Controller SCR, Triac etc to control heater current

I am charged with designing a heat control system that will replace an old system on this companies machine.
It is a "Hot Wire" system that trims a plastic netting material.


In the past this company has used a 1500W transformer (120VAC / / 12VDC) with a simple DPDT SSR to turn on/off the primary voltage, Looking at the old drawings I see nothing to control the heated wire current. It was connected straight from the Xformer secondary to the two ends of the wire via a ceramic terminal block.
Circuit consist of the transformer secondary through a 6" piece of alloy 800 S.S. Wire. (.617ohms per foot) The wire will heat up and act as a hot knife to cut netting material. So I assume any change in primary voltage likely had a negative effect on the heated wire.....As you can imagine, they claim this system is unreliable and very flaky..


I need a smarter circuit than this. I am thinking of using the transformer. Maybe a FWBR (Rectifier) to convert it to DC. Then some type of controller to control the heated wire..

My question is what type of controllers are available for this type of application?

I recommend Control Concepts http://ccipower.com/ for an SCR power controller. Control the primary side of the transformer. Then connect the transformer secondary to the heater. AC will be just fine, no need for a rectifier.

what is the benefith of being at 12vDC VS 12v ac? Easier to read current ?

I just know about temperature controller based on temperature probe...

I think it would be easier to keep it like it is now and replace the on/off SSR per a variable one driven by a current controller. (Devide that would read current and reflect an output to the varible SSR. When the cutting wire heat up, it's resistance increase making tha amps to go low and self regulate a little bit and same thing while cutting, temperature drop and resistance also. If the TFO is only designed to that duty, if you regulate TFO primary current, you will regulate the heat and also regulate for any 120v swing that could happen in the input line...

TConnolly said:

I recommend Control Concepts http://ccipower.com/ for an SCR power controller. Control the primary side of the transformer. Then connect the transformer secondary to the heater. AC will be just fine, no need for a rectifier.

Click to expand...

That sounds like a good concept and retain the simplicity!

Jeff23spl said:

what is the benefith of being at 12vDC VS 12v ac? Easier to read current ?

I just know about temperature controller based on temperature probe...

I think it would be easier to keep it like it is now and replace the on/off SSR per a variable one driven by a current controller. (Devide that would read current and reflect an output to the varible SSR. When the cutting wire heat up, it's resistance increase making tha amps to go low and self regulate a little bit and same thing while cutting, temperature drop and resistance also. If the TFO is only designed to that duty, if you regulate TFO primary current, you will regulate the heat and also regulate for any 120v swing that could happen in the input line...

Click to expand...

I like the idea. I'll look at some controllers and post back...

THANK YOU GUYS !!

Some of the models that I linked have the built in ability to measure power and some can control power to a setpoint, eliminating the need for a second controller and power metering instruments. If you are controlling power instead of current then you minimize the problem of changing resistance in the heating element.

I have requested a quote for the 2022 AC Model [ http://ccipower.com/products/controllers/2022-ac-single-phase-power-controller ]
Seems quite simple and easy to implement into the existing system. I could even control it from an analog output..

milmat1 said:

In the past this company has used a 1500W transformer (120VAC / / 12VDC) with a simple DPDT SSR to turn on/off the primary voltage, Looking at the old drawings I see nothing to control the heated wire current. It was connected straight from the Xformer secondary to the two ends of the wire via a ceramic terminal block.

Click to expand...

In this old system, are you sure it was 12VDC? A transformer does not make DC, it transforms AC to AC. You would have needed a rectifier to make it DC. If it was 12VDC, the 120VAC would have to be transformed down to about 8-1/2VAC (because the DC votlage would be the peak voltage, not the RMS.

Circuit consist of the transformer secondary through a 6" piece of alloy 800 S.S. Wire. (.617ohms per foot) The wire will heat up and act as a hot knife to cut netting material. So I assume any change in primary voltage likely had a negative effect on the heated wire.....As you can imagine, they claim this system is unreliable and very flaky.

Click to expand...

Pretty good assessment. The heat was likely controlled just by doing on-off control of the primary to the transformer. Cheap and reliable, but not terribly accurate. Accuracy may not be important for a hot wire cutter cutting screen material however.

I need a smarter circuit than this.

Click to expand...

Are you sure you need it to be smarter? Are you doing something different?

I am thinking of using the transformer. Maybe a FWBR (Rectifier) to convert it to DC. Then some type of controller to control the heated wire.

Click to expand...

Still easier to control it on the AC side. You can't control DC with an SCR, it has to be a transistor. More complicated control scheme, no added benefits.

My question is what type of controllers are available for this type of application?

Click to expand...

AC power controllers, like those you have already looked at now, come in two basic formats, Zero-Cross Variable Time base (aka Time Proportional controllers), and Phase Angle Controlled.

The Zero Cross controllers fire full pulses of AC, but vary the average voltage seen by the load by changing the number of on and off cyles of the SCRs. They create very little electrical noise and harmonics because they always switch at the zero cross point of the sine wave, but cannot be used when there is a transformer behind the controller, the transformer magnetizing inrush and inductance causes problems. You use those when you are directly sending AC power to the heating element and the hot-to-cold ratio of resistance is low.

For systems where you control the power to the primary of a transformer, you have to use the Phase Angle Controller type. That fires on every sine wave, but for only a PORTION of each sine wave to reduce the RMS voltage seen by the load. They can soft-start into the transformer winding, but create harmonics and line noise on the power feeding them, as well as RF interference.

So the unit you are looking at is a Phase Angle controller, should work fine. But understand that you will be introducing harmonics and line noise into a system where there was none before. Might be fine, might not.

humm i forgot about tfo inrush and noise to adress tfo primary....If 120 is fed from a local 480-120 tfo it isn't an issue but if it also power electronic devices it is something to look for too.

You bring a good point, what is the reel problem with the actual basic setup that you want to upgrade? Why it isn't enought?

In the past I have retrofitted this exact application and I used SSRMAN-1P with excellent results at very reasonable price. SSR goes on xfmr primary, I did not experience any issues with the inductive load even with no snubber; still running today, all day every day for 3 years with no snubber, no issues.

TConnolly said:

I recommend Control Concepts http://ccipower.com/ for an SCR power controller. Control the primary side of the transformer. Then connect the transformer secondary to the heater. AC will be just fine, no need for a rectifier.

Click to expand...

I 2nd this. One of my first projects was a heat control project using a Control Concepts SCR on the primary side. Worked great!

TConnolly said:

I recommend Control Concepts http://ccipower.com/ for an SCR power controller. Control the primary side of the transformer. Then connect the transformer secondary to the heater. AC will be just fine, no need for a rectifier.

Click to expand...

Second that for Control Concepts.
I have used SCRs for years and have found Control Concepts (plus their support) to be top notch.
Most of the time, the delivery is very good also.

I was designing a control circuit to "outgas" the getter and filament of an x-ray tube during assembly.
I needed 0.1 amp control capability in a range of 0.0 to 10.0 amps @ 5 Volts with a 4-20ma control input. They designed a system that ended up being readable, controllable, and stable in the range of .01 amp, even reading it at .001 amp was very good.

The original system was / Is AC Straight from the transformer to the heated wire. I fail to see how they controlled the heat. Unless it was simply calculated from the wires resistance. (.617 ohm / ft) and we have 6" wire so there would be close to 40 amps. I know they had much trouble keeping the system working in the past. That is why I am looking at a different approach.
I had considered rectifying it to DC, but that may not even be needed.

If I place the control concepts SSR in front of the transformer. Then as they recommend double the voltage rating of the transformer, rather than a 120//12 I would use a 240//24 to prevent saturation. The 10:1 turns ratio would still yield the correct voltage. And this would stabilize fluctuations in the supplied voltage as well.
And I can get this unit with POT control so in the field it would be a set it and forget it operation.

can the trouble comes from bad connections between TFO secondary and heating element ? At this low voltage it is very critical like a loose battery post that prevent starting the car

I do something similar on a larger scale. We have a softstart that is modified for use as a current controller feeding the primary of a transformer. The secondary is connected to 3 wheels via slip rings and copper wire that is being drawn travels around the 3 reels.

Same concept but a bigger scale. We push close to 3000A through the transformer secondary.

Drawing

Trying to Attached a Drawing of the original system used.

The problem it seems was they used the length of the wire (very Critical) to adjust the heat / current. The process of changing the wire could take all day to get right.

I'm now looking at a phase angle controller that is set with a simple POT adjustment Or even a 4-20ma signal...