140 COMMERCIAL ELECTRICAL TESTING 



heating. On large machines the rotor winding some- 

 times becomes quite hot, so that the temperature of 

 the end rings and bars of the winding should be taken 

 and recorded. 



Starting Resistance 



The form L motor has a starting resistance in the 

 rotor which in the smaller sizes is controlled by means 

 of a rod sliding within the shaft and in the larger machines . 

 by a lever and ratchet combination. The resistance 

 of the different starting steps must be measured. 



The rod should be pulled out to the full extent by 

 means of the knob handle, thus putting all the resistance 

 in circuit. The rod is then divided into five equal parts 

 and from the impedance test the voltage that will 

 give about 125 per cent normal amperes when the rod 

 is in the running position (resistance all cut out) is found. 

 Apply this voltage, and with the rod in the first position 

 read volts and amperes stator. Similar readings should 

 be taken on each of the five different steps marked on 

 the rod. The same procedure holds good for the larger 

 machines, where the resistance is cut out step by step. 

 These readings, with the resistance in circuit, must be 

 taken as quickly as possible, otherwise the resistance 

 becomes unduly heated and may be injured. 



Table XVIII shows the form used in calculating 

 stationary torque on induction motors. 



Efficiency 



Input-output efficiency and power-factor tests can 

 be made by either the "string brake" or "pumping 

 back" methods. Neither of these methods are particu- 

 larly accurate nor are they recommended. 



In the "brake method" the size of the brake limits 

 the size of the motor tested. In Fig. 42, L is a lever or 

 scale beam suspended at the point X. From T the small 

 platform A is suspended, on which calibrated weights 

 are placed. P is a flat faced pulley on the shaft of the 

 motor, running in the direction shown by the arrow; 

 i.e., toward the lever L. One end of a small rope is 

 attached at B, which is wound one or more times 



