8-A] OPERATION AND LOAD TEST. 26,9 



stant while readings are taken ; furthermore, except in the case 

 of small motors, there is difficulty in disposing of the heat. In 

 place of a Prony brake any form of transmission or absorption 

 dynamometer can be used (see la, Exp. 2-B). 



A convenient method for conducting the test is to use a direct 

 current generator for a load, and this method will be described 

 in detail. 



35. Load Run with Direct Current Generator as a Load. 

 To load the motor, belt it to a shunt generator, preferably with 

 constant separate excitation. The load of the motor is varied by 

 varying the load of the generator and this is done by means of 

 external load resistances. The motor input is measured electric- 

 ally. The motor output is equal to the electrical output (El) of 

 the generator plus the generator and belt losses, which are sepa- 

 rately determined as described later. 



36. Measurements on Motor. For each load, read line volt- 

 age, line current,* watts input, frequency (unless this is constant 

 and known) and speed ; the slip should be measured directly when 

 means are available. The line voltage should, if possible, be kept 

 constant. 



37. For a single-phase motor, the electrical instruments to be 

 read consist of one voltmeter, one ammeter and one wattmeter. 



38. For a polyphase motor, an ammeter and voltmeter may 

 be used on each phase; or, one ammeter and one voltmeter may 

 be shifted from phase to phase by means of suitable switches. 

 Power and power factorf can be determined by any of the watt- 

 meter methods described in Exp. 6-B. The 2-wattmeter method 

 shown in Fig. i, Exp. 6-B, is the best one to use in testing a 

 3-phase motor, or a 3-wire 2-phase motor. 



39. The results, on a polyphase motor, may be worked up in 

 terms of (i) line current and line voltage, as measured; (2) 



* If the secondary is phase-wound, read also secondary current. 

 fFor a 2-phase motor, determine power factor by both the cosine and 

 the tangent methods, 11, 12, Exp. 6-B. 



