Choice of Size of Apparatus

Author: E.E. Kimberly

Fig. 23-1. Graphical Representation of Cost of Motors as a Function of Size

It obviously is economically unsound to to use a 25-hp motor to drive a machine requiring only 10 hp. Not only is the capital investment unjustified, but at such a small fraction of full load the efficiency of the motor is poor. If it is an induction motor, the power factor at small fractional loads will be low. The same conditions exist in an under-loaded transformer; but, when a growing load is to be supplied, it is sometimes more economical to install transformer capacity in excess of the immediate need instead of later replacing a small unit with a larger one.

Fig. 23-2. Graphic Representation of Cost of Motor Starting Equipment as Function of Size

The curves of Fig. 23-1 show the relative purchase costs per horsepower of several types of motors, and the curves of Fig. 23-2 show the costs of their starting control equipment in a range of sizes from 1 to 75 hp. It is obvious from the curve of polyphase-induction-motor costs, for example, that the cost of 10 hp in 1-hp motors is $39.00X₁0 = $390, whereas the cost of the same horsepower in one 10-hp motor is but $120.00. This difference in cost would suggest group drive by the 10-hp motor driving a line shaft.