Dear Paul, Regardless of the rating of the motor, it will draw a current from the mains depending on the needs of the load. If you connect a 10 HP or 60 HP motor to the same load (upto 10 HP of course), power consumption will *almost* be the same. Bigger motors have higher efficiency, but efficiency will increase as the load approaches to the rated power, thus you will see a slight less consumed power difference between the two cases. It is usually not understood that capacity and current drawn are different. Hakan Ozevin
Responding to Hakan Ozevin's comments on the subject: Of course there are many variables to consider, but the following illustrates that the comparable losses of both the 10 and 60 hp motors are NOT "almost the same". Neglecting your reasoning for using the larger motor, and assuming you are using Hi-efficiency motors, then: a) The efficiency of a 10 hp motor operating at full load is between 88.5 and 90.2 percent. Then, for an average of 89.4% the loss will be 746(W/hp)x[(10/0.894)-10] or 885 Watts. b) The efficiency of a 60 hp motor operating at 16.7% (10/60) of full load will be about 82 to 84 %. Thus the loss will be 746(W/hp)x[(10/0.830)-10] or 1,530 Watts. This value is 1.73 times that of the 10 hp case. This case neglects the consequence of lower slip, i.e., the incremental increase in shaft speed. In turn the hp will increase, although slightly. c) Furthermore, the loss difference decreases slightly if you consider standard motor efficiencies that were available some 10-15 years ago. 1,379 and 2,228 Watts for the 10 hp and 60 hp motors, respectively. The loss associated with the 60 hp motor is 1.62 times that of the 10 hp motor. Regards, Phil Corso, PE (Boca Raton, FL)
This have mostly to do with how accurate you want "almost" to be. I would say that the power drawn will be "almost" the same for the two motors, becuse the loss in both cases are at least some 5-10 times smaller than the load. Assume a load of 10 hp. that is 7 460 W output The 10 hp motor needs 8 345 W The 60 hp motor needs 8 989 W (using the numbers below) If this is almost the same or not, depends on what you mean with almost. If I interpret the original post right was if the 60 hp motor would need about 6 times the power of the 10 hp motor, if that would have been the case they would not have needed almost the same power by my way of interpreting that word. The notes above are very good for the topic, regardless of what the original poster intended this is the answer. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------
I have already written that: "Bigger motors have higher efficiency, but efficiency will increase as the load approaches to the rated power, thus you will see a slight less consumed power difference between the two cases." What you have calculated is a slight difference compared to the full load. So I could not understand why you don't agree with me. Hakan Ozevin