An elementary book on electricity and magnetism and their applications . g torque, but they do not regulate so well as do shuntmotors. Cumulatively compound motors are also often usedto drive ventilating fans, and are suitable for driving suchmachines as rock crushers, which may have to be started upfull of rock. 173. Efficiency of the electric motor. One reason for theextensive use of electric motors is their great efficiency, which is sometimes as highfp9 as ninety or ninety-live per cent. 1 heefficiency of a mo-tor, just as of any~Pulley machine, means the Fig. 174. — Prony brake used to me
An elementary book on electricity and magnetism and their applications . g torque, but they do not regulate so well as do shuntmotors. Cumulatively compound motors are also often usedto drive ventilating fans, and are suitable for driving suchmachines as rock crushers, which may have to be started upfull of rock. 173. Efficiency of the electric motor. One reason for theextensive use of electric motors is their great efficiency, which is sometimes as highfp9 as ninety or ninety-live per cent. 1 heefficiency of a mo-tor, just as of any~Pulley machine, means the Fig. 174. — Prony brake used to measure output of raH0 0f output to in-a motor. put. We can easilymeasure the number of amperes and the number of volts sup-plied to the motor and thus compute the watts put in. To get the output of mechanical work, engineers usuallymake a brake test. One simple form of brake is the Pronybrake illustrated in figure 174. It consists of a steel band linedwith canvas which embraces the pulley of the motor and isfastened to a lever; the other end rests on platform DIRECT-CURRENT MOTORS 251 When the motor revolves, friction is developed between thelining of the brake and the pulley, which thus converts thepower of the motor into heat. The brake pressure is controlledby a small handwheel, and thus any desired load is obtained. From the force (F) exerted at the end of the lever and length of thearm (r), we may compute the turning moment or torque (T). Torque = Force X = Fr. This torque gives us the equivalent force acting at 1 foot radius. Inone revolution, then, the work done (foot pounds) is equal to theproduct of the distance (the circumference of a circle 1 foot in radius)and the force (the torque T), and in one minute, if the motor makesN revolutions, the work done is equal to 2wNT foot pounds. Sinceone horse power is equal to 33,000 foot pounds per minute, the generalequation for computing the horse power from the Prony-brake testbecomes: 2*NFr 33,000 Fina
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