Cooker and hob...1 feed or 2?

EFLImpudence said:

Ah, fair enough. However, you did say it first:

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I did - but I put it in quotes, which (to me, but probably not you!) meant that I was using the phrase in a technically incorrect manner. Looking back, I did you similar phraselogy previous to that and, to be clear correct, I probably should have written something like "initial period of energy usage', thereby making it clear that I was talking not only about power, but also how long that power had to be drawn in the 'post switch-on period' (to achieved the desired amount of heating).

However, as before, I think we are agreed - that if the traditional diversity calculation were not appropriate, despite the power of the ring being similar, then the only reason could be that the 'full power' had to be drawn for a longer period after switch-on with the induction hob (to achieve the same result) - which to my mind would mean that the induction ring were 'less efficient' (took longer to heat things up with the same power) - which we are told isn't the case.

Kind Regards, John

Some time ago I experimented with the real life load of my induction hob and double oven using a clamp meter.

The situation I tested was 4 pans of cold water on the hob then turning on all rings and both ovens within a few seconds of each other. The pans come to the boil very quickly (a few minutes, no where near 10), but you dont ever leave rings on full very long in real life as you burn, boil over, or boil dry things very quickly. I dont see diversity on an induction hob would be any less than a conventional electric hob, possibly more so as your only having to heat up the pan and its contents rather than mass of a ring as already mentioned.

Below was the info I posted some time ago:

I have a 7.2Kw induction hob and 5.2Kw double oven. If you get the timming perfect in a non real life situation I can get them to draw their maximum load for a relativley short time (a few minutes). Once Ovens are up to temp and pots on hob have boiled and reduced to a simmer the draw was a lot less than half maximum (Can't remember exact figure of top of my head but it was suprising).

Read more: https://www.diynot.com/diy/threads/diversity-factor.532213/page-6#ixzz6uDAc5mMG

reds42 said:

Some time ago I experimented with the real life load of my induction hob and double oven using a clamp meter. .... The situation I tested was 4 pans of cold water on the hob then turning on all rings and both ovens within a few seconds of each other. The pans come to the boil very quickly (a few minutes, no where near 10), but you dont ever leave rings on full very long in real life as you burn, boil over, or boil dry things very quickly.

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Thanks - that's what I would have expected.

reds42 said:

I dont see diversity on an induction hob would be any less than a conventional electric hob, possibly more so as your only having to heat up the pan and its contents rather than mass of a ring as already mentioned.

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Again, as I have been saying, on theoretical grounds that is also exactly what I would have expected. As I've said, the only way in which I could envisage the standard diversity calculation being any different (in a 'bad' direction) for an induction hob would be if it were somehow 'less efficient' than conventional ones - which is contrary to everything we are told, probably mainly for the reason you mention (the amount of energy needed to heat a pan and contents will obviously be the same, no matter of how it is heated - the only difference being energy wasted in 'unnecessarily' heating other things.

Kind Regards, John

My only thoughts on induction hob efficiency would be there is some energy lost from the power electronics running the rings getting hot (my induction hob has cooling fans for the electronics), but I assume those losses must be less than the unnecessary heating of the extra mass in a conventional hob.

reds42 said:

My only thoughts on induction hob efficiency would be there is some energy lost from the power electronics running the rings getting hot (my induction hob has cooling fans for the electronics), but I assume those losses must be less than the unnecessary heating of the extra mass in a conventional hob.

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Sure, that's the (really only) source of 'additional energy loss' with an induction hob - but, as you imply, if that were not at least 'cancelled' by the absence of some of the other 'energy wastage' associated with conventional hobs, I wouldn't think we would be seeing/hearing so many assertions that they are 'more efficient'.

One also has to remember that a fair proportion (perhaps even 'most'?) of the heat dissipated in that 'extra mass' in a convention hob will eventually be transferred to the pan/contents, rather than being 'wasted'.

I also suspect that any 'efficiency advantage' of an induction hob will diminish with increasing 'cooking times' - once that 'extra mass' in a conventional hob has been heated, it will tend to stay hot, hence not needing a lot of energy to keep it up to temp, whereas I imagine that the heat losses due to the induction hob's electronics (the heat blown into the room by a cooling fan) will be more-or-less pro-rata to the duration of cooking.

For those reason, I suspect that the difference in 'efficiencies' is probably pretty small, perhaps not large enough to be of any real consequence - since in both cases the majority of the energy is being used to heat "the pans and their contents" etc. , with those other two phenomena considered above tending to 'cancel'.

Kind Regards, John

Hi, I think you all missed the point about having two generators on the hob. My hob is a 4.5kw Bosch. Each generator is 2.25kw and feeds two rings. It can feed the full power to one ring or both. That's one of the selling points of induction, that you can deliver more power to one ring.

I'm not suggesting the aggregate demand over the cooking period would be any higher. I'm suggesting that it's a lot easier to draw the full rated power of the hob as you can do it with only two rings active and hence using the traditional diversity calcs would be more likely to lead to a thermal trip.

PS I regularly have the hob working at max capacity. Pan of veg boiling, pan of pasta boiling, pan of meat frying. You know when it's struggling because you can hear the relays switching power between rings.

bsr said:

Hi, I think you all missed the point about having two generators on the hob. My hob is a 4.5kw Bosch. Each generator is 2.25kw and feeds two rings. It can feed the full power to one ring or both. That's one of the selling points of induction, that you can deliver more power to one ring. .... I'm not suggesting the aggregate demand over the cooking period would be any higher. I'm suggesting that it's a lot easier to draw the full rated power of the hob as you can do it with only two rings active and hence using the traditional diversity calcs would be more likely to lead to a thermal trip.

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I'm not sure that represents much of difference from conventional (non-induction) hobs, many of which have at least a couple of ~2 - 2.5 kW rings, does it? Furthermore, I've never heard it suggested that "traditional diversity calcs" are not appropriate for cooking appliances with only one or two elements (again, often 2-2.5 kW each) - e.g. ovens and grills.

Kind Regards, John

bsr said:

PS I regularly have the hob working at max capacity. Pan of veg boiling, pan of pasta boiling, pan of meat frying. You know when it's struggling because you can hear the relays switching power between rings.

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Max capacity, yes - but for how long ? generally, if you were to leave veg or pasta on full power then they'd quickly start to boil over and/or boil dry. So it's normal to use full power till boiling, then turn them down considerably.

Also, if you are concerned about max power draw, I know that at least some of them have the option to configure max power - my mate's unit is like that. From memory, the default is to work on a 13A circuit, with options for a 16A circuit, or larger - on initial power up it prompts for the supply type.