In heavy currents and high voltage a.c circuits, the measurement can not be made using the range extension method of low range meters providing adequate leads. Under such conditions, specially constructed precise ratio transformers called instrument transformers. These can be used, independently of the voltage and current ratings of the a.c. These transformers not only extend the range of low-range instruments, but also isolate them from high current and high voltage a.c. circuits. This makes its handling very safe. These are generally classified as
(i) Current transformers and (ii) potential transformers.
Current Transformers (C.T.):
The large alternating currents which cannot be sensed or passed through normal ammeters and current coils of wattmeters, energy meters can easily he measured by use of current transformers along with normal low range instruments.
A transformer is a device which consists of two windings called primary and secondary. It transfers energy from one side to another with suitable change in the level of current or voltage. A current transformer basically has a primary coil of one or more turns of heavy cross-sectional areas. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current.
The secondary of the transformer is made up of a large number of turns of fine wires having small cross-sectional area. This is usually rated for 5A. This is connected to the coil of normal range ammeter. Symbolic representation of a current transformer is shown in the fig..
Potential Transformers (P.T.):
The basic principle of these transformers is same as current transformers. The high alternating voltages are reduced in a fixed proportion for the measurement purpose with the help of potential transformers. The construction of these transformers is similar to the normal transformer. These are extremely accurate ratio step down transformers. The windings are low power rating windings. Primary winding consists of large number of turns while secondary has less number of turns and usually rated for 110 V, irrespective of the primary voltage rating. The primary is connected across the high voltage line while secondary is connected to the low range voltmeter coil. One end of the secondary is always grounded for safety purpose. The connections are shown in Fig..
As a normal transformer, its ratio can be specified as,
So if voltage ratio of PT is known and the voltmeter reading is known then the high voltage to be measured, can be determined
Advantages of Instrument Transformer:
– Their reading do not depend upon circuit constant such as R, L & C
• As in the case of shunts and multipliers As in the case of shunts and multipliers
– Possible to standardize the instrument around their ratings.
• This makes the replacement of instrument transformer very easy.
– The measuring circuit is isolated from the power circuit
– Low power consumption in the metering circuit
– Several instrument can be operated from a single instrument transformer