Energy Efficient Motor Drive Systems

Motor Electricity Use• Motors consume about 75% of all the electricity used by

industry. • Their popularity is a testament to their reliability,

versatility and efficiency. • Despite these attributes, the cost of powering motor

driven systems in the US is over $90 billion per year. • Thus, increasing the efficiency of motor drive systems

can lead to significant savings.

Motors: The Nature of Wealth

James Watt observed that a horse pulling 180 pounds of force walked at 181 feet per minute.

Thus, the horse generated 33,000 ft. lbs. per minute, which Watt called one “horsepower”.

Generating 1 hp required: – 1,000 lb horse – 6 ft tall– costs $5,000 /yr to board

Today, generating 1-hp requires:– 32 lb motor (30x less)– 4 x 6 inches (12x less)– costs $250 /year (20x less)

Inside Out Approach to Energy Efficient Motor Drive Systems

End Use– Turn off motors when not in use– Move motor use to off-peak shift

Distribution– Motor drives

Primary Energy Conversion– Right size motors– Purchase ‘Premium Efficiency’ motors

Turn Off Motors When Not In Use!

Stamping press motors – 80% loaded while stamping– 65% loaded during idle– 65% of power dissipated as

heat due to friction!

Example: Turn off 50-hp stamping press for 2,000 hr/yr.– 50 hp x .65 x .75 kW/hp x

2,000 hr/yr x $0.10 /kWh = $4,875 /yr

Turn Off Motors When Not In Use!

Hydraulic system motors – 8 kW while loaded– 5 kW while unloaded– Draws 63% of loaded power

when unloaded.

Example: Turn off 20-hp hydraulic motor for 2,000 hr/yr.– 5 kW x 2,000 hr/yr

x $0.10 /kWh = $1,000 /yr

Move Motor Operation to Off-Peak Shift

Motor used only during first shift

Move motor use from 1st to 2nd shift to reduce electrical demand

Example: Move use of 50-hp, 80% loaded, 90% efficient, grinder to off-peak shift– 50-hp x 0.75 kW/hp x 80% / 90% =

33 kW– 33 kW x $14 /kW-mo x 12 mo/yr =

$5,544 /yr

Inside Out Approach to Energy Efficient Motor Drive Systems

End Use– Turn off motors when not in use– Move motor use to off-peak shift

Distribution– Motor drives

Primary Energy Conversion– Right size motors– Purchase ‘Premium Efficiency’ motors

Replace Smooth with Notched V-belts

Notched V-belts– 3% more efficient than smooth belts– Last 50% to 400% longer than smooth belts– Cost only 30% more than smooth belts

Example– 25-hp motor, 91% efficient, 75% loaded– Savings = 25 hp x 0.75 kW/hp x 75% / .91 x

(1/.92 - 1/.95 ) = 0.5 kW– Savings = 0.5 kW x 6,000 hours/yr = 3,000

kWh/year– Savings = 3,000 kWh/year x $0.10 /kWh =

$300 /year

h = 92%

h = 95%

Inside Out Approach to Energy Efficient Motor Drive Systems

End Use– Turn off motors when not in use– Move motor use to off-peak shift

Distribution– Motor drives

Primary Energy Conversion– Down size under-loaded motors– Purchase ‘premium efficiency’ motors– Replace rather than repair older failed motors

Down-size Under-loaded Motors

Efficiency declines at low loads Power factor declines at low loads

Motors: Energy Cost >> Purchase Cost

• 20-hp, 93% eff, 75% loaded, 8,000 hrs/year, $0.10 /kWh, cost = $1,161

• Annual energy cost = 20 hp x 75% x .75 kW/hp / 93% x 8,000 hr/yr x $0.10 /kWh = $9,677 /yr

• Over 1 yr, energy cost is 8x greater than purchase cost• Over 12-yr life, energy cost is 100x greater than purchase

cost!

Purchase and Energy Costs(20 hp motor at 8,000 hours/year over 20 years)

0

30,000

60,000

90,000

120,000

150,000

Purchase Energy

($)

Purchase Premium Efficiency Motors

Consider– 15 hp motor, 80% loaded, 6,000 hr/yr, $0.10 /kWh– Standard Eff = 0.91 = $889 Premium Eff = 0.93 = $1,010

Cost of electricity– Savings = 15 hp x .8 x .75 kW/hp x 6,000 hr/yr x $0.10 /kWh x

(1/.91 – 1/.93)– Savings = $127 /yr

Incremental Cost of Premium Efficiency Motor– $1,010 - $889 = $121

Simple Payback– $127 / $121 /yr = 1 year

Replace or Repair Older Failed Motor?

Assuming 80% loaded, 6,000 hr/yr, $0.10 /kWh

Size Efficiency Cost Efficiency Cost Rew-Rep Rep-Rew S. P.(hp) Rewound Rewound ($) Engy Eff Engy Eff ($) ($/yr) ($) (yr)

1 73 220 84.6 275 68 55 0.85 82 330 89.8 432 191 102 0.510 84.7 500 91.7 686 324 186 0.615 85.5 550 92.6 911 484 361 0.720 87.3 600 93 1,071 505 471 0.930 88.2 760 93.8 1,553 731 793 1.150 90.6 980 94.4 2,482 800 1,502 1.960 90.8 1,116 94.8 3,280 1,004 2,164 2.275 91 1,320 95.3 4,476 1,339 3,156 2.4100 91.2 1,650 95.4 5,645 1,738 3,995 2.3150 91.8 2,400 95.5 8,624 2,279 6,224 2.7200 92.3 2,650 95.7 10,680 2,771 8,030 2.9250 92.9 2,860 95.8 13,043 2,933 10,183 3.5300 93.1 3,080 96.1 14,084 3,621 11,004 3.0500 92.8 4,400 96.6 25,725 7,630 21,325 2.8

Replace Rather than Rewind Motors

Source: US DOE Motor Master+ 4.0

Operating Hours: 8,000 hrs/year

0

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250Motor HP

Sim

ple

Pay

bac

k (Y

ears

)

$0.05 /kWh

$0.08 /kWh

$0.11 /kWh

U.S. D.O.E. Motor Master Software Over 25,000 motors from 18 manufacturers

Rapid data entry, sorting by condition, and rewind/replace recommendations.

Technical data to help optimize drive systems, such as:– Motor part-load efficiency, power factor

– Full-load speed, locked-rotor, breakdown, and full-load torque.

Motor purchasing information, including list prices, warranty periods, etc.

Capability to calculate savings, payback, return-on-investment, etc.

http://www1.eere.energy.gov/industry/bestpractices/software.html#mm

Employ Energy Efficient Flow Control

Inefficient Flow Control

By-pass loop(No savings)

By-pass damper (No savings)

Throttling valve(Small savings)

Inlet vanes(Moderate savings)

Fan w/ Inlet Vanes

Efficient Flow Control

Trim impellor for constant-flow

pumps

Slow fan for constant-flow

fans

VFD for variable-flow pumps

or fans

Close Bypass Valve

dP

VFD

Power and Flow Control

10% 20% 30% 40% 50% 60% 70% 80% 90% 100%0%

20%

40%

60%

80%

100%

By-pass Outlet Damper Variable Inlet Vane

Variable Frequency Drive

Volume Flow Rate (%)

Po

wer

(%

)

Case study: Large Cooling Towers

Large Cooling Loop Pumps

Worlds Largest Bypass Pipe

For Constant Flow Pumping: Trim Pump Impellor and Open Throttling Valve

For Constant Flow Fan: Slow Fan Speed by Increasing Pulley Diameter

For Variable Flow:Install VFD & Control with Difference Pressure

• W2 = W1 (V2/V1)3

• Reducing flow by 50% reduces pumping costs by 87%

warm water

cool water

cooling tower

city water make-up

7.5 hp pump

25 hp pump

reservoir

process water return

bypass / pressure

relief valve

cooling water to process loads

dP

VSD