Instrument Transformers
The d.c. circuits when large currents are to be measured, it is usual to use low-range ammeters with suitable shunts. For measuring high voltages, low-range voltmeters are used with high resistances connected in series with them. But it is neither convenient nor practical to use this method with alternating current and voltage instruments. For this purpose, specially constructed accurate ratio instrument transformers are employed in conjunction with standard low-range a.c. instruments.
The purpose of Instrument Transformers is to reduce the line current or supply voltage to a value small enough to be easily measured with meters of moderates size and capacity. In other words, they are used for extending the range of a.c. ammeters and voltmeters. Instruments transformers are of two types :
(i) Current Transformers (CT) —for measuring large alternating current.
(ii) Potential Transformers (VT) —for measuring high alternating voltages.
Advantages of using instrument transformers for range extension of a.c. meters are as follows :
(1) The instrument is insulated from the line voltage, hence it can be grounded.
(2) The cost of the instrument (or meter) together with the instrument transformer is less than that of the instrument alone if it were to be insulated for high voltages.
(3) It is possible to achieve standardisation of instruments and meters at secondary ratings of 100–120 volts and 5 or 1 amperes
(4) If necessary, several instruments can be operated from a single transformer and
(5) Power consumed in the measuring circuits is low.
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In using instrument transformers for current (or voltage) measurements, we must know the ratio of primary current (or voltage) to the secondary current (or voltage). These ratios give us the multiplying factor for finding the primary values from the instrument readings on the secondary side.
However, for energy or power measurements, it is essential to know not only the transformation ratio but also the phase angle between the primary and secondary currents (or voltages) because it necessitates further correction to the meter reading.
For range extension on a.c. circuits, instrument transformers are more desirable than shunts (for current) and multipliers (for voltage measurements) for the following reasons :
1. Time constant of the shunt must closely match the time constant of the instrument. Hence, a different shunt is needed for each instrument.
2. Range extension is limited by the current-carrying capacity of the shunt i.e. upto a few hundred amperes at the most.
3. If current is at high voltage, instrument insulation becomes a very difficult problem.
4. Use of multipliers above 1000 becomes almost impracticable.
5. Insulation of multipliers against leakage current and reduction of their distributed capacitance becomes not only more difficult but expensive above a few thousand volts.
Read article – alternating current
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