DRIVES AND CONTROLS 



211 



ANALYSIS OF MOTOR LOAD FOR TORQUE REQUIREMENTS 

 Starting Torque and Time Required to Start the Machine 



Condition at stoirHng 



Rg rs = Scarfing . 



torque of A^'S of 



motor I "^°^°r 



I I shaft \ 



^•*^r^-m- P >: 



in.ff: 



P in lbs. 



Pp =(/-/o/d/ng torque ofsfatic/oad 

 on driven macf>.)^ (holding fricfional 

 torque of machine parts) 



To start mach. Rsrs must be greater than Pp 

 fhe excess being available for accelerating 

 the machine 



To start a machine, the motor torque must overcome all frictional resistances of 

 bearings, sliding parts, and transmission elements, and also the resistance of any con- 

 nected load. Where the load is not imposed until the machine has come up to working 

 speed, the load resistance is zero. However, machines such as compressors, piston 

 pumps, and hoists without unloading devices may be required to start under full load. 

 With machines of these types, the resistance should be determined for the point of 

 maximum starting torque in the machine cycle. 



The motor torque delivered in excess of that required to overcome running fric- 

 tion at start plus starting load on the machine is used in bringing the machine up 

 to speed. 



Accelerating Torque and Time Required to Bring Machine Up to Speed 



-Linear 

 acceler- 

 ation per 

 sec. 



Pad. of aeration 

 ofmacn. parts 



Equivalent mass of ro- 

 tating driven mach. parts 



^ .---Torque to accel- 

 • ' 1^ > erate load on 

 machine 



Rg r^ = Accelerating torque delivered by motor 

 To accelerate mach. Rq fc must be greater than 

 algebraic sum of combined torques resisting 

 acceleration in machine 



Condition when 

 accelerating 



Torque to overcome 

 frictional forces 



Axis of 4. 



motor' 



shaft 



The amount of torque needed to accelerate the machine and the rate at which it 

 should be dehvered by the motor mil depend upon the moments of inertia of the 

 masses contained in the moving parts and their radii of gyration about or -Rath refer- 

 ence to the motor axis. Flywheel members added to make the load on the motor 

 uniform increase the WW- of the machine and, consequently, increase the accelerating 

 torque which must be dehvered by the motor. (For a discussion of these factors, see 

 page 208.) Other factors that determine the torque needed are loads on machine 

 that must be accelerated before full speed is attained. The time allowed for acceler- 

 ation is an important factor in determining the heat developed in the motor windings. 



