Induction Voltmeter
- Its construction is similar to that of an
induction ammeter except for the difference
that its winding is wound with a large number of turns of fine wire. - Since it is connected across the lines and carries very small
current (5-10mA), the number of turns of
its wire has to be large in order to produce
an adequate amount of m.m.f. Split phase
windings are obtained by connecting a high
resistance R in series with the winding of
one magnet and an inductive coil in series
with the winding of the other magnet as
shown in Figure
Errors in Induction Instruments
There are two types of errors (i) frequency error and (ii) temperature error.
- Since deflecting torque depends on frequency, hence unless the alternating current to be
measured has same frequency with which the instrument was calibrated, there will be large error in its readings. Frequency errors can be compensated for by the use of a non-inductive
shunt in the case of ammeters. In voltmeters, such errors are not large and, to a great extent,
are self-compensating. - Serious errors may occur due to the variation of temperature because the resistances of eddy
current paths depends on the temperature. Such errors can, however, be compensated for by
hunting in the case of ammeters and by combination of shunt and swamping resistances in
the case of voltmeters.
Advantages and Disadvantages Induction Voltmeter
- A full-scale deflection of over 200° can be obtained with such instruments. Hence, they
have long open scales. - Damping is very efficient.
- They are not much affected by external stray fields.
- Their power consumption is fairly large and cost relatively high.
- They can be used for a.c. measurements only.
- Unless compensated for frequency and temperature variations, serious errors may be introduced.