Current Transformer


Also found in: Acronyms, Wikipedia.

current transformer

[′kər·ənt tranz′fȯr·mər]
(electricity)
An instrument transformer intended to have its primary winding connected in series with a circuit carrying the current to be measured or controlled; the current is measured across the secondary winding.

Transformer, Current

 

an instrument transformer for the measurement and monitoring of large currents by means of standard measuring instruments and automatic control and monitoring devices. It also serves to insulate apparatus from the potential of the circuit that is being measured or monitored. A distinction is made between AC transformers (usually known simply as current transformers) and DC transformers.

The primary winding of an AC current transformer (seeINSTRUMENT TRANSFORMER, Figure 2) consists of one or several turns of wire w1 of relatively large cross section; it is connected in series with the circuit whose current is to be measured or monitored. The secondary winding consists of a large number of turns of wire w2 of comparatively small cross section; instruments having negligible internal resistance—for example, ammeters, meters, and relays—are connected to it.

The distinguishing feature of a current transformer is that the current I1 in the primary winding is independent of the operating conditions of the secondary (in practical terms, the secondary winding is short-circuited). The primary magnetomotive force I1w1 is balanced by the magnetomotive force I0w1 (which gives rise to the main magnetic flux in the core) and by the magnetomotive force I2w2, which determines the demagnetizing effect of the current I2. Under normal operating conditions, I0w1 is usually 1–3 percent of I1w1—that is, I1w1I2w2. If the transformation ratio is known, this relationship makes it possible to determine the large current I0 by measuring the relatively small current I2. Since, nevertheless, I0w1 is nonzero, the measured value of I1 has a current error determined by the relative value of I0w1 and an angular error determined by the phase shift between the currents I0 and I2. In some transformers, called compensated current transformers, the measurement errors are canceled out. The rated value of the current I2 in most current transformers is 5 amperes. Since current transformers are used in circuits where short-circuit currents may occur, the windings of such transformers are also required to withstand short-duration currents that substantially exceed the rated values.

Current transformers are made for various purposes; types distinguished on this basis include instrument, protective, line, and laboratory transformers. Transformers may be of the outdoor or indoor type, or they may be built into electrical apparatus and machines or attached in various ways, such as by means of a bushing; they may also be portable. Other bases for distinguishing various types of transformers include the number of stages (single-stage or multistage), the method of attachment (through-type, including hook-on meters and pole transformers), the number of turns in the primary winding (single-turn, or rod, and multiturn), the operating voltage (low-voltage or high-voltage), and the type of insulation on the windings (dry, resin-paper laminate, and poured insulation). (For DC current transformers, see.)

REFERENCES

Bachurin.N. I. Transformatory toka. Moscow, 1964.
Elektricheskie izmereniia: Obshchii kurs, 4th ed. Edited by A. F. Fremke. Leningrad, 1973.

M. I. OZEROV

References in periodicals archive ?
For medium voltage motor, current measurements are done be using current transformer clipped on the secondary circuit of the motor main current transformer, and voltage measurements are done by clipping the voltage probes on the secondary terminals of motor potential transformer.
The circuit breaker and current transformer power losses have to be calculated using the actual breaker amperage.
Current transformers with ratio 300/5 are used on feeders in real network.
Discrete designs typically use current transformers that suffer from mechanical variations.
Tenders are Invited for Implementation of research for the development and implementation of economic activities optical current transformer new generation.
Virginia Transformer Corp (VTC) has launched its own on load tap changer product, the PKJ LTC III, which will be bundled with its current transformer offerings.
New features distinguish it from other electronic meters in its class including a real-time clock with multi-tariff capabilities, a current transformer connection, RS-232 or RS-485 communications, advanced security features, instrumentation data and installation diagnostics.
Tenders are invited for Instrument Current Transformer.
0, communications software applications, Type CMV-HT Current Transformer, and Type CBT-H Current Transformer at Metering Americas 2002 in New Orleans, in Booth 408.
Tenders Are Invited For Supply of 33Kv Class & 11 Kv Class Current Transformer And 11 Kv Class & 66 Kv Voltage Transformer
Tenders are invited for Indoor Resin Cast , Ring Type, Current Transformer Shall Have The Following Technical Requirements.
Surge Arrestor For Current Transformer for 3 phase loco to CLWs Specification.

Full browser ?