144 COMMERCIAL ELECTRICAL TESTING 



5 = speed in revolutions per minute. 



watts 

 Power-factor = r ^ 



volts and amps. 



2irR 

 Then horse-power = 



PX12XSX33,000 



horse-power output X 746 

 Efficiency = - 



watts input 



Considering Fig. 43, let M be the motor and L the load 

 machine, the latter being a direct current machine of 

 about the same capacity as the motor, belted to 

 the motor and separately excited from a suitable 

 source. 



Ammeter 



To Measuring 



ine 



r To Water Resistance 

 or suit able Motor 



Source of Current. 



Fig. 43. Connections for Measuring Input-Output by 

 Pumping Back 



To make the efficiency test, connect M so that the 

 total input can be obtained. The necessary connections 

 are not given, as they vary widely, depending on whether 

 M is a one, two or three-phase machine. Separately 

 excite the field of L and connect an ammeter and a 

 variable resistance in circuit. Connect the armature 

 of L to a water-box or to a motor, the load of which 

 can be varied. An ammeter should be placed in the 

 circuit and a voltmeter across the brush terminals. If 

 the test involves a considerable range of speeds, run M 

 over that range and hold the field current of L constant, 

 its value being such that the speeds or loads required 

 for M can be obtained. 



Having made the necessary connections, etc., keep the 

 field current of L constant at its predetermined value. 



