Geyser 4kw to 1kw draw

[Axle]

Hi Guys,

First post here, I have an interesting problem that I hope someone can assist me with. Out here in sunny South Africa we go through fairly regular power cuts, now I have a small battery / inverter system that copes with my lights / tv etc but our biggest problem is the geyser. One knows that generally when there is a power outage the geyser is at the temperature one has set it at, but due to no power it slowly starts to cool. I have a 1Kw generator and I would like to know if there is something that can be put into the wiring between the generator and the geyser’s element (4kw) that will only allow the element to draw one Kw, thus keeping the temperature stable and allowing the generator to run?

Is this possible?

 

[duke37]

If the generator is AC, then putting the power through a transformer giving out half voltage, the power will be reduced to a quarter. The transformer will be quite a big lump.

The power could also be reduced by a phase control circuit but the generator may object to this.

 

[Axle]

I've thought of the transformer route and I also don't think it's pratical, could you explain a little more on the circuit route and why the generator would 'object' it is AC (220v 50hz in SA). Many thanks for the assistance so far.

 

[duke37]

An AC supply has a voltage which varies as a sine wave, going positive and negative. It is possible to delay the connection to the load at some point in the sine wave. For instance if the load is connected at the peak voltage until the voltage crosses zero, the power will be halved. This is the principle of the light dimmer. A very simple circuit is used which contains a triac which is triggered to conduct and stays conducting until the current drops to zero which it does in an AC circuit.

The distortion of the waveform can cause problems and the utilities specify limits of harmonics put into the supply. I used to use a large DC motor fed from AC mains with a chopped supply. If I turned up the torque, I could blow all the lighting fuses in the complex!

I do not know how the method that your generator uses to control its voltage but a chopped waveform is likely to upset it. To get 1kW instead of 4kW will still need a large peak current at switch on. It is many years since I integrated over part of a sine wave - a little bit of maths for you?

Light dimmers come with about 4A capability, perhaps you could get larger or perhaps you could drive a bigger triac (20A?) with one.

Taking the power over part of the waveform will increase the resistive loss in the generator so the generator may overheat. Considerable experimentation and measurement would be necessary to get a working system.

 

[(*steve*)]

The generator may object because it is being askes to provide 400% of the power 25% of the time (or thereabouts).

It probably peaks out well under this.