Functions of a Current Transformer
Varying the Current of Electricity in AC Power Supplies
Jun 2, 2009
Safe, cost-effective delivery and use of electrical power requires an ability to change the current and voltage transmitted through a circuit. Current transformers perform this function using electromagnetic fields.
Definition of a Current Transformer
A current transformer raises or lowers the current of an AC circuit in a predictable manner dependent on the ratio between the windings. For example, a 100:1 ratio current transformer, 100 amps flowing throw the primary transformer winding will result in a current of 1 amp through the secondary winding. According to Ohm's Law, power is conserved; therefore, the effect of a current transformer on voltage is inverse to its effect on current.
How Does a Current Transformer Work?
A current transformer consists of primary and secondary coils of wire wrapped around a core, usually made of steel or a nickel alloy. Transformer windings are electrically insulated from each other and from the core. The winding connected to the power supply is called the primary winding. The transformer winding in which current is induced is called the secondary winding.
As current passes through the primary winding, an electromagnetic field is produced which induces a current in the electrically isolated secondary winding.The number of coils in a winding determine the amount of current and voltage in the circuit. By altering the coil ratio between the primary and secondary winding, a current transformer can alter the current, either stepping it up or down, while conserving power.
Uses for Current Transformers
Current transformers are used in the AC power supplies of appliances, in ground fault circuit interrupter circuit breakers, and in AC power supply grids.
What are the Functions of a Current Transformer?
A current transformer can function in:
- metering of power to track energy use
- monitoring of current flow through a circuit
- relay of power through an energy grid
- control of the state of a circuit (open or closed) in a ground fault circuit interrupter
- protection of instruments and appliances connected to AC power supplies
Metering Functions of a Current Transformer
Current across the secondary winding of a transformer is directly proportional to the current across the primary winding. Direct measurement of current across the second winding allows an indirect calculation of the current conducted across the primary winding. Measurements may be used for calculating energy usage in power supply billing.
Monitoring Functions of a Current Transformer
Current measurements across the secondary winding may be used to monitor and display the current conducted along a circuit or power line to check for spikes and drops.
Relaying Functions of a Current Transformer
Power grid transformers may be used to increase or decrease the current of the power supply. A step-up transformer increase the current and decrease the voltage of power from an energy plant generator before transmission over long distance power lines. This reduces the costs of transmission by permitting use of smaller power lines with less energy loss. Step-down transformers lower current at the point of use.
Control Functions of a Current Transformer
Measurements from the secondary winding of a current transformer may trigger switches in controllers such as ground fault circuit interrupter (GFCI) circuit breakers when measurements exceed allowed parameters.
Protection Functions of a Current Transformer
Metering, monitoring, and control equipment used with AC power supplies can be damaged by high currents. Transformers can step-down current so that appliance circuitry is protected. For this reason, current transformers are often termed instrument transformers.
Current Transformers Perform Critical Roles in the Use of Electricity
Without current transformers, long distance transmission of electricity would not be cost-effective. Home use of AC power would not be safe. The metering, monitoring, relaying, control and protection functions of current transformers enable the essential role electricity plays in high technology societies.
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