POWER LOSSES IN GENERATORS AND MOTORS. 143 



load, and such a motor is therefore designed to give its maximum 

 efficiency at about ^ full rated load, as shown in Fig. 94. When 

 a motor is to be used most of the time at full load, it is always 

 designed to give its maximum efficiency at or near full load. 



The maximum efficiency of a shunt motor (in which the stray 

 power loss is approximately constant) occurs for that particular 

 load for which the armature loss, RJ*> is equal to the constant 

 loss, namely stray power loss plus field loss. The proof of this 

 proposition is very similar to the proof of the corresponding 

 proposition concerning the efficiency of a shunt generator (see 

 article 67). 



The efficiency of a motor depends also upon its size. Thus a 

 very small motor of ^-horse power may have an efficiency under 

 the best conditions of about 60 per cent, while a 20- or 3O-horse 

 power motor may have an efficiency of 90 per cent, or more. 



71. The use of the separately excited motor as a dynamometer. 



One of the most convenient methods for determining the 

 amount of mechanical power required to drive a machine of any 

 kind, is to drive the given machine by a separately excited motor, 

 measure the electrical input of power to the armature of the 

 motor, and subtract from this observed input the stray power 

 loss and the armature loss in the motor, thus finding the net 

 mechanical power delivered to the given machine. 



Example. The power required to drive a certain ventilating 

 fan is to be determined. With this end in view, the fan is belted 

 to a shunt motor. The field winding of the motor is connected 

 to the i lo-volt supply mains. The armature of the motor is also 

 connected to the supply mains with an ammeter and a rheostat in 

 series with it. The rheostat is adjusted until the fan is driven at 

 the desired speed when the armature current is observed to be 

 16.4 amperes, and the electromotive force between the brushes, as 

 measured by a voltmeter, is observed to be 92. 5 volts. 



The belt is then thrown off, a much larger resistance is placed 

 in series with the armature, and this resistance is adjusted until 



