Induction Hobs

[davelx]

There are several posts on this forum that say that diversity can't be applied to induction hobs as they boost the output of one ring when not all are in use.

This does not make any sense to me at all! Think of the thermodynamics:

Once the item(s) being heated is up to the temperature the cook requires, e.g. the water is boiling, the oil is at 170C, the last thing that is needed is to boost the output as that will make it boil dry/boil over/catch fire.

The hob needs to control the energy input to each ring to achieve the desired effect, so the hob must either reduce the power to the rings to maintain the required energy input into the food, or it must somehow dissipate (waste) the additional energy and prevent it getting into the food - which I find highly unlikely

So if you have an induction hob with any number of rings and some or all of those are in use to simmer water in pans, etc, the hob MUST be running at less than full output just like a normal hob. Bearing this in mind, can somebody explain why diversity cannot be used?

 

[ericmark]

I can see what you are saying there must be a maximum time it will take to boil a pressure cooker and then it will start on the mark space power reduction.
But with standard cooker you are looking at 6500 watts until first pan boils and by time two pans have boiled it is down to half the output with odd burst as two rings with small pans switch on.
With induction you are looking at 6800 watts but only drops to 6000 watts after first two pans boil so it will draw 6000 watts for a lot longer than a standard hob so standard hob in first 5 min will drop from 28 amps to 14 amp but induction in first 5 min will drop from 29.5 amp to 26 amp.
So allowing for diversity you may be able to consider it as being 26 amp instead of 29.5 max but on a 32 amp MCB this will only give 6 amp to play with and adding an oven especially if double oven could cause a 32 amp MCB to open.
But at 4.4 Kw that the standard is likely to use on two rings you have enough left over for the oven.
With some makes getting to 7.2Kw total and 6.5Kw with two rings these units do have a lot bigger draw than old units.
If you look at MCB graph as long as all rings are cycling within 5 mins you will likely get away with it. But are you sure the spikes causes when all the rings disconnect together will not harm the electronics if not is it worth taking a chance?

 

[davelx]

Ah, so it's the hob's cycling frequency vs the MCB's integration time that makes the difference.

I'd quite like to try and plot this sometime - will have to find someone with an induction hob willing to let me use it for 1/2 hour.

Thanks.

 

[ban-all-sheds]

Eric - is all this documented somewhere? You're the only person I have ever known to mention it....

 

[ericmark]

From Bosch web site "Boost setting for all zones - increases the power output of the selected zone by 50%, it achieves this by borrowing power from another cooking zone. When boost is used on one zone, the contributing zone cannot be used. This function is particularly useful if you need a high heat for example on stir frying with a wok, or making toffee."
From Fagor
# - Front right 2.8 / 1.4 kW (180 mm)
# - Front left 2.8 /1.4 kW (180 mm)
# - Right rear 2.8 / 1.4 kW (180 mm)
# - Left rear 2.8 / 1.4 kW (180 mm)
Total 5.6kw but with that boost no diversity really!

The problem one of the makes on fitting instructions asked for semi-conductor fuses but there is nothing about this on web sites and it is a little late once one comes to fit them to see fuses not MCB is required.
Also some are rather big and need more than the standard 32A supply.
Some are designed for French market and have two phases. But I will have to say they work very well. Only problem is with touch controls and a splash of water on touch controls can shut down hob until wiped clean.

The guy demonstrating did not believe this would happen and his face was redder than the hob once shown!

 

[ban-all-sheds]

You can't beat gas...

 

[ericmark]

For the first time yes electric can beat gas, quicker, cleaner, can be greener, and it does not heat the kitchen it heats the food.

There are some things against induction hobs but not speed.

I don't like touch controls, and I think there are some EMC problems, not convinced on reliability, also there is a restriction on types of pan one can use, lets face it a wok can only be used with gas, but for the first time they are faster, they have always been safer, and they heat the food not the room. Not a problem in Winter maybe but in the summer a very big plus point.

As electricians however they do present a problem needing bigger supplies than before and we have the usual house wife demand "will you 'just' connect this up" and they will not take no as an answer. Neither do they seem to accept that the oven should be disconnected so that the hob can have the supply it requires.

However if we look at the consumer unit with 12 ways often they exceed the diversity allowed but we know there will not be a problem. This may be the same with the induction hob and in real terms there may not be a problem. In another 5 years we will know.
I read the word induction and think of back EMF. What worries me is if the hob becomes disconnected from the supply what will happen to that back EMF especially when we also know there are semi-conductors and I for one would not want the bill if the overloaded trip opens and back-EMF kills the hob and manufactures claim you have not followed installation instructions. I hope this is a false worry but do you want to take the chance? I have already heard of some arguments between customers and manufactures over locations of the induction hobs so I would be wary. Remember if it fails the customer will often call manufacture first and by time you know anything they are armed with all the facts.