Odd power usage by freezer.

[winston1]

Take an extreme example. 240 v at 10 amps for 6 minutes, or 480v at 20 amps for 3 minutes. Average current is the same.

 

[ericmark]

Not according to Mr Ohm.

The heavy loads in a house are generally heating appliances which certainly obey Ohms law. Smaller electronic items will generally take less current as voltage increases. I would think overall as voltage increases, current increases.

Although current for a 2.8 kW kettle goes up, making it a 3 kW kettle the time goes down, 2.4 minutes to 2 minutes so with a load of items used randomly the net current goes down as the voltage goes up if there is some form of thermostatic control.

 

[JohnD]

Take an extreme example. 240 v at 10 amps for 6 minutes, or 480v at 20 amps for 3 minutes. Average current is the same.

take a much less extreme, and much more sensible, example.

twenty houses down my street cooking their sunday dinner in electric ovens

At 240v:
click - one comes on
click - ones goes off
click - one comes on
click - ones goes off
click - one comes on
click - ones goes off
At 230v:
click - one comes on
click - ones goes off
click - one comes on
click - ones goes off
click - one comes on
click - ones goes off

it doesn't matter if they are on for 6 minutes or for 5 minutes at a time, due to slightly faster heating as a result of slightly higher voltage.

Not only is average power the same over an hour, with so many ovens clicking on and off, average power is the same over a minute or a second. But average current is slightly lower at the higher voltage.

The same goes for electric heaters.

 

[ericmark]

Sorry post between my posting 4.3% increase in current and a 16% decrease in time. So net amps of many items is lower assuming some thermostatic control.

 

[ericmark]

And yes very sad I actually worked it out. Used 4.19 kJ per kc. And freezer is thermostatically controlled although not sure what happens to a motor over voltage?

 

[SimonH2]

Not according to Mr Ohm.

The heavy loads in a house are generally heating appliances which certainly obey Ohms law. Smaller electronic items will generally take less current as voltage increases. I would think overall as voltage increases, current increases.

Yes, perhaps I missed off some detail - which others have filled in. For loads which do increase power taken as voltage goes up - then the DNO gets to carry more metered power at the same I²R losses per unit carried. For loads which are constant power - either because they are switch mode with a constant load, or they are thermostatically controlled and so reduce the duty cycle - then the DNO gets to carry the same metered power for less I²R losses.
Outside of industrial users with large quantities of (eg) motors running DOL and huge workshops/offices full of flouro tubes, I think you'll find that a very significant proportion of the load is now in the latter category.

 

[JohnW2]

Take an extreme example. 240 v at 10 amps for 6 minutes, or 480v at 20 amps for 3 minutes. Average current is the same.

Did you really mean to write that, or do you want to reconsider? Whilst what you say is all correct, you are comparing chalk and cheese - two different amounts of energy (0.24kWh and 0.48kWh respectively). Even given that, you ought to be able to see that, in terms of your example, with the higher voltage you are getting twice as much energy for the same average current.

In terms of the discussion, it is more meaningful to compare for the same amount of energy - say 0.24kWh, and say that with a 240V supply that was achieved by 10A for 6 minutes. To get the same 0.24kWh of energy with 480V, that might be 10A for 3 minutes, or 5A for 6 minutes or whatever- but, in all cases, the average current over 6 minutes would be 5A - half of that with the 240V supply.

Kind Regards, John

 

[JohnW2]

Yes, perhaps I missed off some detail - which others have filled in. For loads which do increase power taken as voltage goes up ....

Can you think of any high-energy loads (high power for long periods of time) that fall into that category, at least in non-industrial settings?

Incandescent bulbs/lamos were an example (although not massive energy) but they are a dying breed. Almost all heat-producing (or 'cold-producing'!) loads are thermostatically controlled, so I'm not sure what's left.

Kind Regards, John

 

[EFLImpudence]

I'm getting confused, but -

Did you really mean to write that, or do you want to reconsider? Whilst what you say is all correct, you are comparing chalk and cheese - two different amounts of energy (0.24kWh and 0.48kWh respectively). Even given that, you ought to be able to see that, in terms of your example, with the higher voltage you are getting twice as much energy for the same average current.

Isn't the average current with varying voltage the subject being queried?

In terms of the discussion, it is more meaningful to compare for the same amount of energy - say 0.24kWh, and say that with a 240V supply that was achieved by 10A for 6 minutes. To get the same 0.24kWh of energy with 480V, that might be 10A for 3 minutes, or 5A for 6 minutes or whatever- but, in all cases, the average current over 6 minutes would be 5A - half of that with the 240V supply.

Yes, but that's not how resistive loads work.

If the voltage is doubled the current will be doubled.

 

[SimonH2]

Can you think of any high-energy loads (high power for long periods of time) that fall into that category, at least in non-industrial settings?

I was thinking about that as I wrote, and in a domestic setting I think the only ones would be electric heaters. As you say, these days almost all are thermostatically controlled, but I think you'll find that some are used in an "I'm cold, put the fire on" mode. In fact, the "fire" (2kW on max) in my flat is non-thermostatic. Similarly, my mate has an Optimist which I think is also non-thermostatic.
Neither of those is a primary heating method since the properties both have gas central heating - the fires are largely decorative.

I wonder how many people still have, and use, "shall we have one bar or two on" electric fires ?

 

[SimonH2]

Take an extreme example. 240 v at 10 amps for 6 minutes, or 480v at 20 amps for 3 minutes. Average current is the same.

Actually, it would be 20A @ 480V for 1.5 minutes to get the same amount of energy out. Thus, over 6 minutes the average current would only be 5A - half that taken at 240V.

 

[EFLImpudence]

Ah, that's it.

 

[JohnW2]

I'm getting confused, but - Isn't the average current with varying voltage the subject being queried?

Indeed so, but ....

Yes, but that's not how resistive loads work. If the voltage is doubled the current will be doubled.

You don't seem to have been following the flow of the discussion Winston's comment/example was, I presume (he didn't quote anything), a response to my previous post in which I said that nearly all large resistive loads (that's large loads which are resistive, not loads with a high resistance ) are thermostatically controlled, and hence consume roughly the same amount of energy (losses may vary) regardless of voltage. It therefore follows that with such a load, increasing the voltage will reduce the average current (for the same, thermostatically controlled, energy consumption). As SimonH2 has just agreed with me (in response to my question) non-thermostatically controlled large resistive loads are fairly uncommon, and gradually disappearing, at least in non-industrial environments.

Kind Regards, John

 

[JohnW2]

Ah, that's it.

Why are you agreeing with Simon when he says that, but challenging me when I said essentially the same thing?

Kind Regards, John

 

[winston1]

Did you really mean to write that, or do you want to reconsider? Whilst what you say is all correct, you are comparing chalk and cheese - two different amounts of energy (0.24kWh and 0.48kWh respectively). Even given that, you ought to be able to see that, in terms of your example, with the higher voltage you are getting twice as much energy for the same average current.

In terms of the discussion, it is more meaningful to compare for the same amount of energy - say 0.24kWh, and say that with a 240V supply that was achieved by 10A for 6 minutes. To get the same 0.24kWh of energy with 480V, that might be 10A for 3 minutes, or 5A for 6 minutes or whatever- but, in all cases, the average current over 6 minutes would be 5A - half of that with the 240V supply.

Kind Regards, John

Yes, I got it wrong. Of course if you double the voltage and current you quadruple the instantaneous power so the time should have been a quarter not half. Then the average current would have been half. Apologies.