For measuring high values of current or voltage, it is desirable to use standard low-range measuring instruments together with specially-constructed Instrument Transformers, also called Accurate Ratio Transformers.
There are two types of instrument transformers:
• Current - Used with an ammeter to measure current in AC voltages.
• Potential - Used with a voltmeter to measure voltage (potential difference) in AC voltages.
Current 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 to be measured. The secondary coil is made up of many turns of fine wire, which must always be connected across the ammeter terminals.
A clamp-on ammeter works in a similar way. By 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 to be gauged accurately.
wiring coneection of clam meter as below
Potential Transformer
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An accurate ratio transformer does just as the name suggests. It transforms at an accurate ratio to allow an attached instrument to gauge the current or voltage 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, Voltmeter or Wattmeter.
There are two types of instrument transformers:
• Current - Used with an ammeter to measure current in AC voltages.
• Potential - Used with a voltmeter to measure voltage (potential difference) in AC voltages.
Current 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 to be measured. The secondary coil is made up of many turns of fine wire, which must always be connected 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 (closed-circuited) to react with the primary, to prevent the core from becoming completely magnetized. If this happens, the instrument will no longer read accurately.
wiring coneection of clam meter as below
Potential Transformer
A Potential Transformer is a carefully designed, extremely accurate step-down transformer. It is normally used with a standard 120-volt voltmeter. By multiplying the reading on the voltmeter (called the Deflection) by the ratio of transformation, the user can determine the voltage on the high side. Common transformation ratios are 10:1, 20:1, 40:1, 80:1, 100:1, 120:1, and even higher.
Wiring Diagram of a Potential Transformer
In general, a potential transformer is very similar to a standard two-winding transformer, except that it handles a very small amount of power. Transformers for this service are always of the shell type because this construction has been proven to provide better accuracy.
For safety, the secondary circuit is extremely well-insulated from the high-voltage primary. It is also grounded. This protects the operator from a shock hazard, in case of accidental contact with the wiring.In general, a potential transformer is very similar to a standard two-winding transformer, except that it handles a very small amount of power. Transformers for this service are always of the shell type because this construction has been proven to provide better accuracy.
3 comments:
Hello,
Interesting blog. It would be great if you can provide more details about it. Thanks you.
Current Sensor
The measurement of very large electrical quantities, it can be made possible by using the Instrument transformers with these small rating measuring instruments.
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