Current transformers or CTs are indispensable anywhere high currents are measured and processed. These instrument transformers effectively diminish high-voltage current flows for a convenient way to securely evaluate and monitor real electrical current through an ac transmission wire relying on a conventional ammeter. CTs perform this task by generating alternating current in a secondary winding that’s proportional to the current in a primary winding.

In fact, the fundamental purpose of a current transformer is different from that of a standard voltage transformer. Current transformers have only one to a few primary-winding turns — which takes the form of a single flat turn, a core wrapped in a coil of heavy-duty wire, or just a bus bar or conductor placed through a hole. Due to this circuit arrangement, it’s no wonder CTs are also called series transformers.

Shown here is a DIN-rail voltage-tap installation example.

In contrast with this ultra-simple primary-winding construction, the secondary winding of a CT has a number of coils turns wrapping around a laminated core of low-loss magnetic material. The laminated core features a large cross-section to minimize magnetic flux density — and relying on wire having a small cross section. The exact geometry depends on the magnitude of current that must be reduced as the wire attempts to output a constant current … regardless of the connected load.

During operation, the secondary winding sends current into a short circuit (such as an ammeter) or into a resistive load — unless the voltage created in the auxiliary winding is sufficient to immerse the core … or spur a failure because of voltage breakdown. In contrast to a voltage transformer, the essential current of a current transformer isn’t reliant on the secondary load current … it’s managed by an external load.

Auxiliary current is typically evaluated at a standard 1 A — or 5 A for higher ratings. Current transformers can decrease or step down current levels from thousands of amperes down to a standard of some known proportion … again, to either 5 A or 1 A for ordinary applications. So delicate and high-accuracy components as well as control devices are well served by current transformers because the latter renders them effectively protected from the effects of any nearby cables carrying high voltage power.

Metering applications and other uses abound for current transformers. For example, current transformers work in power-factor meters, Watt meters, Watt-hour meters, and protective relays. Current transformers also work as trip coils in magnetic circuit breakers or MCBs.

In contrast with voltage transformers, voltage taps easily and safely connect into circuits to measure the voltage of an existing electrical system. In more advanced variations, transformer voltage taps also serve as adjustment connections on transformer coils through which engineers can control voltage. These voltage taps adjust voltage to hold the value at the secondary at some nominal value.

More specifically, tap-connection adjustments can regulate a transformer’s output voltage to ensure full-voltage output. Where line voltages are either lower or higher than a primary transformer’s rated voltage, the difference has a proportional effect on the secondary voltage … which in turn can cause inaccurate current and voltage output. Using a voltage tap can change a transformer’s voltage ratio so its secondary voltage stays at its target output voltage. On large transformers, taps on the primary compensate for inputs that are higher or lower than normal. Such voltage tap connections are typically set by the component supplier some set line-voltage value. In cases for which the facility or site voltage is unique, voltage taps can be adjusted accordingly by the supplier before they’re sent out.

Voltage taps make direct connections with transformer windings. A whole number of turns is needed between voltage-tap leads or else one will be on the transformer’s wrong side.

Voltage-tap switches are typically included in an installation to allow for the execution of actions requiring that a transformer be shut off. The machine operator should first deenergize the transformer and deploy safety grounds on the transformer’s terminals. Then he or she should move the tap switch from the current position to the suitable position.

Lisa Eitel has worked in the motion industry since 2001. Her areas of focus include motors, drives, motion control, power transmission, linear motion, and sensing and feedback technologies. She has a B.S. in Mechanical Engineering and is an inductee of Tau Beta Pi engineering honor society; a member of the Society of Women Engineers; and a judge for the FIRST Robotics Buckeye Regionals. Besides her motioncontroltips.com contributions, she also leads the production of the quarterly motion issues of Design World.

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