Motors matched to load profiles lead to high-efficiency service

Pulp & Paper, Aug 1995 by Cole, Richard, Thome, Terry

ALERTED BY THE ENERGY ACT OF 1992, MANY users of AC induction motors seem to be eyeing premium-efficiency motors with mixed feelings, perhaps some confusion. When the October 1997 deadline arrives for compliance with the 1992 standards, the clearest winners will be the environment (due to the reduced need for more power generating plants), and the petroleum economy (because of reduced dependency on foreign oil).

Most obvious is that the potential for the pulp and paper industry involves smaller electricity bills. Motors account for approximately 64% of the electricity consumed in the U.S., at a yearly cost of $112 billion. Simple arithmetic shows that every 1% reduction in motor demand cuts .64%--or $716,800,000--off the total industrial bill. Higher efficiency motors also run cooler, which lightens the load on air conditioning systems and further reduces plant demand. Lower total demand can lead to lower rates by helping to minimize peak-demand surcharges. As rates increase over time, these savings increase.

Pulp and paper producers will claim more of these benefits than many other industries because typical mill operations fit the ideal profile for premium-efficiency motors. Compared with many other industries, pulp and paper production requires a relatively large number of motors, most of them within the size range that shows the biggest savings and most of them running around the clock. In addition, paper mill environments are typically hot, humid, and corrosive, and prone to periods of low voltage and motor overload. Here, the premium-efficiency motor's high reliability pays extra dividends because the continuous nature of the papermaking process makes downtime very expensive.

EVALUATING PAYBACK. Because they are built better, premium-efficiency motors typically cost up to 25% more than a comparably rated standard motor. Buying higher-priced equipment always needs to be cost-justified. The following example provides that justification.

Standard-efficiency motors typically incur yearly operating costs of 10 to 20 times the purchase price, compared with 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 hours/year at 0.08/kWh, can be expected to rack up $57,450 in one year's electricity bills. A 100-hp, premium-efficiency motor with an efficiency of 95.4 and a list price of $4,404 costs $54,800/year to operate under the same conditions (Figure 1). (Figure 1 omitted)

The $619, high-efficiency price premium returns an annual $2,650 savings, so in this ex. ample, it pays back the price difference in 2.8 months. The savings pays off the full price of the premium-efficiency motor in less than 20 months. Note that this example does not factor in any "hidden" savings in peak-demand rate reduction, lower air conditioning and motor maintenance costs, or the additional payback offered by local utilities through energy rebate programs, which can drop the payback period down closer to one year.

Note, however, that maximum savings will not always accrue to every installation, for several important reasons which will be discussed. Anyone considering a switch to premium-efficiency motors should be aware of those reasons before planning their changeover strategy.

First, it is important to look at what makes a motor energy-efficient and why that costs more. Understanding this is an important part of the essential cost justification.

WHY EFFICIENCY RAISES PRICE. Motors lose energy in several ways. Biggest among them are the "copper" losses that result naturally from current passing through the copper-wire windings (Figure 2). (Figure 2 omitted) Premium-efficiency design employs larger-diameter wire, increasing the volume of copper by 35 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 .025 in. thick, rated for electrical loss at 3.0 w/lb. Premium-efficiency motors use high-grade silicon steel laminations around .018 in. thick, having an electrical loss of 1.5 w/lb. The chemical makeup and thinner gauge 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, a result of complex molecular magnetic alignment properties, are reduced in premium-efficiency 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 higher-grade bearings, and windage losses in fan-cooled motors are reduced by smaller, more efficient fan designs. Over all, generally tighter tolerances and more stringent manufacturing process control are applied to minimize losses from unplanned conducting paths and stray load phenomena, both of which are common among motors. 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.