212 



HANDBOOK OF MECHANICAL DESIGN 



Running Torque over Time Interval Required by Local Cycles on Machine 



Condition when running 

 Axis of mofor shaft , 



Running 

 forque fo 

 \ \ldo work 

 \\ a flood 



t. 



Running forque fomatce 

 up for fricfional and in - 

 herenfmacfi. losses 



Rp ff = Running torque of motor 



To keep mach. running R,- r,. must be greater 



than algebraic sum of combined resisting 

 running torques in mach. Limiting value of 

 Rf r^ h motor pull-out torque 



When operating at rated speed, the torque supplied by the motor is that required 

 to do useful work and to make up for frictional and inlierent machine losses. 



In calculating the running torque required to keep the machine operating, it is 

 desirable to add something on the safe side to take care of unexpected loads and circuit 

 variations. It is poor practice to plan to use the excess torque that a motor can deliver 

 over its nominal rating, because such overloads cause a rise in winding temperature 

 with consequent depreciation in insulation properties and shortening of motor hfe. 



Work Load Applied After Motor Is Running 



Clufch /ever - 

 Mofor 



-n 



Counfershaff 

 drive for 

 machine ^ 



Time 



With a disengaged clutch or unloading device between motor and machine, the 

 conditions at starting favor the motor since it is then free to start and to come up to 

 speed against little resistance. 



When clutch is engaged, the machine load imposed on the running motor may be 

 applied almost instantaneously if the clutch is of the jaw or the magnetic types, or 

 the load may be apphed to the running motor gradually over a short time range if the 

 clutch is of the frictional or the spring-separated plate type that permits shpping. 



However, the abOity of the running motor to start and accelerate the driven 

 machine when the clutch is engaged is limited by the torque- value at which the motor 

 will stall, usually called the break-down or pull-out torque. 



If applying the machine load slows the motor, an accelerating torque will be 

 required of the motor to bring the machine up to the desired speed. Thereafter, the 

 machine load will determine the running torque required of motor. 



