2 7 8 



INDUCTION MOTORS. 



[Ex*. 



EXPERIMENT 8-B. Predetermination of the Performance of 

 an Induction Motor by Means of the Circle Diagram. 



i. Data. To determine the complete performance of an in- 

 duction motor without loading, two readings with voltmeter, 

 ammeter and wattmeter are required : first, when the motor is run- 

 ning at normal voltage without load; second, when the motor is 

 locked, and is supplied with reduced voltage so that the current 

 is not more than, say, ij4 the normal full-load current. These 

 two tests correspond, respectively, to the open-circuit or core loss 

 test and the short-circuit or copper loss test of a transformer. 



In addition to these two sets of readings, the primary resistance 

 is to be measured, as nearly as possible under normal working 

 temperature conditions. 



2. Example. A test is made of a 5 horse-power, 8-pole, 60 

 cycle, 3-phase motor having a wound secondary (rotor). The 

 synchronous speed is 900 R.P.M. The primary is Y-connected* 

 for a rated line voltage of 190 volts, or no volts from line to 

 neutral. The test is made, however, at a measured line voltage 

 of 185 volts, corresponding to 107 volts from line to neutral. 



The average hot primary resistance, as measured from line to 

 line, is 0.51 ohm; hence, the primary resistance per phase is 

 R : = 0.255 ohm. 



3. No-load Readings. The readingsf are 



* (2a). In testing a 3-phase motor, it is not necessary to know how 

 the primary or secondary is connected; as a matter of fact, it is usually 

 impossible to tell. The results are not affected by assuming it either Y- or 

 delta-connected. The results are here worked up as K-connected. All 

 results may be expressed in terms of equivalent single-phase quantities to 

 advantage ; see 28, Exp. 6-A. 



t(3a). When there is a variation in the readings of current and 

 voltage for the different phases, average readings are taken. The watts 

 are the total watts of all phases, irrespective of whether the phases are 

 alike or not, and are determined preferably by two wattmeter readings. 

 (In using the 2-wattmeter method for testing a 3-phase motor, it is to be 

 remembered that one reading may be negative; see 25, 31, Exp. 6-B.) 



For an investigation of motors on badly unbalanced voltage, see paper 



