Switchboard measuring instruments for continuous and polyphase systems

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...coils placed in the other phases. It is easily seen from the theory that these phasemeters can indicate either lag or lead. Let Fig. 67 represent the condition of a circuit possessing a lag, that is, a circuit in which the minimum value of the current occurs at a later time than that of the voltage. If, however, there were a lead between current and voltage, then at the instant considered, the vector O R would be on the opposite side of O M by an angle f, say, or in other words, the forces Fi, F3, F5, F7, would be reversed in direction, and consequently, also the motion of the coil. By a similar line of argument, it can be shown that the unbalanced load instrument also indicates lag or lead. Errors in Balanced-Load Instruments.--I. These instruments, as in the case of all air-core instruments, are greatly affected by stray fields, consequently they must be properly shielded. 2. The generation of eddy currents in metal parts should be avoided, as described in air-core ammeters and voltmeters, as a serious error may be introduced from this source. 3. As seen from the theory, the indications of the instrument depend on the maximum value of the current, it will therefore be inaccurate if used on a circuit whose wave form differs from that on which it was calibrated. 4. An alteration in the frequency will produce a slight error, since it does not affect the fixed and moving systems to the same extent, but for practical purposes this error is so small that it may be neglected. These instruments are independent of current or voltage since any alteration in either will produce a corresponding effect in both sets of coils. Errors in Unbalanced-Load Instruments.--These instruments suffer from the same errors as the balanced-load instruments except that 3 is entirely ...