Energy-Efficient Motors - P2 InfoHouseinfohouse.p2ric.org/ref/17/16897.pdf · INNAVERS L ENERGY-EFFICIENT MOTORS INTRODUCTION More and more industries are switching to energy-eff

I N N A V E R S L

ENERGY-EFFICIENT MOTORS INTRODUCTION

More and more industries are switching to energy-eff icient motors. Why? Energy-efficient motors lower production costs. Inefficient or improperly-sized motors waste money and energy. By becoming more familiar with different types of motors and their applications, an industrial facility can significantly reduce the energy costs per unit of production.

ciency can significantly lower production costs. A motor can cost thousands of dollars a year to oper- ate, so savings of even a few percent add up quickly. An energy-efficient motor generates savings whenever it is running and as long as it lasts, which may be 20 years or more.

Energy-eff icient motors’cost more than standard motors, but purchase price pales in comparison to a motor’s operating costs (see Figure 1). In fact, since the annual operating cost of a motor is often 5 to 10 times its initial cost , the typical 3 to 5 percent higher efficiency of an energy-eff icient motor can more than offset its 15 to 20 percent higher initial cost over its life.

As a real-life example, Figure 2 compares two 100-horsepower motors in a high-use application. Although Motor B is only 3.4 percent less efficient than Motor A, its losses total more than Motor A’s losses and purchase price together. In other words, Motor B wouldn’t be the most economical choice even if it were free.

In addition to costing less to oper- ate, energy-eff icient motors generally last longer, require less maintenance, and reduce system amperage draw.

MOTORS AND THEIR EFFICIENCIES

for all the power consumed by a

Just a small increase in motor effi-

Because the motor user is charged

motor, whether it is converted into useful work or losses, it pays to be concerned about motor efficiencies.

The efficiency of a motor refers to the power it puts out divided by the power it consumes. Motor efficiency varies depending on load. In general, standard motors operate most eff i- ciently when fully loaded, and lose efficiency noticeably below 80-percent loaded 2 . For that reason, it is important to match motor ratings with loads as closely as practical.

As motors convert electrical energy to mechanical energy, they give off inefficiencies as heat. Because heat adversely affects a motor’s capability, efficiency, and life, losses must be

Automobile Purchase Price $1 6,000 Annual Use 12,000 mi les/yr. Efficiency 30 miles/gallon FueVEnergy Cost $1.10 /gallon Annual Operating Cost $4401~ r.

Operating cost as YO of Purchase Price

limited to keep the operating temper- ature below a reasonable limit. This also means that larger motors must be more efficient than smaller motors, because they have proportionally less surface area through which they can release heat.

Because larger motors generally have higher efficiencies, smaller motors may represent larger potential savings: ten 10-horsepower motors waste more power than one 100- horsepower motor.

I 60 HP* Motor $4,000

4,000 hrs * *

I 89 2% 5 6c kwh

$1 O,036/yr

250% I

First Cost Is Not the Last Cost.

3r. equivalent to 50 18 kw capacity ihift operation Ontario Hydro

3Ec H CAROLINA WATIW ENERGY ORATION

Motor A Motor B

Efficiency Efficiency 95% 91.6%

fl Purchase price (100 HI? 1800 RPM, TEFC) Net present worth cost of losses @ .05/kwh and 10% cost of capi- takmotor life assumed 40,000 hrs.

Option A: Purchase price $3,660; NPW of losses $5,686

Option B: Purchase price $3,150; I NPW of losses $9,552

THE BASICS OF MOTOR SELECTION

Basic motor selection starts with matching the motor characteristics to the load requirements. These include starting loads, running loads, intermit- tent loads and shocks, environmental temperature and contamination, physical space and mounting require- ments, drive takeoff requirements, and available voltages. Obviously, a motor incorrectly matched to load requirements will waste energy, and may fail prematurely or cause other problems in the production line.

Having matched the motor type to the load requirements, the next step is to consider standard versus energy-eff icient motors. There are three basic types of motors available - new energy-eff icient motors, new standard motors, and repaired (“rewound”) motors.

ciency during the rewind process and/or when they fail, but are quick and cheap replacements.

A standard motor sacrifices eff i- ciency, endurance, and other positive attributes for lower initial costs. Stan- dard motors may be economical where annual operating hours are low.

ally the most economical choice where annual operating hours are high. Energy-efficient motors also offer other advantages, such as longer insulation life, longer lubrica- tion interval, greater inertial acceleration, and higher service fac- tors. In addition, they may be better equipped to handle heavy starting loads, large sustained loads, phase imbalances, voltage excursions, high ambient temperatures, and impaired ventilation 3 . Energy-efficient motors also do not add as much to air-conditioning loads as standard motors, since they do not give off as much heat.

Rewound motors usually lose effi-

The energy-eff icient motor is usu-

MOTOR COSTS On top of initial purchase price,

motor costs include operating costs,

~~~

maintenance costs, and replacement costs.

In addition, oversized motors may result in power factor penalties from the utility. Power factor is defined as the ratio of the active, or real, power required to serve the load (measured in watts) to the apparent power that is being drawn from the line (measured in volt-amperes). Properly-sized induction motors have a power factor of 0.85 or better. For an underloaded motor, power factor can be as low as 0.30 to 0.40.

While watt-meters and the utilities’ billing meters generally record only the active power used, lines and transformers must be sized by the volt-amperes drawn, which is larger than the active power if the power fac- tor is low. Therefore, many utilities charge a penalty for a lower power factor to cover the expense of the added equipment.

Calculating total energy-related motor costs can be done with a plant- wide survey. A motor survey can pro- vide an accurate idea of how much energy is currently going to power motors, and how much could be saved if energy-eff icient motors were in place. Such a survey can help answer such questions as, “Are motors appropriately matched with their applications and loads?” “How much money is the plant spending to operate motors each year?” “Are other alternatives more attractive?”

After a motor survey, decisions can be made about what motors will replace current motors when they fail. That way, when motors fail, they can be replaced with the best and most economical motor for the job, rather than just the quickest replacement avai I able .

More general questions can be addressed during a motor survey as well. For example, “Are the motors running during the factory’s peak demand inefficient, incurring extra demand costs?” “Are any motors being used during the peak demand that could be moved off peak?” “How often do particular motors fail?” “How much does downtime cost when a motor fails?”

Another benefit of a motor survey is e that hidden maintenance problems, such as machinery in need of lubrica- tion or adjustment, may be discovered.

DOING A MOTOR SURVEY Doing a motor survey means

checking to make sure all motors being used have the appropriate design characteristics for their appli- cation, and that motors are properly- sized and have good power factors. Oversized motors waste energy and have low power factors.

In addition, a motor survey means taking electrical measurements of voltage, current, and power factor. Voltage and current can easily be measured; power factor can be deter- mined with instruments costing less than $300. This data can be col- lected from 25 to 50 motors per day during a normal motor survey.

These three measurements can be used to calculate the actual operating kilowatts, or active power (see Figure 3). Multiplying active power by the hours the motor runs gives kilowatt- hours. Now total energy-related costs for motors can be calculated 4 .

Multiplying the electrical load in kilowatts by utility rates yields demand costs. Similarly, multiplying the kilo- watt-hours with utility rates yields energy costs. The total energy- related costs equal the demand costs plus the energy costs plus, if applica- ble, penalties for poor power factor.

The next step after calculating the total energy-related costs is to do some economic comparisons of stan- dard motors and energy-eff icient motors4. Figure 3 shows the com- ponents of the calculations for energy-related savings from using energy-eff icient motors.

C

ECONOMtC ANALYSIS Although simple payback calcula-

tions can give a general comparison between two motor choices, they should not be used to evaluate long- term projects. Simple paybacks can- not take into account the long-term

costs and benefits of the various alternatives and the time-value of money. Net present value and internal rate of return give a better measure of the lifetime benefits of the alternatives.

The economics of motor choice depends on utility rates, motor effi- ciencies, motor power factor, initial motor cost, maintenance cost, and the cost of money to the industry.

in the main cost comparisons that must be made. Different types of costs (see below) must be deter- mined for each alternative motor installation. Those costs that occur over time must be adjusted for the time-value of money. The alternative with the least total life-cycle costs should be chosen.

1. Initial cost of the complete installation, including any required capacitors, starters, etc.

2. Expected cost of energy used in each year.*

3. Expected demand cost in each year.*

4. Expected maintenance cost in each year.*

These factors determine the values

'Discounted back to the present value in the year of initial installation so that direct com- parisons can be made.

EN E RGY -E FFl Cl ENT MOTOR RATINGS

Methods of rating and testing energy-eff icient motors vary greatly. Motor purchasers should specify a minimum efficiency to be guaranteed by the manufacturer. In addition, motor purchasers should require that the efficiency tests meet IEEE-112, Test Method B, which is applicable for motors up to 500-horsepower; it uses dynamometer testing and segregates losses by type. This rating method is specified in NEMA Test Standard MG1-12.53a. Other rating methods may overestimate motor efficiency.

SUMMARY

motors cost more to buy than Even though energy-eff icient

Inputs: %EH~ = % Efficiency of premrum motor %Eb = % Efficiency of standard motor HP = Rated full load horsepower of

motor LF = %of full load the motor

actually runs V = Measured voltage between

supply conductors AMPS= Measured current flowing in

supply conductors ActP = Active power in kW PF = Power factor H = Annual running hours ec dc

Motw Load

= U t i l i energy charge in SkWh = U t i l i demand charge in SkW

RebtionshipS

Measured ActP = VxAMPSxPFx 1.732/1000 (for three-phase system)

Calculated LF = Measured ActPx % Efficiency of Motor4746 xHP)

or

Estimated ActP = 0.746xHPxLF (estimated)/%Efficiency of motor

Demand Cost = dc x ActP x 12 mohr Energy Cost

Energy savings = LF x HP x

A n d operaling costs

= ec x ActP x H

savings Between Motor choices

(100/%Eb - 100/%EHi) x .746 x H x ec

Demand savings = LF x HP x (100/%E~, -

dc x 12 mohr IW/%EHi) X .746 X

Total savings = Energy savings t Demand savings

I I

Figur03. dcavlngs CIPleulat Dons

_--

standard motors, the money saved in operating costs often makes imple- menting energy-eff icient motors an attractive alternative. Figure 4 shows a cost comparison of replacing a 10- horsepower motor with a rewound, a new standard and a new premium motor.

In addition to reducing operating and maintenance costs, energy-effi- cient motors usually offer other advantages, such as longer life, less maintenance, and better power factor.

NEW STANDARD

88% EFFICIENCY

8.5 kw 5100 kwh

$75.65

$178.50

$5.28 22 Months

NEW HIGH-EFFICIENCY

92% EFFICIENCY

8.1 kw 4860 kwh

$72.09

$170.10

$1 7.24 12 Months

HELP For additional information about

energy-eff icient motors, contact:

0 Your local utility representative 0 Your local contractorkonsultant 0 Major motor manufacturers 0 North Carolina Alternative

Energy Corporation Industry Program Secretary PO. Box 12699 Research Triangle Park, N.C. 27709 (919) 361 -8000

These organizations can offer or recommend assistance programs, workshops, and other information to help industries evaluate energy- improvement alternatives.

REFERENCES AND NOTES

fives for Electric Drives, Oak Ridge National Laboratory, Oak Ridge, Tennes- see, 1982

2 Colby and Flora, Measured Efficiency of High Efficiency and Standard Induction Motors, IEEE paper presented at Indus- trial Applications Conference, Seattle, Washington, October, 1990.

3 Montgomery, How to Specify and Eval- uate Energy-Efficient Motors, General Electric Company Motor Business Group, Schenectady, New York, 1984.

4 A Lotus 123 spreadsheet is available from North Carolina Alternative Energy Corporation (see address below) to assist in plant-wide motor surveys and economic analyses.

SUPPLEMENTARY RESOURCES Clapp, Primer on Project Economics, North Carolina Alternative Energy Corpo- ration, Research Triangle Park, North Carolina, 1984. Fink and Beaty, Standard Handbook for Electrical Engineers, McGraw-Hill Book Company, New York, 12th edition, 1986. IEEE Standard Dictionary of Electrical and Electronics Terms, ANSlIlEEE Standard 100-1984, The Institute of Electrical and Electronic Engineers, New York, 1984.

1 Comnes and Barnes, Efficient Alterna- -

-

NORTH CAROLIhA ALTERNATIVE ENERGY CORPORATION

Established by the North Carolina Utilities Commis- sion in cooperation with the state s major electric utilities North Carolina Alternative Energy Corpora- tion (AEC) is an independent, nonprofit organization promoting energy efficiency in North Carolina

- P. 0. Box 12699

Research Triangle Park, NC 27709