Current Transformer - Basic Properties, Terminology and Theory
What is a Current TRANSFORMER?
Current Transformer is used with an ammeter to measure current in AC voltages. Current Transformers are also known as instrument transformers or accurate ratio transformers. They transform at an accurate ratio to allow an attached instrument to gauge the current without actually running full power through the instrument. It is required to transform relatively small amounts of power because its only load, called a burden, is the delicate moving elements of an ammeter.
The transformer that measures the potential difference in AC voltages and uses a voltmeter is known as a potential instrument transformer.A Current Transformer has a primary coil of one or more turns of heavy wire. It is always connected in series in the circuit in
which current is measured. The secondary coil is made up of many turns of fine wire, which must always connect across the ammeter terminals.
The secondary of a current transformer must never be open-circuited. This is because the primary is not connected to a constant source. There is a wide range of possible primary voltages because the device can be connected to many types of conductors. The secondary must always be available (close-circuited) to react with the primary, to prevent the core from becoming completely magnetized. If this happens, the instrument will no longer read accurately.
A clamp-on ammeter works in a similar way. Opening the clamp and placing it around a current carrying conductor, the conductor itself acts as a single turn primary. The secondary and the ammeter are conveniently mounted in the handle of the device. The dial allows a number of current ranges for accurate gauging.
Current Transformers are available in non-ANSI-rated versions. They range from 50-400Hz and in current ratios from 50:5-2000:5 in a variety of window diameter openings. Work with a Current Transformer specialist to determine the best product for your application.