2-A] SPEED CHARACTERISTICS. 29 



radius or lever arm to which the force is applied. It accordingly 

 follows that: torque is proportional to armature current and to 

 the flux density of the field; this is irrespective of whether the 

 armature is rotating* or not. A reversal of either the current 

 or the flux alone reverses the direction of the torque. 



Of the total torque, part is used in overcoming friction, wind- 

 age and core loss ; the remainder is useful torque and is available 

 at the pulley. 



4. Automatic Increase of Current with Load. The counter- 

 electromotive force E' is always a few per cent, less than the sup- 

 ply voltage E. The difference is due to the resistance drop in 

 the motor armature, including brushes, brush contact and con- 

 nections, and the series field (if any) ; that is 



and speed; thus, if R.P.M. is revolutions per minute and T is torque in 

 pound-feet 



H.P. = 27r X R.P.M. 

 33,000 



If power is known, torque may be found by dividing power by speed. 

 In pound-feet, torque is 



T 31' V H - R 



27T A R.P.M. ' 



When power is in watts, it is frequently convenient to express torque in 

 " synchronous watts "; thus, 



Watts 



T= 



R.P.M. * 



(One synchronous watt = 7.04 pound-feet.) 



(One pound-foot = 0.142 synchronous watt.) 



Torque is also expressed in "watts at 1,000 R.P.M."; thus, 



Watts 

 r= i ; ooo XO;H: . 



*(3c). Torque with the armature at rest (static torque) can be de- 

 termined for various field currents and for various armature currents by 

 means of a lever arm attached to the armature and a spring balance or 

 platform scales. 



