INDUCTION INSTRUMENTS 



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By proper design this term is kept as small as possible in watt- 

 hour meters, and the armature is seldom allowed to make more 

 than one revolution per second. 



Induction Ammeters and Voltmeters. As an illustration of 

 the application of these principles to measuring instruments, the 

 Westinghouse induction ammeters and voltmeters may be 

 taken. 2 - 3 Fig. 260A shows the electrical and magnetic circuits 

 of the ammeter. The line current enters by the terminals TT and 

 flows through the coil P, giving rise to the flux, <J> P , which crosses 

 the air gap in the horizontal direction. The magnetic circuit 

 carries a second winding, S, which with the coils A A' forms a 

 closed circuit. 



FIG. 260. Westinghouse induction ammeter. 



The current induced in this secondary circuit sets up a flux, 

 'Px, which crosses the air gap in the vertical direction. The two 

 fluxes are in the proper space relation to produce a torque on 

 the thin aluminum cup which forms the movable element. They 

 must also have the proper time-phase relation. This is attained 

 by the transformer action of the two windings, P and S. 



Referring to the vector diagram accompanying Fig. 260A, the 

 flux $>p links both P and S. It is therefore the mutual flux and 

 induces in the winding S an e.m.f., E s , which lags 90. The 

 flux, 3>. s , threads the secondary winding, S, but not the primary 

 winding, P, consequently it is the secondary leakage flux; it will 



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