For all problems assume the cost of electrical demand is $14 /kW-mo, the cost of electrical energy is
$0.02 /kWh and the power factor charge is $0.60 /kVAr-month.
1) The measured current, voltage and power factor of a premium-efficient, three-phase, 125-hp air compressor motor are shown in the table below. Assume motor is 95.3% efficient at all loads.
Loaded Unloaded
Current (Amps) 127 63
Voltage (Volts) 476 476
Power Factor (kW/kVA) .92 .81
A) Calculate the power draw of the motor in kW when loaded and unloaded.
B) Calculate the power output of the motor in hp when loaded and unloaded.
C) Calculate the fraction loaded of the motor when loaded and unloaded.
D) If the service factor is 1.05, is the motor in danger of premature failure?
E) Calculate the annual cost of electrical demand and electrical energy use if the motor cycles between running loaded for 40 seconds then unloaded for 60 seconds, and operates 5,000 hours per year.
2) A 93%-efficient 100-hp motor runs 6,000 hours per year and produces power for a fan that requires 75 hp of shaft work.
A) Calculate the annual operating cost of the motor if the pulleys use 92%-efficient standard V-belts
B) Calculate the annual operating cost of the motor if the pulleys use 96%-efficient notched V-belts.
C) Calculate the annual electricity cost savings from switching from standard to notched V-belts.
3) Calculate the simple paybacks of replacing a failed 75%-loaded, 15-hp, standard-efficiency motor with an energy-efficient motor rather than rewinding it, if the motor runs 2,000, 5,000 and 8,000 hours per year.
4) A 20-hp motor runs 5,000 hours per year, produces 4 hp of shaft work, and operates at 50% efficiency with a power factor of 60%.
A) Calculate the power (kW), energy (kWh) and reactive power (kVAr) produced by the motor.
B) If motor were replaced with an energy efficient 5-hp motor operating at 90% efficiency with a power factor of 85%, calculate the power (kW), energy (kWh) and reactive power (kVAr) produced by the "right-sized" motor.
C) Calculate the power, energy, power factor and total cost savings from rightsizing the motor.
D) Using MM+ data (but not the software!) for the cost of the new motor, calculate the simple payback for right-sizing the oversized motor.