kW-h can reach up to $57,450 in one year’s electricity bills. A 100-hp premium- efficiency motor with an efficiency of 95.5% and a list price of $4,404 costs $54,800 per year to operate under the same conditions.

The $619 list price difference returns an annual $2,650 in operating cost savings. In this example, the price difference is paid back in 2.8 months. Maximum savings, though, will not always accrue to every installation for reasons discussed later in this article.

According to the Energy Act, you do not have to immediately replace existing motors with premium-efficiency motors. By 1997, motor manufacturers will no longer be able to offer standard-efficiency motors, so it is assumed that as motors wear out, replacement motors will be the high-efficiency offerings.

Why efficiency raises prices

The benefits are attractive, but an essential part of cost justifying premium-efficiency motors involves understanding what makes them energy-efficient and why that costs more. Motors lose energy in several ways. Biggest among them are the “copper” losses that result naturally from current passing through the copperwire windings.

Premium-efficiency design uses larger-diameter wire, increasing the volume of copper by 34 to 40%. To accommodate larger wire, the steel laminations that support the windings need larger wire slots, which reduces the amount of active steel in each lamination. To compensate for the loss of steel, more laminations must be added. Consequently, the rotor and stator core must be lengthened, and the motor’s shell length increased. More metal adds more cost.

Next comes magnetic core loss — technically divided into eddy-current and hysteresis losses. In premium-efficiency designs, the longer rotor and core generally help decrease magnetic losses, but the makeup of the laminations is the key factor.

Most standard-efficiency motors use low-carbon steel laminations around 0.025 in. thick, rated for electrical loss at 3.0 W/lb. Premium-efficiency motors use high-grade silicon steel laminations around 0.018 in. thick, having an electrical loss of 1.5 W/lb. The chemical makeup and thinner gage of premium-efficiency laminations, plus a coating of inorganic insulation on each piece, combine to greatly reduce eddy current losses. However, better steel costs more.

Hysteresis losses are a result of complex molecular magnetic alignment properties. They can be reduced in premiumefficiency motors by special annealing and plating of rotor and stator components, plus use of high-purity cast aluminum rotor bars.

Friction losses are reduced by highergrade bearings, and windage losses in fan-cooled motors are reduced by smaller, more efficient fan designs. Overall, generally tighter tolerances and more stringent manufacturing process control minimize losses from unplanned conducting paths and stray load phenomena.

While all the above differences in material and manufacturing discipline combine to increase motor price, they also combine to make premium-efficiency motors run cooler than their standard-efficiency counterparts.

Aside from cutting down the ambient air conditioning costs, cooler operation lengthens the motor’s service life in two important ways. For every 10 C reduction in temperature, motor insulation life doubles; premium efficiency motors tend to operate 10 to 20 C cooler than their standard-efficiency counterparts. Heat also is the primary cause of grease breakdown shortening bearing life; premiumefficiency motors tend to run 10 to 15 C cooler at the bearings. Consequently, premium efficiency motors can provide up to four times longer winding life and twice the lubricant life of standard motor designs.

Best and worst candidates for change

When evaluating a changeover to premium- efficiency motors, look first at two factors — cost of electrical power and the hours of operation.

The utility cost benefit of premium-efficiency motors begins to diminish when industrial power rates drop below $0.06 per kW-h. The true kW-h cost is often 1 to 2 cents higher than the actual base rate because of peak demand charges and other penalties. True kW-h cost is most easily determined by dividing the facility’s total electric cost by its kilowatthour usage.

The economic argument for changeover gains strength wherever the true kW-h cost moves above $0.06. The hours of operation will indicate which motors in your plant might provide the quickest and best opportunities to save money with higher efficiency. Motors that run at least 2,000 hours per year (eight hours per day, five days per week) are your best candidates. As operating hours increase, payback period shrinks.

But lower rates and shorter running times should not automatically rule out a change. Many local utilities offer energysaving rebate programs that might shorten payback sufficiently to make premium- efficiency motors more cost effective. Also, consider that because of the other benefits of better construction — cooler operation and longer life — premium- efficiency motors can be attractive regardless of how much they help cut utility bills.

The third factor to review is the size and type of motor involved. The potential for cost savings is greatest in motors rated 1 through 125 hp, including TFrame, NEMA Design A, B, and C motors that operate at 3,600, 1,800, 1,200 or 900 rpm, Figure 1.

For those industrial facilities that use older U-Frame motors, a changeover to higher-efficiency T-Frame motors should be considered. If, however, changing to a T-Frame with its smaller mountings and shafts proves difficult, adaptation mounts are available. Alternately, you may be able to use premium-efficiency direct replacements for the older U-Frame motors. Some manufacturers are beginning to offer high-efficiency U-Frame motors, eliminating the need to adapt to the original mounting dimensions. Applications involving seasonal or peak-load requirements, or where the motor runs at less than 50% of rated load throughout most of its duty cycle, may not be justifiable on utility cost basis alone. Perhaps the least likely place to consider switching is in harsh environments where a motor’s life expectancy is so short that neither payback period nor the long-life benefits of better construction have any real relevance. Calculating your savings Deciding whether to switch to premium- efficiency motors begins with calculations of the savings that will result. Standard PC-diskette programs are available for this purpose, usually built around the following formula:
= 0.746 x P x Cx
[(100 ÷ Es) - (100 ÷ Ep)]

= Savings per year, dollars
P = Motor rating, hp
= Energy cost, $/kW-h
= Running time, hr/yr
= Efficiency of standard-efficiency motor, %
Ep = Efficiency of premium-efficiency motor, %

Caution: this formula (and some diskette programs) presumes constant load — it does not account for load variation — and it assumes that horsepower will be a true horsepower value, or the worst-case horsepower requirements of the driven load. You cannot simply plug in the horsepower nameplate rating of the existing motor, for two reasons:

• The motor presently operating may be an inadvertent or incorrectly specified replacement for what was originally there.
• The motor presently operating, even if identical to what was originally specified, may not be operating close to its rated nameplate horsepower.
• In order to deliver their promised economies, premium-efficiency motors must run between 75% to 100% of rated load. Operated at less than 50% load, they might use even more electricity than standard motors in the same service.

To determine the correct horsepower value to plug into the formula, someone intimate with the driven process must profile it to determine how long the motor works hardest. This profile should include the regular cycles of varying difficulty that occur during a normal day and the seasonal motor load and operating conditions. Likewise, someone should profile when the motor workload is lightest, and for how long.

Note any unusual aspects of the candidate motor’s load profile; intermittent or pulsating loads, such as a reciprocating pump or compressor, may need a motor specially built for that purpose. Replacement with a premium-efficiency motor not built for that purpose may result in higher current pulsations and an apparent low power factor.

Once all the profile information is collected, compare measurements taken during the times of hardest service to the motor’s nameplate rating to determine how closely that relates to fully loaded condition. This can be done within reasonable margin of error by measuring the motor’s:

• RPM and comparing it with nameplate full-load speed. or
• Current and comparing it with nameplate current rating.

For both measurements, motor input voltage must be checked at the same time to make sure the motor is receiving full rated voltage. Because motor torque varies with the square of the voltage change, even a mild brownout can make a difference in the measurements. If voltage is chronically erratic or off nominal at the measuring site, consult the motor manufacturer for assistance in determining performance variations during voltage fluctuations.

If speed and current measurements taken under full voltage fall close to their respective nameplate values, the motor is running at or near rated load. If they aren’t close to nameplate values, these measurements can be used with standard Amp/Watt/Horsepower charts to approximate the motor’s horsepower output at the time of measurement. It’s a good idea to graphically plot the motor’s duty cycle to show how much of that cycle keeps the motor above 75% of rated load. The savings produced by premium-efficiency motors will vary according to how much of the motor’s operating time is spent above 75% of rated load.

System considerations

Premium-efficiency motors tend to run a bit faster than their standard-efficiency counterparts. When applied to centrifugal loads— where the load horsepower varies by the cube of the speed — this small increase in speed will demand a large increase in motor output horsepower, causing the motor to draw more power and lose part or all of its energy-saving benefit. In belt-driven applications, this increase in motor speed can be compensated for by variable-pitch sheaves to adjust the fan or pump back to specified speed. Direct-drive loads typically require additional engineering evaluation to assure energy-saving benefits.

Premium-efficiency motors also draw higher locked-rotor (stalled) current. Installations using thermally protected motor starters usually need to be checked for properly rated heater elements. Likewise, circuit breakers with higher trip ratings may be needed in the motor service line.

With the benefit of higher power factors than offered by standard-efficiency motors, Figure 2, premium-efficiency designs require less total current for an equal amount of work. Lower current demand on the line means that less energy is wasted in all feeder circuits serving the motor, thereby enhancing the efficiency of the distribution system. This power-factor advantage begins to erode as motor operation drops below 75% of rated load and declines sharply below 50% of rated load, Figure 3, which accounts for the general recommendation to operate the motor at or above 75% of rated load.

Dayton Electric Mfg. Co. is one of the manufacturers of high-efficiency motors.

Richard Cole is Product Manager and Terry Thome is Senior Product Engineer for Industrial Motors, Dayton Electric Mfg. Co., Niles, Ill.

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