115. Experimental data required for construction of circle diagram. Determination 

 of hysteresis loss 116. Short-circuit test 117. Construction of torque circle 

 and slip line. Scales for torque and slip 118. Applications of circle diagram 

 119. Brake tests. Soames brake 120. Air friction dynamometer 121. 

 Eddy-current brakes 122. Measurement of slip. Stroboscopic methods 123. 

 Methods applicable to slip-ring rotors 124. Measurement of friction loss 125. 

 Efficiency tests. Methods of Meunier, Sumpner, and Weekes 126. Alexanderson's 

 method 127. Heating test 128. Induction motors with large starting torque. 



115. Experimental Data required for Construc= 

 tion of Circle Diagram. Determination of 

 Hysteresis Loss 



THE circle diagram furnishes us with an easy method of completely 

 investigating the properties of an induction motor, without requiring 

 any very elaborate tests. In order to be able to construct the 

 diagram, we have to carry out the following measurements. 



The motor is supplied at the normal p.d. and frequency, and is 

 allowed to run light. The current I and total power W are care- 

 fully measured. If V ? = normal line p.d., then phase p.d. for a 



three-phase star winding * = = V ( 16). The power per phase 



v3 



is pVo, and hence the power factor at no load is 



cos o = 



(1) 



We may now commence the construction of our diagram by draw- 

 ing a horizontal line O'V (Fig. 131), and from 0' drawing a line 

 OT = I , making an angle with O'V, being given by (1) ; P 

 will obviously be a point on the circle. 



The next step consists in determining the position of the point O 

 in Fig. 129. Since O'O is that part of the power component of the 

 current which corresponds to the stator core loss, we must measure 

 this loss. Now, when the motor is running light, the power taken 



* In what follows, we suppose that we are dealing with a three-phase motor ; the 

 necessary modifications in the formulae for a two-phase motor are quite obvious. 



