142 



COMMERCIAL ELECTRICAL TESTING 



around the pulley. The other end is made fast to a spring 

 balance G. A strip bearing a mark is located at K so 

 that when the point of the lever L comes opposite to 

 the mark, the lever is in a horizontal position at an 

 angle of 90 degrees to the force exerted by the pulley. 

 Since the stress along a rope is transmitted through 

 its center, adjust the brake until the points M and TV 

 are a distance apart equal to the diameter of the pulley 

 plus the diameter of the rope, one-half the diameter of 

 the rope being added to each side of the pulley. This 

 adjustment must be accurately made and care taken to 

 see that nothing moves to throw the brake out of line 

 or proper adjustment. When ready, slip the rope off 



Fig. 42. Diagram of Apparatus Used in Taking Input-Output 

 by the String Brake Method 



the pulley but leave it attached at B and G, then balance 

 the lever until the pointer on the end comes to rest at 

 the mark K . This balancing of L must be repeated each 

 time the rope is changed. 



The motor should be run light for at least one hour 

 before the test proper is commenced, so that friction 

 may become constant. Since speed is one of the impor- 

 tant factors in the output of the motor, it should be 

 taken very carefully. The slip should be taken with 

 the slip machine. 



Running light readings should now be taken on the 

 motor, the impressed voltage, as well as the frequency, 

 being held constant. A small weight is next attached to 

 the spring balance to give enough tension on the spring 

 for a reading on the balance of a quarter or half a pound. 



