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An AC Current Generator, Part 1

I wrote an article on this topic in 2000 that was published on Circuit Cellar Online. Despite what the alarmists may tell you that nothing is ever deleted on the Internet, I have evidence to the contrary. I believe that the basic information that I wrote about is still relevant and so I am re-writing the article as a blog. I must acknowledge the role Ernesto Gradin, my mentor, played in the realization on the magnetics of the project. Any errors are as a result of my misunderstanding.

My current employer, Emphatec Inc., has been through several guises, but we still design and manufacture several sensors for an AC current. The AC current is measured with either a current transformer, or in-line resistor and then converted to a signal friendlier to instrumentation like a 4-20mA current loop. If you look through my blogs both here on Planet Analog and on MCU Designlines, you will find several references to such devices. The latest was Figure 2 in Potentiometers: Mechanical & Electronic, Part 2. Integral to the manufacturing process is calibration and test necessitating the generation of an AC current to stimulate the input of the module under test.

The currents that need to be generated for our products can, in some cases, exceed 100Amps AC. Resistive techniques typically generate excessive heat. In addition, they are also potentially dangerous to the test technician since there could be 120V present at the module inputs. A further disadvantage is that the test is restricted to the line frequency and subject to the unpredictable changes of the line voltage.

One solution in the industry is to use a toroidal transformer with only a few turns on the secondary. For all transformers, the power presented at the primary is equal to the power delivered at the secondary, and the output voltage is proportional to the turns ratio of the secondary winding to the primary winding. It is possible by using these two properties to construct a toroidal transformer to generate a high output current at a low output voltage. I am simply going to refer to this as a “transformer” from now on.

We started out using an existing transformer with a 120VAC primary and drove it with a variac. A few turns were made through the toroid as the secondary and shorted together, and this is a very simple start, but we could never get the output current to be stable over any length of time. We attributed this to mains variations, but we never had the instrumentation to get to the bottom of it.

The next attempt was to use a sine wave oscillator driving a power amplifier that was connected to the primary of the transformer. In this configuration it was possible to change the AC frequency, but it suffered from another problem. The transformer secondary current is affected by the loop resistance, which includes the terminal resistance of the module and the wire length. In addition with higher current, the secondary loop warmed and affected the overall loop resistance.

We decided to control the output in a closed control loop built on dedicated PCB that included PC setup and monitoring, an 8051 micro, and a couple of chips that are now obsolete. Because of the obsolescence I will not include that in my description, and if I were to repeat this project, I would certainly consider a PID loop made up of commercially available parts.

Figure 1-1

A block diagram of our vision. Things may change in a new system.

A block diagram of our vision. Things may change in a new system.

Figure 1-2

Here is the original system. Because this unit was to be used on multiple products we created two transformers. The 10A is on the lower right with the black secondary winding. To its left, seen mounted vertically, is the 100A transformer with a car battery lead as the secondary. The other three pink/red toroids are current transformers used to measure the current as part of the closed loop control. The shunt resistors you can see are used for unit calibration.

Here is the original system. Because this unit was to be used on multiple products we created two transformers. The 10A is on the lower right with the black secondary winding. To its left, seen mounted vertically, is the 100A transformer with a car battery lead as the secondary. The other three pink/red toroids are current transformers used to measure the current as part of the closed loop control. The shunt resistors you can see are used for unit calibration.

Because we were going to use a custom driver, we had to wind our own transformer. We will cover that in the upcoming parts of this blog.

9 comments on “An AC Current Generator, Part 1

  1. EMCgenius
    December 4, 2014

    Your current transformer desciption is backward.  The primary should be not more than a few turns, and is often just the high amperage wire to be sensed inserted through the center of the toroid, which equals a single turn.  The secondary comprises many turns at commensurately lower current, (nearly) shorted by a burden resistor (often just a few ohms) that converts the scaled lower secondary current into voltage for measurement purposes.

  2. antedeluvian
    December 4, 2014

    EMCgenius

    Your current transformer desciption is backward. 

    With all due respect, I believe you are missing the point of this blog. The “current transformers” that I mention do indeed work with a single turn primary and multi-turn secondary. What I am trying to describe here is a method to generate an AC current so that I can test the equipment that includes the Current Transformer.

     

  3. EMCgenius
    December 5, 2014

    OK, I see what you mean.  It's hard to read the text in the schematic on my laptop screen, as the letters are about 4 pixels high.  Thanks for the clarification.

  4. uchiha
    December 6, 2014

    Advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages.

  5. antedeluvian
    December 7, 2014

    uchiha

    Advantage that AC electricity has over DC electricity is that AC voltages can be readily transformed to higher or lower voltage levels, while it is difficult to do that with DC voltages.

    You may find this interesting “Top 5 Reasons to be thankful for AC power

  6. vasanjk
    December 8, 2014

    AK,

    In the age of optically coupled current converters, transformer based sensing is still very much relevant. I recall a 2000A AC power supply we designed for testing a MCCBs with a range upto 1800A. It was a nightmare to measure such high currents with CTs and still we could do a reasonably good job.

     

  7. samicksha
    December 10, 2014

    The only key point i find with AC is that is safe to transfer over longer city distances and can provide more power, rather doing same with DC.

  8. dassa.an
    December 10, 2014

    @samicksha: Yes very true plus the stability too is assured in shorter distance

  9. uchiha
    December 10, 2014

    @antedeluvian

    Thank you very much. This is a good article about AC power. Got a better idea about growth of AC power.

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