Induction hob operation.

Bendy_House

Hi.

Do I take it that induction hobs don't use thermostats to control their temp, but electronic control that adjusts the hob's output? So a 1.8kW 'plate', for example, would be adjustable from, ooh, ~500W to the full 1.8kW? It doesn't 'click' on and off at the 1.8kW rate?

Is that the principle?

Cheers.

FreeBear

Can't comment on the top spec hobs, but mine pulses on and off. The higher the power setting, the longer the pulses. So average draw at slow simmer could be 500W whilst full power would be 1.8KW. Peak draw when pulsed on, 1.8KW.

In addition, there are sensors in each heating zone that should shut it off if the temperature rises to silly levels. e.g. when a pan boils dry and is left unattended.

fenwick458

induction hobs just use a different way of heating the pan,  any hob (induction of ceramic) has power control and this directly controls the amount of power sent to each ring.
I'm not entirely sure but I think they will both use thermostats (or similar) to control the temp
the extra efficiency comes from the fact that induction doesn't waste energy by heating the whole of the cooktop.

an electric shower in comparison is different. it will use the full 8.5kw to heat water when it's turned on, and the temperature is adjusted by altering the flow of the incoming cold water. more flow, colder, less flow warmer

grumbler

fenwick458 said:

I think they will both use thermostats (or similar) to control the temp

AFAIK, majority of them certainly don't. Only some from the most expensive ones control the temperature. Otherwise it's the power.

Bendy_House

Thanks all.

My understanding is, since it's such a fast-reacting method of providing heat, it cannot use a 'thermostat' as such, since the 'on' and 'off' periods would be too rapid. But, as FB says, his hob uses a 'pulse' method, and that is effectively the same thing, and is often used for, say, motor speed control; 'pulse width modulation'. 

I wonder why it doesn't (IF it doesn't) just use an actual 'power output' control such as a Triac? Ie, as you turn it down to '1', the outputted power would drop to, say, a steady 100W or so? Or, is a Triac just a rapid switcher too?

I guess it's effectively the same thing!

Just curious.

grumbler

Don't ignore the thermal inertia of a pan. Power is fast-reacting, but temperature changes less fast. You don't even need modulation - like in irons that are thermostatic and controlled by switching on and off. Yes, you can argue that it's still sort of 'pulse-width'.

Triac is just a switch that can be used for the same pulse-width modulation or contactless switching alternating voltage/current on and off.

Bendy_House

grumbler said:

Don't ignore the thermal inertia of a pan. Power is fast-reacting, but temperature changes less fast. You don't even need modulation - like in irons that are thermostatic and controlled by switching on and off. Yes, you can argue that it's still sort of 'pulse-width'.

Triac is just a switch that can be used for the same pulse-width modulation or contactless switching alternating voltage/current on and off.

Ah, I'd assumed that a Triac was like a transistor - a variable input-output. But it's just an electronic on-off switch?

bob_a_builder

A triac is used in things like light dimmers - don't know about induction hobs
it is effectively pulse width modulation, in that the load is switched on earlier or later in the cycle of 50 hz sine wave of  the ac mains to give greater or lesser % of power

FreeBear

Bendy_House said:

grumbler said:

Don't ignore the thermal inertia of a pan. Power is fast-reacting, but temperature changes less fast. You don't even need modulation - like in irons that are thermostatic and controlled by switching on and off. Yes, you can argue that it's still sort of 'pulse-width'.

Triac is just a switch that can be used for the same pulse-width modulation or contactless switching alternating voltage/current on and off.

Ah, I'd assumed that a Triac was like a transistor - a variable input-output. But it's just an electronic on-off switch?

Nope. Once triggered, you get full current flow across a triac.

If you want to control the current flow through a device, an IGBT is better suited for high voltage high current applications. But even with IGBTs in use, if you are varying the current in order to control temperature, you are going to generate a lot of heat within the device. Much better to use Pulse Width Modulation as there is greater thermal inertia, and the switching losses much lower.

An induction hob will (probably) be using IGBTs to control all the high power stuff in an iduction hob - They are pretty cheap nowadays and easier to interface with digital control systems.

grumbler

Bendy_House said:

grumbler said:

Don't ignore the thermal inertia of a pan. Power is fast-reacting, but temperature changes less fast. You don't even need modulation - like in irons that are thermostatic and controlled by switching on and off. Yes, you can argue that it's still sort of 'pulse-width'.

Triac is just a switch that can be used for the same pulse-width modulation or contactless switching alternating voltage/current on and off.

Ah, I'd assumed that a Triac was like a transistor - a variable input-output. But it's just an electronic on-off switch?

Any transistor-like device, operating not in a switch (full on/off) mode, effectively controls power by dissipating it.This is not just a waste of energy, but a technical problem as well (overheating). AFAIK, all modern high-power devices use some sort of pulse modulation. Switches are either fully opened (~ zero voltage drop on the switch)  of fully closed (zero current). Hence, almost no power to lose/dissipate.

Bendy_House

Fab, folks - thanks.