218 ELECTRICAL ENGINEERING 



motor is used but if it is overloaded its speed falls to a value too 

 low for satisfactory operation. 



In generators the regulation is a voltage regulation. As the 

 load is increased the voltage falls off and a point is soon reached 

 where any increase of load will cause so great a decrease in voltage 

 that the power supplied is unsatisfactory. 



(2) The efficiency of a machine increases with increasing load 

 to the point where the variable copper losses are equal to the con- 

 stant losses. Above this point the efficiency decreases due to the 

 rapid increase of the variable losses. 



With properly designed machines the output is limited by either 

 heating or sparking before the regulation or efficiency become too 

 bad. 



(3) All the losses of power in a machine are converted into heat 

 and raise the temperature of the various parts until the point is 

 reached where the rate at which heat is being radiated or carried 

 off by the ventilating apparatus is equal to the rate at which heat 

 is being generated. The temperature will then remain constant. 

 When a machine is overloaded its losses increase and consequently 

 its temperature rises above normal. 



If a machine operates at a high temperature for any length of 

 time permanent injury to the insulating materials will result. 



(4) Sparking will occur in a machine when the field cut by the 

 coil which is being commutated is not strong enough to reverse 

 the current in the time of commutation. Sparking will therefore 

 occur in generators or motors at heavy load when the armature 

 m.m.f. is so great that it wipes out the field under the pole tip or 

 weakens it to such an extent that it cannot produce the required 

 commutating e.m.f. Motors will also spark at high speed since 

 the time of commutation is reduced, especially when the high speed 

 is produced by field weakening. Take for example a shunt motor 

 rated at normal speed as 10 h. p., 110 volts, 80 amperes, 400 

 r. p. m. and suppose the temperature rise to be 50 C. above 

 standard room temperature of 25 C. If the motor is operated 

 at half speed of 200 r. p. m. by reducing the impressed voltage 

 to half, the rating may be taken as 10 h. p., 55 volts, 80 amperes 

 but the temperature rise will be greater than before because 

 the armature copper loss is the same, the field copper loss is the 

 same and the iron and friction losses are less due to the low speed, 

 but the ventilation is only about half as good as before. 



