Induction hob technical query

[Marvin]

There are two views about what happens when you use a large induction ring and a smaller pan. Assuming everything is equal and the pan is large enough to turn the ring on. Will the ring use the same amount of electricity if it was a large pan or a small pan. I.e. as the pan does not cover the entire ring, will the same amount of electricity be used even though the whole ring is not covered. Whilst it seem easy to answer I have found many conflicting opinions.

 

 

 

[SteamyTea]

Interesting question.

I shall see if I can test it, right now.

 

Well, weird results.

Both pans had pint of water in them.

 

Half Power: Small pan, small ring, 0.869 kW

Half Power: Small pan, large ring, 0.805 kW

 

Full Power: Large Pan, small ring, 2.28 kW

Full Power, Large Pan, Large Ring, 2.65 kW

 

More testing needed, but warming supper now.

Edited January 3, 2022 by SteamyTea

[Marvin]

4 minutes ago, SteamyTea said:

Interesting question.

I shall see if I can test it, right now.

I suppose the question is: 

 

When the induction ring is fully energised, is the energy all used up producing and electromagnetic field? And therefore no more, or less, energy is used when heating the pan?

 

[SteamyTea]

4 minutes ago, Marvin said:

I suppose the question is: 

 

When the induction ring is fully energised, is the energy all used up producing and electromagnetic field? And therefore no more, or less, energy is used when heating the pan?

 

Maybe I can time a fixed amount of water until it boils.

But going to eat some supper instead.

[Onoff]

16 minutes ago, SteamyTea said:

Interesting question.

I shall see if I can test it, right now.

 

Well, weird results.

Both pans had pint of water in them.

 

Half Power: Small pan, small ring, 0.869 kW

Half Power: Small pan, large ring, 0.805 kW

 

Full Power: Large Pan, small ring, 2.28 kW

Full Power, Large Pan, Large Ring, 2.65 kW

 

More testing needed, but warming supper now.

 

So from that we can see a small ring needs more power...

[SteamyTea]

2 minutes ago, Onoff said:

 

So from that we can see a small ring needs more power...

Or the better fitting pan takes more power.

[Marvin]

1 hour ago, SteamyTea said:

Interesting question.

I shall see if I can test it, right now.

 

Well, weird results.

Both pans had pint of water in them.

 

Half Power: Small pan, small ring, 0.869 kW

Half Power: Small pan, large ring, 0.805 kW

 

Full Power: Large Pan, small ring, 2.28 kW

Full Power, Large Pan, Large Ring, 2.65 kW

 

More testing needed, but warming supper now.

 

 

I think there are too many variables for this to qualify as an answer to the question.  

 

 

[SteamyTea]

38 minutes ago, Marvin said:

I think there are too many variables for this to qualify as an answer to the question.  

Not so fast.

 

Second set of results.

I have reset my energy monitor as it was miss reporting for some reason, and now I have noticed that there is a 0.15 kW fan on when the hob is running.

I also timed how long it would take to heat 500 g of water.

 

Full Power: Small pan, small ring, 1.63kW, 3m 20s (200s)

Full Power: Small pan, large ring, 1.93kW, 2m 16s (136s)

 

Full Power: Large Pan, small ring, 1.63kW, 3m 35s (215s)

Full Power, Large Pan, Large Ring, 1.93kW, 2m 25s (145s)

 

To just boil 500 g of water should take 0.0511896 kWh (assuming incoming is at 12°C), this is 184,282 Ws.

Multiplying the time taken by the power of the ring, in watts, but minus the 150 W for the cooling fan gives:

 

Small Pan, Small Ring, 296,000 Ws, 60% more energy needed

Small Pan, Large Ring, 242080 Ws, 30% more energy needed

Large Pan, Small Ring, 382700 Ws, 73% more energy needed

Large Pan, Large Ring, 258100 Ws, 40% more energy needed

 

Some of that extra energy is needed to heat the pan and lid, and when the large pan was on the smaller ring, it only boils vigorously in the middle of the pan.

 

So, to boil faster, and use less energy, match the pan to the ring, and the task.  Don't boil peas in  frying pan, try and use a smaller diameter pan if you can.

 

There is not a huge difference between my rings, ~300W.

Edited January 3, 2022 by SteamyTea

[Marvin]

Hi @SteamyTea Thank you for your trials, but not so fast.

 

I cannot understand your conclusion. Surely the 242080 Watts: (Notice I'm now using the capital W in Watts...) used in the 

34 minutes ago, SteamyTea said:

Full Power: Small pan, large ring, 1.93kW, 2m 16s (136s)

test used less energy than the other alternatives (although it took longer which was not part of the problem.)

 

But even so, is there not a scientific calculation that will identify what the answer should be?

 

When the induction ring is fully energised, is the energy all used up producing and electromagnetic field, and therefore no more, or less, energy is used when heating the pan?

 

Someone must know the scientific answer. 

 

 

[SteamyTea]

1 hour ago, Marvin said:

Watts

It is Ws, for watt second, which is energy, not power.

Why we should really use J, joule, for energy and not any other units.

 

1 hour ago, Marvin said:

When the induction ring is fully energised, is the energy all used up producing and electromagnetic field, and therefore no more, or less, energy is used when heating the pan

Assuming all things being equal in the pan materials, and allowing for different masses due to size, I will take a guess that there will be an optimum size of pan base for a particular induction coil.  If the pan base is smaller than the coil, then some back electromotive force (BMF) may happen in the 'uncovered' portion of the coil, reducing efficiency.  There may also be a reduction in power, but overall the efficiency will be less.  To test this would mean going out and buying more pans, but others may like to give it a go with what they have, and then we can see what comes out of the data.

 

 

A quick plot of this very limited data shows a power ratio (exponent has the same R² value as well).

So:

Measured Value = 0.0078x2.1118 

Where x is the theoretical value.

 

Now what was the question.

(We used to call these Silly Sunday Experiments)

 

Edited January 3, 2022 by SteamyTea