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Thermocouple operating in a powerfull magnetic field.

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dr pepper

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I have a bearing induction heater I use occasionally, its basically a C I transformer core, the primary (3.5kw mains) is on the C section, and the I section is removeable, you slide the bearing over it then place it on the C section, set the time and turn on, the bearing acts as a shorted secondary turn and heats up quickly, and sounds like an angry wasp on steroids.

Later ones have a plug in magnetically attached thermocouple that monitors the temp, then if you cant get back to the heater when its ready at temp it will sit there maintaining temp for you, I'd like to add this function to mine with a microcontroller, as I cant allways be ready to remove the bearing esp on repetitive jobs.
The question is what steps will they have taken to make the system immune from noise, which I suspect is going to be huge from a thermocouple attached to a shorted secondary turn.
I'd have a look inside one I use now and again at another 'shop but it cost £££'s and is pretty much new, knowing me I'd bust it.

Heres the one 'borrow', the connector for the temp rpobe looks like a K type:
 

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My guess is that the 50hz noise will not be as much as you expect.

Why not get a thermocouple and measure the noise with a scope?

Also, add a low pass filter with a nice low cut-off frequency, say 1hz, to the thermocouple amplifier.
Depending on how fast the bearings heat up, this low frequency LPF should not cause a problem to the heating controller.

JimB
 
What is the absolute minimum and absolute maximum temperature doc?

spec
 
I was wondering about the ground line getting noise too, I spose thinking about it I could use an amp that has differential i/p's so a lot of induced noise would be rejected by the fact its common mode and like you say a low pass, it takes min 30 secs for a small bearing to warm up so that might work.
The heat controller on mine is a relay, and I think I'd keep that, just implement a really slow Pwm.
I spose I'm a little overcautious as this machine has claimed the lifes of 2 watches and a mobile, fotunately not mine, the previous owners.
 
You can buy grounded TC probes or ungrounded ones. The grounded ones are better for noise, but can cause other issues with measurements especially with multiple thermocouples. If you care about the process a separate over-temperature cut out is employed using another thermocouple. So, the safety system disconnects power with a real relay. Control was done with an SCR/TRIAC.

In one general system I designed, ON/OFF controlled a contactor and so did the over-temperature thingy. Same contactor). The device has a drop out on power failure that could be enabled/disabled. It was an IR heater in a vacuum system. Power fail - and come back on- not good. Over-temp - not good either.
 
How about just cutting the power for a second or so (Zero crossing solid state relay.) to take the reading. Another possibility may be an IR thermometer. I don't know if they available with an output other than the LCD display.

Edit I have just read post #4 and realise that shutting the power off for a second may be too long as the total time is only about 30 seconds.

Les.
 
2 things.
I'm planning to use slow pwm with a relay, so if I limit maximum on time I could implement power off - measure - power on.
I'm going to stick a dead bearing in the machine, then stuff a cheap lcd thermometer with its sensor on the bearing, if it blows up then I know I have an emi problem.
 
I'm going to stick a dead bearing in the machine, then stuff a cheap lcd thermometer with its sensor on the bearing, if it blows up then I know I have an emi problem.
Nothing like a practical test to find the reality of the situation! :eek:

JimB
 
Well I conducted a scientific test, I found a housing to heat instead of a expensive bearing.
For some reason my camera filtered out the racket this thing makes.
Anyway heres a K type thermocouple probe and display unit working perfectly, even with the display unit held right in the field of the machine, I was surprised it didnt blow up never mind work perfectly.
I tried a couple of diffrent units too, and also verified that the temp probe that came with the machine is a K type, that display I'm using can do J, K or T type.

 
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