25-04-2014, 11:57 AM
Instrument Transformer
Instrument Transformer.ppt (Size: 1.12 MB / Downloads: 117)
Introduction
These are special type of transformers used for the measurement of voltage, current, power and energy. As the name suggests, these transformers are used in conjunction with the relevant instruments such as ammeters, voltmeters, watt meters and energy meters.
Types of Instrument Transformer
Such transformers are of two types :
Current Transformer (or Series Transformer)
Potential Transformer (or Parallel Transformer)
Current transformers are used when the magnitude of AC currents exceeds the safe value of current of measuring instruments.
Potential transformers are used where the voltage of an AC circuit exceeds 750 V as it is not possible to provide adequate insulation on measuring instruments for voltage more than this.
Uses of Instrument Transformer
It is used for the following two as:
To insulate the high voltage circuit from the measuring circuit in order to protect the measuring instruments from burning
To make it possible to measure the high voltage with low range voltmeter and high current with low range ammeter.
These instrument transformers are also used in controlling and protecting circuits, to operate relays, circuit breakers etc. The working of these transformers are similar as that of ordinary transformers.
Advantages of Instrument Transformer
The measuring instruments can be placed for away from the high voltage side by connecting long wires to the instrument transformer. This ensures the safety of instruments as well as the operator.
This instrument transformers can be used to extend the range of measuring instruments like ammeters and voltmeters.
Core type
It is rectangular form core type. The laminations are of L shaped assembled together.
The winding are placed on one of the shorter limbs, with the primary usually wound over the secondary. The main advantage of this type of core is that sufficient space is available for insulation and is suitable for high voltage work.
Ring type
Ring type core is commonly used when primary current is large. The secondary winding is distributed round the ring and the primary winding is a single bar.
It is a joint less core and there is very small leakage reactance.
Burden of Instrument Transformer
The operation of current transformer differs slightly from the power transformer. In case of current transformer, the secondary winding has a very small impedance or “Burden” , so the current transformer operates on short circuit conditions.
The rated burden of this Instrument Transformer is the volt- ampere loading which is permissible without errors exceeding the limits.