1 APPENDIX 7

- A. ENERGY USE SCENARIO FOR ELECTRIC MOTORS WITH

HIGHER OPERATING SPE EDS

TABLE OF CO NTENTS

7

- A.1 BACKGROUND

................................ ................................ ................................ ........... 2

7

- A.2 METHOD FOR DETERMINI NG ENERGY SAVING IN VARIABLE TORQUE APPLICATIONS

................................ ................................ .......................... 2

7

- A.3 ASSUMPTIONS TO DETER MINE ENERGY SAVINGS IN VARIABLE TORQUE APPLICATIONS

................................ ................................ .......................... 3

7

- A.3.1 Share of motors negatively impacted by higher operating speeds

................................ . 3

7

- A.4 SENSITIVITY ANALYSIS

................................ ................................ .......................... 42 APPENDIX 7

- A. ENERGY USE SCENARIO FOR ELECTRIC MOTORS WITH

HIGHER OPERATING SPE EDS

7

- A.1 BACKGROUND

The installatio n of a higher efficiency motor alone m ay increase the energy consumption

for a particular application, instead of realizing energy savings. A more efficient squirrel

- cag e induction motor usually has less slip than an older less efficient motor because of a reduction in

the resistance of the rotor . This results in higher operating speed and potential overloading of the

motor. The U.S. Department of Energy ( DOE ) acknowledges that the cubic relation ship between speed and power requirement in certain fan, pump, and centrifugal compressor applications can affect the benefits gained by efficient motors which have a lower slip. This appendix describes

the methodology DOE used to estimate this effect as a sensitivity analysis in the Life

- Cycle

- Cost spreadsheet at http://www1.eere.energy.gov/buildings/appliance_standards/commercial/electric_motors.html

.

7

- A.2 METHOD FOR DETERMINI NG ENERGY SAVING IN VARIABLE

TORQUE APPLICATIONS

DOE based its methodology on a previous publication i which states the following:

In the cas e where there is a cubic relationship between the power and the speed,

Po EE ( L ) = Po BE ( L )

. ω EE ( L ) 3

ω BE ( L ) 3 Where:

L is the load in percentage

Po EE (L) is the output power of the energy efficient motor

Po BE (L) is the output power of the baseli ne efficiency motor

ω EE (L) is the operating speed of the energy efficient motor

ω BE (L) is the operating speed of the baseline efficient motor

When the operating speeds are the same then:

Po EE ( L ) = Po BE ( L )

If the more efficient motor has a higher speed then it produces more output power then required by the application:

Po EE ( L ) > Po BE ( L )3 If the only useful power is that generated by the baseline motor ( Po BE ( L ) ), then the

“effective” losses a of the EE motor are:

Losses ( L ) = Pin E E ( L )

− Po BE ( L )

Where:

Pin EE (L) is the input power of the energy efficient motor.

The efficiency of the EE motor is η EE ( L ) and Pin EE ( L ) is:

Pin EE ( L ) = Po EE ( L )

η EE ( L )

And:

Pin EE ( L ) = Po BE ( L )

. ω EE ( L ) 3

ω BE ( L ) 3 . 1

η EE ( L ) Then the “effective” losses of the EE motor are:

Losses ( L ) = Po BE ( L ) ( ω EE ( L ) 3

ω BE ( L ) 3 . 1

η EE ( L ) − 1 ) [Equation 1]

If the end

- user does not adjust for the higher speed of the EE motor, then the losses experienced will be greater than if the operating speeds remain constant.

DOE calculated “effective” losses vs. load tables based on Equation 1 and used these values to estimate the energy use of higher efficiency motors in variable torque applications which would not benefit from higher operating spee ds.

7

- A.3 ASSUMPTIONS TO DETER MINE ENERGY SAVINGS IN VARIABLE

TORQUE APPLICATIONS

No sufficient solid data was found to estimate the share of motors which are negatively impacted by higher operating speeds. DOE therefore considered a scenario described by th e two following main assumptions: (1) the share of motors which are negatively impacted by higher operating speeds, and (2) the actual operating speed of the motor in the field.

7

- A.3.1 Share of motors negatively impacted by higher operating speeds

DOE assumed th at 60 percent of pumps, fans and compressor applications are variable torque applications.

a The “effective” losses experienced are not losses, they include the increased load imposed by increased speeds associated with variable torque applications.4 Of these 60 percent , DOE assumed that all fans and a majority (70 percent) of compressors and pumps would be negatively impacted by higher operating speeds; and t hat 30 percent of compressors and pumps would not be negatively impacted from higher operating speeds as their time of use would decrease as the flow increases with the speed (e.g. a pump filling a reservoir). DOE assumed this revolutions per minute ( RPM ) effect did not impact fire

pump motors.

When choosing to run the life

- cycle cost ( LCC ) spreadsheet based on the “RPM

scenario” the LCC results are based on the “effective” losses for 60 percent of all fans and 42 percent of all compressors and pumps appl ications. This does not account for the share of users who adjust for increased motor speed.

7

- A.4 SENSITIVITY ANALYSIS

The results provided by applying this methodology do not account for motors which are positively impacted for higher operating speeds and re ly on two major assumptions: (1) the share of motors which are negatively impacted by higher operating speeds, and (2) the actual operating speed of the motor in the field. DOE believes the data supporting these assumptions is not sufficiently robust to in corporate this effect in the main analysis and therefore incorporated it as a sensitivity scenario in the LCC spreadsheet.

i P. Pillay. Practical considerations in applying energy efficient motors in the petrochemical industry

. Petroleum and Chemical Industry Conference, 1995. Record of Conference Papers.,

Ind ustry Applications Society 42 nd Annual