3. Revolutions per minute.
Example.—If the armature pull of a motor having a two foot pulley, be such that a weight of 500 lbs. attached to the rim, is just balanced, and the speed be 1,000 revolutions per minute, what is the horse power?
Here, the distance that the pull acts from the center of the shaft is one foot, hence for each revolution the resistance of 500 pounds is overcome through a distance equal to the circumference of the pulley or
π × diameter = 3.1416 x 2 = 6.2832 feet.
Fig. 427.—General Electric type CQ Motor. These motors range in capacity from 1/6 to 20 horse power. The small sizes are bipolar, and the larger sizes have four poles. For installations where the motor is exposed to dust, mechanical injury or moisture, it may be partially or entirely enclosed by means of hand hole covers. The standard voltages are 115, 230 and 550.
The work done in one minute is expressed by the following equation:
| { | work per minute | } | = | { | weight in lbs | } | × | { | circumference of pulley in feet | } | × | { | revolutions per minute | } | = foot pounds |
| = | 500 | × | 6.2832 | × | 1,000 | =3,141,600 |
Hence, the power developed is
3,141,600 ÷ 33,000 = 95.2 horse power.
Ques. What is "brake" horse power?
Ans. The net horse power developed by a machine at its shaft or pulley; so called because a form of brake is applied to the pulley to determine the power.