You might try a simple infinite heat control that is used on many electric ranges. Make sure it isn't a solid state type.
repairclinic.com is a great source and they would probably help you find one. It would be great just to get a used one and try it out to see if it's what you need.

 

As far as I'm aware all modern stove controls are solid state.

It would still cause the same problems, I don't think it's EMC that's the cause here. He's using zero crossing so it should be better EMC wise than switching the heater with a relay.

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Actually there are two versions of the same product.
one uses a 220V heater, the other uses the low voltage, high current heating ribbon through a transformer.
My customer says that he sees the problem with both variants.

Meanwhile I took a shunt resistor of 1R rather than 50mr. Since then I did not see the "cut sinus" phenomenon again. Of course after not to long the shunt starts to smoke as a result of high dissipation. So until I get a high power shunt I can only test for short periods.

I disconnected the heater from the transformer and measured the current. This peaks unregularly during transformer startup. The most powerful ones are about 30A. This sound like a good explanation for dimming lights.

However, with the resistive-only heater this cannot explain. I am starting to believe that the zero crossing detector might not work correctly.
I will try testing this this week.

If so, it might be that it also uses half phases, giving a rectifier-like effect. In that case I can imagine the transformer current going out of hand.

If that is not the case, I might have to conclude that it is not possible to frequently switch a transformer.
Or I should go to phase cutting. But in that case I would definitely need a big inductor to keep it EMC.

 

Yes, the zero crossing might not be working properly.

Also make sure you have the same number of positive and negative cycles going through to avoid the rectifier effect.

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Ok, will do some testing this week and tell you what I found

 

Ok, it was indeed the zero crossing detection which failed.

I attached a schematic. The problem is, that I get some sort of drop-outs in the resulting signal.

I added C27 today. This should have been there all along (shame on me), but it does not seem to completely solve the problem.

I set the software to supply a single phase (= one positive+ one negative) each 200ms. appr. 19 out of 20 times this works and then either a pulse is skipped, or a double pulse is provided.

I am also having troubles to get the oscilloscope to trigger on the mains current measurement.
I ordered for a current sense transformer so I can trigger on the zero crossing signal and view the mains current at the same time.
Hopefully I will be able to find out more soon.

Attached Thumbnails

 

 

I think the problem might be caused by phase shift in the transformer aupplying the circuit and the current not being zero when the voltage is zero due to the fact that the trasformer is inductive.

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I don't think so. I also tried the controller on a purely resistive heater. The same effect is seen here.

Also I found that with the load attached the transformer actually behaves a lot like a normal resistive load. I even do not see the rather large 6A startup peak when the load is attached. In that case the current amplitude is 3A in all cases.

 

I'm reading with excitement all about the heat controll. Since I'm designing a big 16kw heated platen for vacuum laminator I would like to confirm if i understand it well.
The heart of my system wil be mitsubishi alpha 2 controller. It has a pid controll function block. From it I go (using function bloc) to PWM and set period of 2 seconds. On the output i get pwm that varies accordingly to MV value of the PID function. From the output of the alpha I go to crydom 50A zero switched 3 phase SSR. Finaly the multiple incoloy heating elements will be conected to delta configuration and heat the aluminium platen.
Thermocouple finishes the close controll setup with signal converted by ad595cq to useable 0-10V range for alpha2 controller. The platen will be about 1.5m2 area and working temperature about 155C.
Is this a possible setup?

 

I am not familiar with all of the hardware you are using.

Basically it should be possible using a PID. The system will be rather large, so I am not sure about the regulation speed you can achieve.
It might be useful to make the platen rather thick. This added thermal capacity will allow for slower regulation.

Also be cautious that if you use many interfaces (thermocouple to 0-10V to controller to switch) there might be a significant lag. A slower control loop might be more stable.
Also be sure that the measured temperature is accurate. Put the thermocouple at a location where the temperature measurement is representative for the whole platen. Ie. not to far away from the heater, and not too close to it.
A thicker platen will probably have a better equality.

Hope it helps.

 

Hi there,


Yes sometimes switching a transformer on and off causes continuous line surges
which isnt always good either. It's a nice idea though.

Another idea is to use DC to power the heater element. Using a rectifier and
PWM power supply you can control the heat to a very specific level and very
smoothly. If you dont mind the extra complexity this works out pretty good.

A little less complex is to try switching AFTER the transformer instead of BEFORE it.

Another idea is to build an AC chopper circuit, where you actually chop up the
AC line with one or more transistors. This is a bit more complex too but also
offers excellent power control.

Perhaps you can tell us what your load (heater) actually is, and how many watts.

 

After I got my problems with the zero crossing fixed, I was rather satisfied with switching the transformer. Not funny spikes anymore.

I would recommend against the triac after the transformer, because all TRIACS have an appr. 1,5V voltage drop.
1,5V at 1A (before transformer) = 1,5W
1,5V at 16A (after transformer) = 24W = very hot Triac + relatively high losses.
In that case I would recommend rectifying and using a FET instead.
Or you could use the TRIAC to do the actual heating ;-)

About the 16kW heater. I am not sure about phase cutting a 16kW device. This will cause such a large phase cut load to the net that it might even be illegal.
You might have to use some kind of power factor correction.
It might work with something like frequency controller motor driver. This can probably also be controlled witha 0-10V control voltage.

 

Hi again,


You mean the OP is using a 16kwatt heater?

 

For the 16kW heater I was referring to nemo50 who is working on something like this.

 

Hi again,


Oh ok, i was hoping the OP would come back with the power of his heater
so we would have a better idea what we are dealing with.
Yeah, 16kw is a lot of power.