How to reduce power to a heating device.

[Dave Edwards]

I suspect the slow cooker has 2 heating elements, one is switched on at low power, the other is switched on at medium power, both are switched on at high power.

The electric drill is a different type of load, it has a motor in it which can have its speed altered by changing the magnetic fields inside it. The speed is not controlled by simply changing the power input as far as I know, but motors are not really my area so I may be wrong.

Whatever you do I suspect the device you have built will chew through relays pretty quickly as it will often reach the point where it enters rapid on/off switching cycle you have described.

The device I've made already doesn't do rapid switching. The shortest is around 1.5 to 2 seconds between on and off. That's mainly down to the LED in the meter which reacts quite slowly to the change between importing & exporting. But that's also part of the reason I'm still using grid power, as the power used during switching is metered.

 

[plugsandsparks]

Yes it can be done with power electronics. Is there a product out there, dunno.
You could build it yourself if you have time to research it , get the components and build it into a box.
Obviously its all dangerous voltages, i would research PWM cct with 50% duty cycle through a solid state relay. Good Luck

 

[davesparks]

I really just wanted to know if you can take a 240V 13A supply and somehow change it to a 240V 6A supply.
I should have just asked that question really.
My Bad!
Trouble is I still don't have the answer really.
It seems variable resistors or the like are going to generate lots of heat and waste power.
Leaves me wondering how other equipment that varies output power seems to do it without those problems.
I guess there's no easy answer.
Thanks for all replies and help and ideas though.
Much appreciated.

There are many ways to vary power output, with simple heaters the answer is often to have multiple smaller heating elements and switch them on/off to add up to the required power.
However true variable control of the power is possible, after all that's how the iboost works, by varying the power.

The circuit in the iboost is effectively a dimmer, but it has been designed for the specific load and installation conditions. It has heat dissipation and airflow designed in and will have safety circuits to prevent overheat etc.


Looking at products that could be used for the power control you are looking for I have found that they exist more commonly than I had expected.
I am hesitant to suggest any particular product as I would want to take it apart and see how well protected against overheat it is before using it. I don't want to advise any particular product in case it causes a fire and burns your house down.
But you could look at the devices used to control infrared heaters in vivarium's, stables and, apparently, pub gardens. There appear to be a number of 3kW devices, mostly looking a bit too 'home made' for my liking.
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The device I've made already doesn't do rapid switching. The shortest is around 1.5 to 2 seconds between on and off. That's mainly down to the LED in the meter which reacts quite slowly to the change between importing & exporting. But that's also part of the reason I'm still using grid power, as the power used during switching is metered.

A 2 second on/off cycle is still not good and the really won't last very long with a 3kW load!

 

[pc1966]

The "perfect" way to adjust the power is to use a variac - a variable ratio autotransformer.
But that is not cheap, light, small, or automatic. For example:

MVT3.3k/12A-1E | Carroll & Meynell 1 Phase 3.3kVA Variac, 1 Output, 240V | RS

uk.rs-online.com

@davesparks suggestion of a using proven diverter is the most sensible route. I have fitted the earlier version of this to a friend's home who has 4kW of panels (less Scotland's weather...) and they, and another guy I know, have had them work well for a number of years:

SolarImmersion Surplus Solar PV Water Heating Solar Immersion Controller Switch

SolarImmersion Intelligent solar PV energy storage or solar immersion controller switch diverts surplus solar PV power to heat water for free. Simple, efficient & affordable.

solarimmersion.co.uk

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It may or may not be relevant, but you can also get various powers of immersion elements as well for a fixed change:

Products

 

[Dave Edwards]

There are many ways to vary power output, with simple heaters the answer is often to have multiple smaller heating elements and switch them on/off to add up to the required power.
However true variable control of the power is possible, after all that's how the iboost works, by varying the power.

The circuit in the iboost is effectively a dimmer, but it has been designed for the specific load and installation conditions. It has heat dissipation and airflow designed in and will have safety circuits to prevent overheat etc.


Looking at products that could be used for the power control you are looking for I have found that they exist more commonly than I had expected.
I am hesitant to suggest any particular product as I would want to take it apart and see how well protected against overheat it is before using it. I don't want to advise any particular product in case it causes a fire and burns your house down.
But you could look at the devices used to control infrared heaters in vivarium's, stables and, apparently, pub gardens. There appear to be a number of 3kW devices, mostly looking a bit too 'home made' for my liking.
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A 2 second on/off cycle is still not good and the really won't last very long with a 3kW load!

I take full sole responsibility for burning my house down or not!
That's the main reason I came here to get advice.
I would only want a device that is safe, and the solid state relay I have at the moment I've mounted in a good metal heat sink box, and it'sbeen working for a few months now without giving up or overheating, plus I've not put a load of 3kW on it. That's another reason I want a power supply of up to 2kW max.

So you said :
"Looking at products that could be used for the power control you are looking for I have found that they exist more commonly than I had expected."
Are you now saying you're not willing to point me in the direction of some of them so I can have a look and a think about possibilities?

 

[davesparks]

So you said :
"Looking at products that could be used for the power control you are looking for I have found that they exist more commonly than I had expected."
Are you now saying you're not willing to point me in the direction of some of them so I can have a look and a think about possibilities?

A little later in that same post I told you could look at the infrared heater dimmers being sold for use in vivariums, stables etc

 

[DaveV72]

Reducing the voltage to 110V won't reduce the power to 1500W it will reduce it down to about 600W and the current will be around 6A.

In basic terms the resistance of the element will dictate the current which flows for the applied voltage, this then dictates the power which dissipates. Its a bit more complex than this due to things like the change of resistance due to temperature.


The easiest way to achieve what you want is to use a solar immersion heater controller which is designed for this exact task, such as the solar iboost.
These use a current sensor on the incoming supply to detect exactly how much power is being exported to the grid and then regulate the immersion heater until there is near zero power being exported or imported.
This basically operates like a 3kW dimmer switch which is electronically controlled.

This is what I have been trying to figure out. I'm worried that the iboost will draw power from the grid if the power required for a 3kw element isn't met. So if only 2kw of solar power is being produced, the immersion will draw 1kw from the grid. Or does the iboost prevent that? And will the element still heat up the water at a reduced rate/lower temperature?

 

[w0z]

A 4kW dimmer in series with the (a resistive) heating element (as, FOR EXAMPLE sold by a well known chinese website) would work with a resistive load, you could set the current to be a specific value (if you're able to measure it safely).
It's a cheap manual solution, apologies if I've missed the gist.

 

[bill01803]

11” / 279mm 1.5kW Solar Panel Immersion 09899SP - https://www.backer.co.uk/products/11-279-mm-1-5kw-solar-panel-immersion-09899sp