Why the voltage on the primary coil changes in an induction stoveplate?

  • #1
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Translated from Norwegian high-school physics book:

One of the advantages of an induction stovetop is that it can detect when the cookware is removed from the plate or it boils over. It detects it by changing the voltage in the primary coil underneath the plate. This technique also allows for other functions, such as the cooking plate can adjust so that the food boils so far, or that the plate can turn off automatically when the cookware is removed from the plate.
I wonder how and why does the voltage change on the primary coil when the cookware is removed or boiled over?
 

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  • #3
sophiecentaur
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Translated from Norwegian high-school physics book:
I wonder how and why does the voltage change on the primary coil when the cookware is removed or boiled over?
The reason that you get energy transfer to the pan is that there must be significant resistance in the metal of the pan so that the induced eddy currents will dissipate energy. Pans made of good conducting material like copper will not dissipate enough and, in fact, the 'mismatch' would cause the energy to be dissipated in the high frequency amplifier in the hob (not what's wanted). Iron or steel are best and the system is designed to transfer maximum power into that sort of pan.
He does the hob know then there's an appropriate pan there? The current and volts in the amplifier (it could be called a Transmitter) are self - monitored and the electronics will detect the presence of a suitable Load and turn up the power. A glass or copper pan (or nothing on top) will never register as a Load and the transmitter is turned off. It's an intelligent device that can do something like that but easy to do with modern electronics.
I don't know of a hob that will turn off if a pan boils over. That would need another level of self monitoring.
I used to be in favour of gas cooking but, since having used my daughter's induction hob, I have changed my opinion. It's the way to go - especially if the gas would have to be bottled. (Ours comes from the mains.)
 
  • #4
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With electronics it's possible to make cook plates that monitors itself.

Primary coil has alternating current, so its current changes every time. When its current changes, it produces fluctuating magnetic field, and that fluctuating magnetic field induces the bottom of the cookware. When the cookware is induced, there's current in the bottom of the cookware. But because of the high resistance, the bottom of the cookware heats up.

From what I understand is that, induction stove plate works like a transformer, and the cookware acts like a secondary coil (or secondary winding).

But removing the cookware (or "the secondary coil"), how does it affect the voltage in the primary coil?
 
  • #5
anorlunda
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But removing the cookware (or "the secondary coil"), how does it affect the voltage in the primary coil?
How does changing the secondary current change the primary current/voltage in any transformer?

Hint: You can swap the names primary/secondary and the physics remains the same.
 
  • #6
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How does changing the secondary current change the primary current/voltage in any transformer?

Hint: You can swap the names primary/secondary and the physics remains the same.
So while the AC (alternating current) in primary coil induces the secondary coil (affects its current), the AC in the secondary coil also induces the primary coil (and affects the current of primary coil)?
 
  • #7
anorlunda
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So while the AC (alternating current) in primary coil induces the secondary coil (affects its current), the AC in the secondary coil also induces the primary coil (and affects the current of primary coil)?
Yup. The physics (but not the magnitudes of V and I) are symmetrical primary/secondary.
 
  • #8
sophiecentaur
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So while the AC (alternating current) in primary coil induces the secondary coil (affects its current), the AC in the secondary coil also induces the primary coil (and affects the current of primary coil)?
This (and some other posts) imply that the Transmitter just provides a steady value of Volts (a Voltage source). I doubt if this is true because it would imply that the power input into a pan would depend on the thickness and resistivity of the pan. I would imagine that the design would be capable of providing its maximum input power over a wide range of pans (sizes and actual materials. This makes me suspect that the transmitter would be more sophisticated (it would certainly be easy enough to make it so) and provide something like the same heating power for a range of load by using a regulating circuit. (At the same time giving some temperature protection so that some conductor could be melted when placed on the glass top.)
They do not specify any particular size of pan for the one hob that I have had experience of.
 

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