A meter having a high torque obtained by using a thick and consequently heavy disc, often has a lower ratio of torque to weight than another with lower torque, and is consequently likely to be less accurate over a given range. Furthermore, the heavy disc is a distinct disadvantage because it produces more wear on the bearings and thus reduces the life.
Figs. 2,554 and 2,555. Connections of Sangamo single phase induction meter. Fig. 2,554, 2 wire meter, 5-100 ampere capacity; fig. 2,555, 3 wire meter, 5-100 ampere capacity.
7. The Power Factor Adjustment. This adjustment is necessary to make the phase angle between the shunt and series field components 90° with unity power factor in the metered circuit. Owing to the resistance and iron loss in the shunt field circuit, that field is not shifted quite 90° with respect to the voltage. Yet exact quadrature is necessary to make the strength of the resultant field, and consequently the rotor speed, proportional to the power factor, as explained in the discussion of the field producing element.
Ques. What is the usual construction of the power factor adjustment?
Ans. It usually consists of a short circuited loop enclosing part or all of the shunt field flux.
Ques. How does this loop act?
Ans. It acts like the secondary of a transformer.
The flux induces a current in it which, acting with the current in the shunt coil, produces a slightly lagging field. By shifting the position of the resistance of the short circuited loop, the lag may be so adjusted that the shunt field flux is in exact quadrature with the voltage. It should be noted, however, that this adjustment makes the meter correct at or near one frequency only. This feature is not objectionable if reasonable accuracy be maintained within the limits of normal variation of frequency.