Analogue current and voltage regulation and rectification thereafter

Question

I am a research scholar working on mechanical metallurgy as a part of my engineering study.

For my project (a new kind of electroplating,) I need voltage ranging from minimum 0V to maximum 170V DC. We need a minimum of 130V to trigger the reaction and 150V to effectively carry out the reaction.

I am currently achieving the voltage regulation using a 3 phase auto transformer-bridge rectifier-capacitor setup. To avoid short circuits, I have installed a 32A MCB on the primary and 15A fuse on the filtered DC side.

As my solution heats up, the resistance drops, and current draw increases preventing me from achieving the desired result. Is there a way to regulate or set or limit current to a certain value. For example, 15A DC or 32A DC. while still maintaining the same voltage.

My lab has wiring to handle 45A per phase continuous load, I have access to a 3-phase auto-transformer with 36A/phase capacity corresponding rectifier-filter setup. I do not want to add variable resistor or incandescent lamp in series as it would waste power and reduce the voltage.

Please suggest a suitable way to limit the current and meet my requirements.

Answer 1

Since you have a variable voltage setup already then you can reduce the current by reducing the voltage, obviously, and then crank up the voltage later. If you want this to be controlled by a circuit automatically, then yes you can add a timing circuit that slowly ramps up the voltage.

Now if your asking if there is a way to limit the current but keep the voltage the same.. the answer is, no, not really (though it may seem that way). Remember the liquid in your solution is a resistor. It changes with temperature but at any given temperature its resistance is a fixed value. Because of the fundamental equation \$I=\frac{V}{R}\$ well if you aren't willing to lower the voltage, and since you cant change the resistance of the fluid (short of changing its temperature) you wouldnt be able to effect the current. Of course you also mentioned adding a lamp or resistor in series, but in reality that is the same as lowering the voltage except it would be a more wasteful way of lowering the voltage. Imagine your liquid is 100 ohms and you apply 100 Volts to it, you would have 1 amp flowing through the liquid. Now if you add a 100 ohm resistor in series with your 100 ohm fluid you have now reduced the current to 1/2.. but if you measure the voltage across the fluid it will be 50 volts, and the voltage across your resistor will be 50 volts. So from the perspective of the fluid it would be completely identical to having just lowered the voltage to 50 volts, which would produce the same current, except by putting a resistor in series you are wasting twice the power.

So you have two options:

1. Adjust the voltage to create the current you want, either manually, or you can design the circuit to do it for you. This would be called a current source if you want the circuit to automatically keep a fixed current.

2. Apply cooling to the fluid

If you want is a circuit where you set the current, and it adjust the voltage dynamically to keep the current constant, and is capable of 170V (basically a variable current source rather than a variable voltage source).. then yea you would need to design a circuit that first senses the current, and then a controller to something like a variac that is fed into a rectifier.

Answer 2

Is there a way to regulate or set current to a certain value. For example, 11A DC initially and 30A DC after some time manually; while using an auto-transformer to control voltage.

You can not control both voltage and current. One or the other must be free to change to achieve the other as the load or source changes. You can control one and set a limit for the other. When the limit is reached, the controlled variable will no longer respond to efforts to control it.

For automatic control, better performance and a less complicated control system, it would probably be better to design a controller for the DC output of the rectifier. The AC voltage would then be set to a fixed value and not adjusted.

You can control the current manually by simply installing an ammeter on the output and reducing the AC input voltage when the current gets too high. You can do that automatically by motorizing the transformer adjustment and using a meter relay to turn the motor on and off. That is an easy system to understand; however it may not be so easy to find the right components.