THE INDUCTION MOTOR. 389 



on the load, they may be taken as being constant for any 

 motor. 



From the consideration just given we see that the 

 friction and iron losses are practically constant at all 

 loads, while the copper losses are very small (not more 

 than 1-2 per cent, and due to the magnetising current in 

 the stator) at no load, but increase in proportion to the 

 square of the current taken by the motor. 



If we group together the useful output of the motor and 

 the power spent in overcoming friction and iron losses, and 

 denote their sum by W t , calling the power supplied to the 

 motor W s , we may write 



W A = W t + SvX + 3c 2 2 r 2 . 



If there were no copper losses, we should have 

 W W 



r ' a rr ti 



and the curve showing the relation between total watts out- 

 put and watts supplied would be a straight line passing 

 through zero and inclined at an angle of 45 to the hori- 

 zontal, if the same horizontal and vertical scales are 

 chosen. The effect of the copper losses is to make the 

 watts supplied increase more rapidly at higher loads, the 

 increased power being proportional to the square of the 

 load, so that the line will bend upwards in a curve. 



Keferring to Fig. 185, we see that the curve of watts 

 agrees with the statements just made exactly. The 

 line does not appear to pass through zero, because the hori- 

 zontal scale is useful output only. If the curve were pro- 

 longed backwards to meet the horizontal axis, the distance 

 to the left of the vertical axis would measure the power 

 spent in overcoming friction and iron losses at no load 

 This distance is the same as the height of the point where 

 the line cuts the vertical axis, since the line is inclined at 

 45 deg., as may be seen,. In the present case, therefore, 

 the iron and friction losses may be taken to be 270 

 watts.* 



By drawing a tangent to the watt curve inclined at 

 45 deg. we might measure the power spent in copper 

 losses at any load by determining the vertical distance 

 between this tangent and the curve. 



The curve of efficiency is of the usual shape. If the 



* These losses are not accurately constant at all loads, but sufficiently 

 nearly so for most practical purposes. 



