[Moved]Heater control with relay

[gojkosisa]

Hello,

I'm working in company which produce boilers on the pellet as an electrical engineer. For burning the pellet I use a heater and control circuit is simple: relay controlled by MCU over transistor. My company produce about 1000 boilers per year and company exists about 5 years. At the beginning of production instead of relay Triac was used for heater control but there were a lot of issues so we switched to relay control circuit. This was about 3 years ago. Now, with this control circuit every year we have a progress of broken heaters. We have tried with changing heaters, with better quality heater and this extend heater life but again issue is still there. I'm thinking that the problem is with relay and control circuit. Please check this link (a datasheet of relay):
https://www.schukat.com/schukat/schukat_cms_en.nsf/inde/FrameView?OpenDocument&art=RT424-12&wg=E6371 .
Power of heater is 900 [W].
Heater is switched ON only in the process of ignition, MCU switch transistor (relay) ON and after ignition finished (max 10min) MCU switch transistor OFF. In the winter time relay is switched ON/OFF about 15-20 times per day(max).
Could be problem with control circuit? If someone had a similar behavior please give me some advice.

Best regards.

Edit: I make post in wrong forum section .

 

[alexan_e]

Re: Heater control with relay

You mention broken heaters, how can the relay cause that?

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I can move it, which section do you want?

 

[gojkosisa]

Re: Heater control with relay

I'm not sure how. I have found on the internet that mechanical relay can cause thermal schock (heater expand and contract, getting more and more brittle each time). Move to hardware design or something similar .

 

[ALERTLINKS]

When heater is cold, its resistance is low. On switch_on, a current of manyfolds is passed until heater is hot. Most of the incandescent lamps and heaters fused are fused at this particular moment. Use inrush current limitter and current ramp using PWM. Normal switches and relays can't do this but mcu and switching transistor can easily do it and you are already using them.

 

[gojkosisa]

Hello,

thank you for your post. I'm not sure what you mean, could you please give me more information.

Regards.

 

[ALERTLINKS]

https://en.wikipedia.org/wiki/Inrush_current
https://en.wikipedia.org/wiki/Inrush_current_limiter

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http://en.wikipedia.org/wiki/Current-limiter
http://www.edn.com/design/analog/4316203/Simple-and-effective-inrush-current-limiter-stops-surges

 

[ZASto]

You should monitor the moment when the pellet ignites, that's the moment when you start the blower, wait just a couple of seconds till you have a steady flame and then switch heater OFF. You may have problems with heater overheating.
As you are using uC, while the heater is cold, ramp up the current using PWM.

@ALTERLINKS: When the heater is cold the current through it is NOT manyfolds greater than nominal current. Cold heater does not have the cold resistance manyfolds smaller than hot one. Check data for resistance wires of different compositions.

 

[gojkosisa]

Hello,

that is what I'm doing. I'm monitoring a temperature of smoke, when temperature increase pellet ignites and I switch from stage of ignition to stage of combustion (switch Off heater, increase dosing, speed of turbine and etc..). English is not my first language, please explain what you mean with "ramp up the current".

Regards.

 

[ALERTLINKS]

@ALTERLINKS: When the heater is cold the current through it is NOT manyfolds greater than nominal current.

At least it is true in the case of tungsten filament

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When power is applied, there is a high current 'surge', along with thermal shock and rapid expansion of the tungsten. This doesn't affect the lamp initially, but as the filament ages it becomes thinner and more brittle, until one day it just breaks when turned on. For very large lamps used for theatrical lighting (amongst other things), the solution is to preheat the filament - just enough power is applied to keep the filament at a dull red. Full power is almost never applied instantly - it is ramped up so the lamp appears to come up to full brightness very fast, but this is a simple trick that works because the response of our eyes is quite slow.
The cold resistance of a tungsten filament is typically between 1/12 to 1/16 of the resistance when hot. Based on this, it might be expected that the initial inrush current for a cold filament would be 12 to 16 times the current at rated power. The actual initial inrush current is generally limited to some smaller value by external circuit impedance, and is also affected by the position on the AC waveform at which the voltage is applied.

I measured the cold resistance of a 100W reflector lamp at 41 ohms, and at 230V (assuming the power figure is accurate) the resistance will be around 530 ohms - a ratio of 12.9:1 and comfortably within the rule of thumb above.

The time for the initial inrush current to decay to the rated current is determined almost entirely by the thermal mass of the filament, and ranges from about 0.05 seconds in 15W lamps to about 0.4 seconds in 1500W lamps. [1] This varies with the rated voltage too - a 12V 50W lamp has a much thicker (and therefore more robust) filament than a 230V 50W lamp for example. If incandescent lamps are always either faded up with a dimmer or use some kind of current limiter, they will typically last at least twice as long as those that are just turned on normally.

About nichrome heater wire I see some discussion here.
https://forum.43oh.com/topic/2006-wireless-fireworks-ignitor-sourcing-current/

The resistance of cold nichrome is very different from hot nichrome. So if you measure the current with an ordinary DMM, you will be measuring the hot current and that is much lower than the cold current. Cold nichrome is close to a dead short and you want to pump a lot of current into it fast to get the temperature up. Doing this reliably with solid state will typically cost more

But in some data available it shows only 10% increase in resistance from cold to 1800C like here.

**broken link removed**

Fortunately, it can be measured in a practical case.

To summarise Factors causing failure are.
--Mechanical stress on heater element on sudden rise of temperature when metal expands.
--Corrosion of heater wire on certain spots, loosing its diameter and becoming more susceptible to get burnt at that point after diminished current handling capacity.
--Inrush current if applicable.
Overheating due to turned on excessive voltages. for longer time than necessary or applying excessive voltages.

All these issues can be addressed separately.