AC Current Limiting

[bnfdvn]

I need a way to limit 120 VAC current to 5 Amps. I am building a simulator to connect to a generator controller. The three phases of the generator each have a current transformer installed on the wire. The output of these transformers goes to the generator controller. The input on the generator controller is a 0.25 Ohm, 20 Watt resistor in parallel with a small transformer. The connection I am making is across the resistor/transformer primary. I assume that the resistor is a simple heater for the unit. Each phase has the same circuit.

Any help would be appreciated. Thanks in advance!

 

[Hero999]

Please post a schematic.

You haven't even said what the load is?

A resistor would certainly limit the current but the power dissipation would be 600W which is probably unacceptable.

 

[bnfdvn]

I have attached a .pdf of the load as described in the OP. The generator's current transformers feed into these resistor/transformer circuits. My simulator must also feed into these circuits.

 

[Hero999]

Here's the simplest solution I can think of: connect the three inputs in series (assuming you don't need to control them independently) and connect them all to a 6V transformer in series with a high power 1R variable resistor and 1R fixed resistor in parallel.

 

[Roff]

Something is wrong here. Current transformers are generally in series with a load. They generally have a one turn primary (basically, an AC current-carrying wire passing through the center of a toroid), or at most a few turns. The secondary has many turns, and is paralleled by a burden resistor. In your drawing, if this is what we are looking at, the primaries are on the right side. The burden resistors are on the left side, in parallel with the secondaries. The voltages across the burden resistors will be relatively low, and will be functions of primary currents.
120V across 0.25 ohms will draw 480 amps.Five amps through 0.25 ohms will result in 1.25V. Are you sure this is what you want to do?

 

[bnfdvn]

I am not explaining this very well. Let me try again.

I am building a simulator for the test and repair of generator control modules. Each of the three phases of the generators power outputs has a current transformer installed to measure the load on the generator. At full load on the generator, the current transformers output 5 Amps @ 120 VAC. It is this signal that I am trying to duplicate for my simulator.

The input on the generator control module is the resistor/transformer (one per phase) that I have shown on the drawing. The resistor is across the primary of the transformer. Internal to the control module, the signals from each phase (120 VAC, 5 A) are converted to a DC signal and added together to provide a "Load Signal". At full load the generator has a "Load Signal" of 6 VDC. For testing of the control modules, my simulator will apply the 120 VAC, 5 A to one current transformer input at a time. When applied, the "Load Signal" should be 2 Volts (1/3 of the total, since only one phase is connected).

 

[Hero999]

Going from what you've said, you just need a 5A constant current source to test the current transformer, right?

So the voltage isn't important.

Use a transformer to reduce the voltage to a lower value and add a resistor to limit the current to 5A.

 

[bnfdvn]

I looked on Digikey's website and found a transformer that outputs 10 V @ 10 A. This is the lowest voltage for an "in stock" transformer that will output more than 5 Amps. Now I have some questions about this. The transformer has a center-tap. Do I need to go across the entire secondary (20 Volts??) and not use the center tap or Can I just use the center-tap and one output terminal and leave the other output terminal unconnected??

To limit the current, I would use a 2 ohm resistor (50 Watts or greater) for 10 Volt output or 4 Ohms (100 Watts or greater) for the 20 volt output (depending on the answer above). Do I need to worry about inductance of the WW resistor??

 

[Hero999]

No, you don't need to use the centre tap.

Another question: is the 5A through the current transformer's primary?

If so, you can use a much lower current and simply wrap the wire through the middle several times, for example if you use two turns and have 2.5A, the current transformer will measure the current as 5A.

If you're bothered about accuracy, you'll need to use a high powered variable resistor to trim the current, otherwise it will change, depending on the mains votlage and transformer regulation.

My local supplier sells 6V 20VA transformers, with twin secondaries whcih can be connected in parallel to double the current rating.
**broken link removed**

 

[bnfdvn]

I don't have a current transformer. I am emulating the operation of a current transformer with my circuit.

Thanks for the tip on using parallel output transformers. They are half the price. I found one that has 5 V @ 5 A. With this one, I won't need the resistor.

 

[Hero999]

Yes, you do need the resistor, otherwise the current will be much higher.

Don't forget that the transformer has a really low internal series resistance and if you connect R1 to R3 in series, the total resistance will be 0.75 which will cause 5/0.75 = 6.67A to flow and overheat the transformer and sense resistors. You still need a resistor to limit the current, 0.25Ω will probably do, but it's a good idea to make it adjustable.