FIGURING THE HORSEPOWER RATING OF A MOTOR

motor or a steam or gas engine is with the use of the prony brake. Figure 8-3 shows you the prony brake setup. A pulley wheel is attached to the shaft of the motor and a leather belt is held firmly against the pulley. Attached to the two ends of the belts are spring scales. When the motor is standing still, each scale reads the same 15 points. When the pulley turns in a clockwise direction, the friction between the belt and the pulley makes the belt try to move with the pulley. Therefore, the pull on scale A will be greater than 15 pounds, and the pull on scale B will be less than 15 pounds.

Suppose that scale A reads 25 pounds and scale B reads 5 pounds. That tells you the drag, or the force against which the motor is working, is 25 5 = 20 pounds. In this case the normal speed of the motor is 1,800 revolutions per minute (rpm) and the diameter of the pulley is 1 foot.

You can find the number of revolutions by holding the revolution counter (fig. 8-3, C) against the end of the shaft for 1 minute. This counter will record the number of turns the shaft makes per minute. The distance (D) that any point on the pulley travels in 1 minute is equal to the circumference of the pulley times the number of revolutions or 3.14 x 1 x 1,800 = 5,652 ft.

Change this to horsepower:

Two common motor or engine ratings with which you are familiar are the 1/16-hp motor of an electric mixer and the 1/4-hp motor of a washing machine.

Remember two important points about power:

Power is the rate at which work is done.

Horsepower is the unit of measurement by which power is equivalent to 33,000 foot-pounds of work per minute, or 550 foot-pounds per second.