Butler Winding designs and produces current transformers. A current transformer (CT) is a measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers... can safely isolate measurement and control circuitry from the high voltages typically present on the circuit being measured.
are often constructed by passing a single primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. may be designed to connect directly to an ac ammeter or a voltmeter using a burden resistor across its terminals.	Current Transformers Design Specifications Current Transformers Theory of Operation Split Core Current Transformers

Current transformers are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages.

Design specifications and upper limits of current transformers:

• The CT is typically described by its current ratio from primary to secondary. For example, a 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes.
• The secondary winding can be single ratio or have several tap points to provide a range of ratios.
• The Volt-Ampere, (VA) rating is needed as a design specification.
• Care must be taken that the secondary winding is not disconnected from its load while current flows in the primary, as this will produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer.
• Upper limits: 40 pounds of weight and 2 kilowatts of power.
• More about the design specifications of current transformers and theory of operation of current transformers.

What is the purpose of a current transformer?

To measure alternating current flowing through a conductor, and a current transformer is often classified as a type of instrument transformer. The voltage drop across a known resistor can be measured, and this is okay for low current applications but is often impractical for high current applications. The resistor consumes a lot of power (lowering efficiency) unless the resistor is very low in value, in which case there may be very little voltage to measure. The resistor could be excessively large. The resistor’s heat may affect the resistor value, thereby reducing the accuracy of the measurement. A current transformer can accurately measure the alternating current and put out a reasonable voltage, which is proportional to the current, but without as much heat and size that an appropriate resistor would require. The current transformer can perform its function with very little insertion loss into the conductor current being measured. The current transformer also provides voltage isolation between the conductor and the measuring circuitry. Proper function of a current transformer requires use of a load resistor. The load resistor is often referred to as a “burden resistor”.

What is the core structure of a current transformer?

The best core structure for a current transformer in terms of electrical performance is a toroidal coil. Many toroidal current transformers have only one winding. This winding is usually a “high turns” winding which functions as the secondary winding. In application, the toroidal current transformer is slipped over an end of a high current wire or buss bar, which conducts the primary current. Said wire or buss bar constitutes a one turn primary winding. Split core current transformers are designed so that they can be assembled around a buss bar without disconnecting the buss bar. "C"- cores and "U" core structures are commonly used for split-core current transformers because they are relatively easy to take apart and put back together around the buss bar. Historically, this has not been practical for toroidal coils, but there are now some flexible toroids, which permit the “split-core” feature of installing it around a buss bar. They have limited application. Some printed circuit board applications will utilize bobbin wound current transformers with two or more windings. One winding is an integral part of the circuitry, while the other winding acts the secondary.

Butler Winding can make (and has made) current transformers in a wide variety of shapes and sizes. This includes toroids, “U” and “C” cores for split-core applications; various standard types of "core with bobbin" structures (E, EP, EFD, PQ, POT, and others), and some custom designs.