FINISHING AND TESTING 



335 



transformer suitable for this is shown in Fig. 416. The top half 

 of the transformer contains the exciting coil supplied with alter- 

 nating current, which induces E.M.F. in the form-wound coil under 

 test when in the position shown. 



Location of Faults in Armatures. A faulty coil in a finished 

 armature may be readily detected by placing the armature on a 

 transformer such as that shown in Fig. 417, the principle of which 

 is the same as the one in Fig. 416. For stator armatures a similar 

 transformer is shown in Fig. 418. In this, one will note the 

 difference in diameter of the upper and lower surfaces enabling 



FIG. 417. Testing Transformer for 

 Wound Rotating Armatures. 



FIG. 418. Testing Transformer for 

 Wound Stators. 



the transformer to be used for armatures of very different internal 

 diameter. 



A method suitable for polyphase motors is the following. 

 If there is a fault in the stator winding, replace the rotor by an 

 unwound rotor ; then with the phases independent connect one 

 phase to the alternating current supply and measure the voltage 

 across each coil of the other phase, when the faulty coil will show 

 a low reading. This can be done on each phase in succession 

 until the faulty coils are located. If the fault is in the rotor 

 winding, if coil wound, a similar test can be made. 



