Pump Handbook

Motor Terms

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AMPERE: a unit of intensity of electric current being produced in a conductor by the applied voltage. FREQUENCY: the number of complete cycles per second of alternating current; e.g., 60 Hertz. HORSEPOWER: the rate which work is done. It is the result of the work done (stated in pounds) divided by the time involved. INERTIA: the property of physical matter to remain at rest unless acted on by some external force. Inertia usually concerns the driven load. MOTOR EFFICIENCY: a measure of how effectively the motor turns electrical energy into mechanical energy. Motor efficiency is never 100% and is normally in the neighborhood of 85%. POWER FACTOR: the ratio of the true power to the volt-amperes in an alternating current circuit or apparatus. SERVICE FACTOR: a safety factor in some motors which allows the motor, when necessary, to deliver greater than rated horsepower. SYNCHRONOUS SPEED & SLIP: the speed of an a-c motor at which the motor would operate if the rotor turned at the exact speed of the rotating magnetic field. However, in a-c induction motors, the rotor actually turns slightly slower. This difference is defined as slip and is expressed in percent of synchronous speed. Most induction motors have a slip of 1-3%. TORQUE: that force which tends to produce torsion or rotation. In motors, it is considered to be the amount of force produced to turn the load. It is measured in lb.-ft. VOLTAGE: a unit of electro-motive force. It is a force which, when applied to a conductor, will produce a current in the conductor. APPROXIMATE RULES OF THUMB MECHANICAL FORMULAS At 1800 rpm, a motor develops 3 lb-ft per hp. At 230 volts, a 3-phase motor draws 2.5 amp per hp. Torque in lb-ft = HP x 5250/RPM At 1200 rpm, a motor develops 4.5 lb-ft per hp. At 230 volts, a single-phase motor draws 5 amp per hp. HP = Torque x RPM/5250 At 575 volts, a 3-phase motor draws 1 amp per hp. At 115 volts, a single-phase motor draws 10 amp per hp. RPM = 120 x Frequency/No. of Poles At 460 volts, a 3-phase motor draws 1.25 amp per hp.     Average Efficiencies and Power Factors of Electric Motors kW Efficiency % Power Factor Full Load Amps on 3ph 415V Full Load 3/4 Load 1/2 Load Full Load 3/4 Load 1/2 Load 0.75 74 73 69 0.72 0.65 0.53 2.0 1.5 79 78.5 76 0.83 0.78 0.69 3.2 3 82.5 82 80.5 0.85 0.80 0.73 6.0 5.5 84.5 84.5 83.5 0.87 0.82 0.75 10.5 7.5 85.5 85.5 84.5 0.87 0.83 0.76 14 11 87 87 85.5 0.88 0.84 0.77 20 18.5 88.5 88.5 87 0.89 0.85 0.79 33 30 90 89.5 88 0.89 0.86 0.80 52 45 91 90.5 89 0.89 0.86 0.80 77 75 92 91.5 90 0.90 0.87 0.81 126 Required Value Direct Current Single Phases Two-Phase 4-Wire Three Phase HP Output I x E x Eff/746 I x E x Eff x PF/748 I x E x Eff x PF/746 I x E x 1.73 x Eff x PF/746

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