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AC CURRENT TRANSFORMERS

General:

To reduce danger to personnel, and to reduce the heat generated by high AC current, toroidal AC Current Transformers (also called transducers) are inexpensive, common solutions. Generally speaking, these devices, also known as "donuts", reduce the line current from its rating to 5 amps AC. Thus the receiving instrument never sees anything more than a maximum of 5 amps (amperage is the killer; voltage is of lesser danger).

When using CT's, one must make sure the load or burden that is connected to the secondary is not above the limits of the CT. The load limit is specified by its VA ratings (usually running from 2 to 10 VA). The larger the VA, the more resistance can be tolerated by the CT. When in doubt, utilize a CT and a CT transmitter that converts AC amperage into a 4-20 mA DC signal for long lead lengths (and specify compatible receiving instruments scaled, however, in AC amps).

To calculate load:

1. Resistance of wire (typically, the CT comes with 24" of 16 AWG wire; 16 AWG has a resistance of .0041 ohms per foot: 50 feet out and 50 feet back would 100 x .0041 = .41 ohms
2. Resistance of Analog or Digital Indicator/Recorder/Datalogger, typically .1 ohm
3. VA=I²R (for example, 5 amps² x 100 feet of 16 AWG= .41 + .1 for a meter= 10.5 VA

Therefore it is obvious that any distances over a few feet from the CT should mandate a transmitter.

*See chart

Installation:

The donut transformer can be ordered with mounting feet or flat bottom configuration for surface mounting, or simply hung through the primary conductor with one or more primary turns (see Ratio Modification below).

Care must be taken to ensure the secondary leads are connected at all times when current is passing through the primary conductor.

Multipoint Switches:

In 3 phase systems, the amp meter (0-5 ACA receiving instrument) can be switched externally from phase to phase to monitor the various currents. CAUTION: NEVER BREAK THE SECONDARY CIRCUIT OF A CURRENT TRANSFORMER AS HUMAN LIFE AS WELL AS CONNECTED EQUIPMENT CAN BE ENDANGERED BY INDUCED HIGH VOLTAGES. All Current Transformer Secondaries must be short-circuited when not in use, and the S ampere terminal of the meter-receiver connection must connect to a secondary "BEFORE" the short is removed from that secondary. Switches must "MAKE BEFORE BREAK", and must keep all unused secondaries shorted. (Voltage switching is just the opposite--all voltage switches must "break before make" and all unused potential transformer secondaries should be kept open)

Common Characteristics of CT's

1. Frequency affects a C/T only because the lines of flux generated by the primary current begin to appear as DC as the frequency gets very low; a C/T needs the AC CYCLE changes to induce the secondary current. With anyone's toroidal C/T, you will experience a drop in accuracy as the frequency goes down from 60 Hz. One can manufacture a C/T with an exotic metal core that is not quite as affected as the silicon grain oriented steel most commonly used, but the improvement would be questionable and at high cost.
2. Below 60 Hz, the accuracy will be affected by the drop in frequency and voltage: with Instrument Transformers CT's having the maximum acknowledged accuracy of 0.3% ANSI Rating, you will experience a drop in accuracy at 9 Hz to 5%; at 6 Hz it might be 7.5% of full scale. A Split Core unit might have double the inaccuracy, or more (for example, a 1% Split Core being used at 9 Hz will experience an accuracy rating of 33% - {.3%/5% is as 1%/X or X = 5/.3 = 16.7 x 2}. Remember, it is difficult to come up with test equipment with enough power to test full scale at unusual frequencies. The lesson here is to take the most accurate C/T you can if you are running in lower frequencies than 60 Hz.
3. Exercising the C/T beyond its current rating for short periods is not usually a problem; each CIT has a Thermal Rating Factor (if not published, then you must assume it is 1.0). This is a "continuous thermal current rating factor". The Instrument Transformer model 5A (page 5, Section 2) has a factor of 1.33 at 300C. This means this particular C/T can be operated at 133% of its primary rated current CONTINUOUSLY without overheating (a 200:5 can thus be operated at 200 x 1.33 or 266 primary amps continuously). Other CIT's have thermal rating factors of 1.5 and 2.0 etc. On a momentary basis, any CIT will usually operate at 64 times its primary current rating for 1 second; 150 times its current rating for 1 cycle.
4. Above 60 Hz, a CIT becomes conversely more accurate up to about 4000 Hz. Above this, you must examine the wave shape carefully because it causes the core to saturate. 400 Hz is the published limit with some manufacturers; there is usually no problem with accuracy or heat or saturation at this frequency.
5. 4-20 mA DC Transmitters
   1. For all such transmitters, an independent, stable prime power is a requisite for published operational accuracy and characteristics.
   2. The internal transmitter of the device usually will not operate below 85 volts (43 Hz)
   3. Frequency response with a constant 120V 60 Hz Prime Power starts to fall off at 20 Hz; by 9 Hz it will be off by 5% Full Scale. At 6 Hz it will be off by 7.5% etc.
6. P/T's and Frequency: the ratio of voltage to frequency is important to a P/T (but not to a C/T). It must remain constant, or the P/T will overheat. Lesson: do not power a P/T from a variable frequency drive unless this ratio can be made constant.

AC CURRENT TRANSFORMERS

HOW TO OBTAIN SPECIAL RATIOS FROM STANDARD RATINGS

Window type current tranformers are rated on the basis of a single primary turn. However, other ratios are obtainable by the use of multiple turns. Most window type current tranformers can have its nominal ratio adjusted to a non-standard ratio by the use of primary and secondary turns.

Primary Turn Ratio Modification   Formula: Ka = Kn x Nn / Na Where: Ka = Actual Transformer Ratio Kn = Nameplate Tranformer Ratio Na = Actual Number of Primary Turns Nn = Nameplate Number of Primary Turns The ratio of the current transformer can be modified by adding more primary turns to the tranformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced. Example: A 100:5 current tranformer designed for one primary turn. 1 Primary Turn NAMEPLATE RATIO -- 100:5 ACTUAL RATIO -- 100:5       2 Primary Turns NAMEPLATE RATIO -- 100:5 ACTUAL RATIO -- 50:5       4 Primary Turns NAMEPLATE RATIO -- 100:5 ACTUAL RATIO -- 25:5

Secondary Turn Ratio Modification Formula : Ip/Is = Ns/Np Where: Ip = Primary Current Is = Secondary Current Np = Number of Primary Turns Ns = Number of Secondary Turns Example: A 300:5 Current Tranformer. 300 p / 5s = 60s / 1p (In practicality one turn is dropped form the secondary as a ratio correction factor.) The ratio of the current tranformer can be modified by altering the number of secondary turns by forward or backwinding the secondary lead through the window of the current tranformer. By adding secondary turns, the same primary current will result in a decrease in secondary output. By subtracting secondary turns, the same primary current will result in greater secondary output. Again using the 300:5 example adding five secondary turns will require 325 amps on the primary to maintain the 5 amp secondary output or: 325 p / 5s = 65s / 1p Deducting 5 secondary turns will only require 275 amps on the primary to maintain the 5 amp secondary output or: 275p / 5s = 55s / 1p The above ratio modifications are achieved in the following manner:

TECHNICAL DATA

CURRENT TRANSFORMERS RATIO MODIFICATION

Relatively large changes in ratio may be achieved through the use of primary turns.
For example:

A primary turn is the number of times the primary conductor passes through the CT's window. The main advantage of this ratio modification is you maintain the accuracy and burden capabilities of the higher ratio. The higher the primary rating the better the accuracy and burden rating.

You can make smaller ratio modification adjustments by using additive or subtractive secondary turns. For example, if you have a CT with a ratio of 100:5A. By adding one additive secondary turn the ratio modification is 105:5A, by adding on subtractive secondary turn the ratio modification is 95:5A. Subtractive secondary turns are achieved by placing the "X1" lead through the window from the H1 side and out the H2 side. Additive secondary turns are achieved by placing the "X1" lead through the window from the H2 and out the H1 side. So, when there is only one primary turn each secondary turn modifies the primary rating by 5 amperes. If there is more than one primary turn each secondary turn value is changed (i.e. 5A divided by 2 primary turns = 2.5A). The following table illustrates the effects of different combinations of primary and secondary turns:

Questions? - Call John Perkins at 1-800-767-6051