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What is the current transformer?
Transformer used with an A.C. ammeter
Transformer used with a D.C. ammeter
Transformer used with an A.C. voltmeter
Transformer used with a D.C. voltmeter
Transformer used with an A.C. ammeter
A Current Transformer (CT) is an instrument transformer used to step down high currents to a standardized, measurable level for A.C. ammeters. It isolates the measuring instrument from high-voltage circuits and ensures safety during monitoring.
A Current Transformer (CT) is an instrument transformer used to step down high currents to a standardized, measurable level for A.C. ammeters. It isolates the measuring instrument from high-voltage circuits and ensures safety during monitoring.
IsтАЛ=IpтАЛ├ЧNsтАЛNpтАЛтАЛ тАФ where NpтАЛ,NsтАЛ are number of turns and IpтАЛ,IsтАЛ are primary/secondary currents
CT┬аRatio=Isecondary(rated)тАЛIprimary(rated)тАЛтАЛ тАФ standard definition of transformer ratio
A CT operates on the principle of electromagnetic induction, similar to a power transformer but designed to maintain a nearly constant ratio between primary and secondary currents. Since it is connected in series with the line, the primary winding carries the full load current, while the secondary winding delivers a proportional current (typically 1A or 5A) to the ammeter.
CT secondary must never be kept open-circuited while primary is energized to prevent high-voltage core saturation.
The secondary current is typically standardized to 5A or 1A.
Used extensively in power grids for protection relays and metering.
The primary winding consists of very few turns, often just a single bar.
Isolation of measuring instruments from high-voltage primary circuits.
Allows the use of low-current standard ammeters for high-current measurement.
Secondary open-circuit leads to dangerously high induced voltages causing core damage.
Phase shift between primary and secondary current causes minor measurement errors.
Electrical energy metering in substations.
Overcurrent protection systems for transmission lines and motors.
Options B and D are incorrect because transformers work on the principle of mutual induction, which requires a time-varying magnetic flux produced by A.C., making them incompatible with pure D.C. circuits.
Option C is incorrect as potential transformers (PT) are used for voltage measurement, not current transformers.
A is correct тАФ A current transformer is specifically designed to step down high primary A.C. currents for measurement by standard low-range A.C. ammeters.
Always remember: Current Transformers are connected in series with the load, while Potential Transformers are connected in parallel.
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