Motors and Compressed Air

MAE 406

Energy Conservation in Industry

Stephen Terry

Electric Motors

• In use throughout every manufacturing plant, office, and home

• Consume 64% of all electricity generated.• About 1 billion motors in service today consuming

over 2 trillion kWh of energy per year.

Types of Motors

• Direct Current (DC) Used in small applications and where precise speed is

required. Also used in very large applications (>1,000 hp)

• Synchronous Motors Used for precise speed control and large, slow speed

applications

• Induction Motors Most common type used in industry

Induction Motor Fundamentals

• A voltage is applied to the stator causing a rotating magnetic field

• This induces a counter EMF in the rotor by the flux of magnetic fields

• A rotating field is developed in the windings and the rotor tries to “follow” the magnetic field in the stator

• The rotor cannot spin as fast as the stator because of the load and the motor slips some. An 1,800 rpm motor has a shaft speed of between 1,700 and 1,760

rpm, depending on load.

Motor Fundamentals

• Induction motors have different designs based on application.

• Speed vs. torque• Motors require large power at startup (lock rotor

Amps, LRA) briefly, then operate at running load amps (RLA).

• National Electrical Manufacturers Association (NEMA) sets standards.

Motor Power

• Motor size in US is measured in horsepower (hp) output.

1 hp = 746 Watts = 2,545 BTU/hr• Motors convert electric power to shaft power• Efficiency is a measure of the conversion rate.• Efficiency ranges between 50% and 96% for most

motors.• Energy loss is in the form of heat

High Efficiency Motors

• A few years ago, all motors manufactured will meet minimum efficiency requirements.

• Many old motors are still in service• Efficiency improvements are due to improved

magnets, better heat dissipation, better windings, and lower friction bearings

Rewound motors

• Rewinding motors requires that the windings be removed by heating.

• If this is done at high temperature (quickly) the magnetic loses some of its characteristics

• Rewind losses for a good job = 1%-2%• “Rush” overnight rewind loss = 5% or more

Typical Motor Efficiencies and CostsMotor

hpRewind

CostStandard Motor

Efficiency

High Eff Motor Cost

High Eff Motor

Efficiency

Cost Difference

5 $330 82.5 $375 90.8 $45 7.5 $380 83.9 $525 91.3 $145 10 $500 86 $638 92.3 $138 15 $550 86.6 $825 92.6 $275 20 $600 87.3 $975 93.2 $375 25 $660 88.7 $1,238 93.9 $578 30 $760 89.4 $1,500 94.3 $740 40 $880 88.7 $1,950 94.5 $1,070 50 $980 90.4 $2,325 95 $1,345 60 $1,100 90.8 $3,750 95.2 $2,650 75 $1,320 90.5 $4,500 95.3 $3,180

100 $1,650 91.1 $5,325 95.5 $3,675 125 $2,200 90.6 $7,385 95.2 $5,185150 $2,400 91.8 $8,650 95.5 $6,250200 $2,650 92.3 $10,620 95.7 $7,970250 $2,860 93.1 $13,650 95.7 $10,790300 $3,080 93.2 $15,100 95.9 $12,020400 $3,500 92.6 $20,000 96 $16,500

To Rewind or to Replace

• Depends on motor application. Standard frame motors are good candidates. Specialty motors are too expensive to replace.

• Rewinding increases the cost of electricity to operate the motor.

• Compare extra cost of power to the increased cost of purchasing new motor vs. rewinding.

Example

We have a 25 hp motor that operates 6,000 hrs/yr. at full load. It is a standard efficiency motor that has not been rewound yet. It fails. What do we do?

Current Efficiency = 88.7%

Rewind Efficiency = 87.7%

(good shop)

First, compute the cost to operate the motor after rewinding…

Rewind Power = 25 hp x 0.746 kW/hp / 87.7%= 21.3 kW

Rewind Energy = 21.3 kW x 6,000 hrs/yr.= 127,594 kWh/yr.

Rewind Electric Cost = 127,594 kWh/yr. x $0.03536/kWh+ 21.3 kW x 12 x $11.25/kW

= $7,387/yr.

Note that you’re stuck with having to rewind or replace.

Next, compute the cost to operate a new high efficiency motor…

New Motor Power = 25 hp x 0.746kW/hp / 93.9%= 19.9 kW

New Motor Energy = 19.9 kW x 6,000 hrs/yr.= 119,169 kWh/yr.

New Motor Elec Cost = 119,169 kWh/yr. x $0.03536/kWh+ 19.9 kW x 12 x $11.25/kW

= $6,900/yr.

This is a cost savings of $487/yr.

Determine the cost of a rewind and the new motor.

Rewind Cost, 25 hp = $660Replacement Cost, 25 hp = $1,238

Cost Difference = $578

Compute payback period for installing new motors:

Simple Payback = $578 / ($487/yr.)= 1.2 years

Since motors typically last 50,000 hrs or more (8 years+), installing a new motor is a good idea.

Things to Remember

• Motors above 100 hp are usually better rewound due to large cost difference between rewinding and replacement.

• Motors smaller than 5 hp are best replaced since rewind costs are high, even for small motors.

• Rewind efficiency motors several times before replacement (losses accumulate until the energy costs exceeds replacement cost difference).

Motor Replacement by Hours and Size

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10.0

20.0

30.0

40.0

50.0

60.0

70.0

2000 3000 4000 5000 6000 7000 8000

Operating hours per year

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5 hp 10 hp 15 hp 25 hp 40 hp 50 hp

60 hp 75 hp 100 hp 150 hp

Compressed Air

• Often called a third utility (after electricity and natural gas)

• Almost every manufacturing plant has air compressors that operate.

• Compressed air used for controls, to move things around, to clean work areas, to lift things, and hold things in place

• Can be a significant part of electrical load

Compressed Air Pressure

• Pressure usually measured in pounds per square inch (psi) Usually talk about gage pressure (psig), which is

pressure measured above atmospheric pressure Absolute pressure (psia) used for calculations, pressure

measured against vacuum

Absolute Pressure = Gage Pressure + 14.7 psia

Three Types of Air Compressors

• Reciprocating compressor Utilizes piston / cylinder Usually two stages with an

intercooler (MAE302) Uses oil to seal piston and

for some cooling

k

k

P

P

T

T

1

1

2

1

2

Reciprocating Compressor Advantages/Disadvantages

Advantages

• Good for small application

• Uses no / little power when unloaded

• Cheap and simple to operate

• Operates over a wide range of pressures

Disadvantages

• Noisy

• Maintenance can be an issue

• Not good for larger applications

• Oil-free air units are expensive

Rotary Screw Compressors

• Very common in plants• Good mid-range size

(25 hp – 300 hp)• Models include oil

flooded or oil free• Oil flooded units offer

best efficiency

Screw Compressor Advantages / Disadvantages

Advantages

• Good efficiency for oil flooded models (continuous intercooling)

• Low maintenance costs

• Not usually noisy

• Can run fully loaded for extended periods of time

• Good for heat recovery for space heating

Disadvantages

• Can use 40%-90% of full load power when unloaded

• Not good for very small or very large loads

Centrifugal Compressors

• Uses an impeller to accelerate air to high velocity and then converts velocity to pressure.

• Used for large applications, including water chillers

Centrifugal CompressorAdvantages / Disadvantages

Advantages

• Can be staged for high pressure applications

• Typically used for loads greater than 200 hp

• Can be 1,500 hp or larger

• Low maintenance

• Part load performance is adequate (close off inlet)

Disadvantages

• Intercooling must be provided for higher pressures

• Not as efficient as screws

Compressed Air Dryers

• Air also contains water. The ability of air to hold water is not affected by pressure.

• One cubic foot of air can hold the same amount of water regardless of pressure.

• This is a problem since compressors “concentrate” the amount of water per cubic foot.

• Condensation is a problem, if water not removed

Compressed Air Dryers

• Two types of dryers used Refrigerated air dryers uses small refrigeration cycle to

cool and condense water out before reheating air (using condenser) to plant temperature. Typical dewpoints, around 40 F.

Desiccant Dryers use a chemical that absorbs water vapor. Typical dewpoints, -40F or less.

• Desiccants must be regenerated periodically – using compressed air or electric heat

Other System Components

• Filters to remove oil, particulates• Receivers to help maintain pressure during brief

periods of high air use• Loop compressed air systems• Air regulators (pressure reducing valves)

Sizing Compressed Air Systems

• Consider maximum air demand by summing all end users (see handouts)

• As air travels through piping, the pressure decreases. Air pressure in the system must be high enough for all processes at the point of use

• Don’t forget losses, energy use, and maintenance of filters and air dryers

Typical Measures

• Reduce compressor pressure – 100 psig or less usually adequate.

• Recover compressor waste heat to heat storage areas (80% of total compressor electrical power is typically available as hot air on air-cooled units)

• Repair air leaks

• Install smaller compressor to operate at night / weekends to keep large unit from running unloaded.

• Use outside air for recip / centr compressor inlets