resistance and temperature

[ΩΩΩ]

Curious about the cost of running appliances.
I understand the basic kWh rating to get an idea
But have been playing around with a plug in small oven.
Its rated at 1300 watts, but uses 1400w according to my plug in energy monitor
It draws about 5.8 Amps
I measured the resistance between live and neutral at the plug top when cold got about 34Ω
When the oven had been in use and hot, I unplugged and measured around 200Ω
Thing is watching the current on the monitor it stayed constant at around 5.8A ,unless the oven switch off heating.
So the changing resistance measured doesn't seem to affect current flow?
Wonder why?

 

[bernardgreen]

At switch on when cold the heating element at 34Ω will take 7 Amps (240/34) which is 1,680 Watts (7x240)

When the element heats up it's resistance will increase slightly which will reduce the current to 5.8 Amps that you are reading.
5.8 Amps at 240 volts is 1,392 Watts (5.8x240)

When the element is 200Ω the current taken will be 1.2 Amps which is 288 Watts

I would suggest that 200Ω reading is not the main element but some auxiliary item such as a fan and the main element has been switched out of circuit before you read the resistance, ( thermostat or other control item )

 

[Taylortwocities]

Its rated at 1300 watts, but uses 1400w according to my plug in energy monitor

Pinch of salt on those. Some makers quote their appliances given a 240volt supply, most of us get given less than that.
Does your energy monitor tell you what your supply voltage is?

 

[ΩΩΩ]

Thank you,
Re the voltage. Yes its 239V
PF of 1

 

[EFLImpudence]

1300W @ 230V equates to 1400W @ 239V

 

[SUNRAY]

Thanks to Rodders for highlighting my resistance calculation was at 230V
Well 239V @ 5.8A is 41.2Ω & 1386W
239V @ 34Ω is 7A & 1680W

 

[Rodders53]

Ratings plates for devices like this (kettles, toasters etc.,.) are often quoted at the EU standard of 230 V input.
Put UK typical 240 V on the device and it is more Watts.
Uses little if any more energy but things do heat up quicker.

Well 239V @ 5.8A is 39.6Ω & 1386W

I was taught that V = I * R, W= V^2/R = V * I = R * I^2

239 * 5.8 = 1386.2 W so R = 41.2 ohms by my calculator.

Using that 41.2 ohms at 230 V = 1284 Watts and 240V 1398 Watts = which is in broad agreement with the rounded up 1300 W on the rating plate and what @EFLImpudence wrote, above. Meter inaccuracies notwithstanding.

 

[SUNRAY]

Ratings plates for devices like this (kettles, toasters etc.,.) are often quoted at the EU standard of 230 V input.
Put UK typical 240 V on the device and it is more Watts.
Uses little if any more energy but things do heat up quicker.

I was taught that V = I * R, W= V^2/R = V * I = R * I^2

239 * 5.8 = 1386.2 W so R = 41.2 ohms by my calculator.

Using that 41.2 ohms at 230 V = 1284 Watts and 240V 1398 Watts = which is in broad agreement with the rounded up 1300 W on the rating plate and what @EFLImpudence wrote, above. Meter inaccuracies notwithstanding.

It took me a while to realise you had written W = V²/R = V*I = I²*R.

Using the rating plate figures of 230V & 1300W I make it 40.6923Ω and hence; 1403.7W @239V which is the calculation I assumed he used... and 1415W @240v.

I'm finding a real hotch potch of figures on rating plates these days and on a number of occasions have found the rating of 230V however subsequent tests indicate seem to relate to 220V
As an example a replacement 3KW drinks water boiler kept overheating a 13A plup/socket.
The plate shew 230V, 3KW, 13A however my clamp measured >15A and calculating resistance from actual voltage indicated 3KW @ 220V and 13.7A
An original adjacent machine shew 220-240V, 2500-3000W, 13A and measured <13A
Measuring resistance of the machines with a cheap multimeter gave 17 & 20Ω (probes shorted tends to be around 0.8 to 1Ω so more like 16 - 16.2 & 19 - 19.2Ω) I concluded the machines were actually rated at 220V and 240V.

 

[JohnW2]

I was taught that V = I * R, W= V^2/R = V * I = R * I^2

You were taught correctly

Kind Regards, John

 

[JohnW2]

All of this discussion about temperatures/resistance of elements and power consumption is all very well, but in terms of (total or average) energy usage over a period of time, people seem to be ignoring the most important factor - namely that the thermostat will be turning the element on and off (more often 'off' than 'on' once the oven has warmed up), so that, no matter what the power used when the elemnt is 'on', the average (hence total) over a period of time will be much lower than that.

Kind Regards, John

 

[ΩΩΩ]

Thank you for the comments
Bernard you mention an auxiliary item that could be causing the 200Ω resistance when hot.
I left the meter connected as it cooled down and it very slowly started to go down back to the 30Ω range.

At JohnW2 Yes it took about 2:30 minutes to warm up , then off for around 50s, well, only using 40 Watts. Then on for 50s, and so on. The on of periods were so similar it was if it was timed rather than a temperature sensor.

 

[Alec_t]

I'd guess the resistance variation was due to an NTC surge-supressor in series with the heating element?

 

[bernardgreen]

I left the meter connected as it cooled down and it very slowly started to go down back to the 30Ω range.

Some heating element materials such as molybdenum disilicide have a large difference between their resistance at room temperature and their resistance at 200ºC and a ratio of 200 / 30 ( hot / cold ) might be possible.

That said is it highly unlikely that Molybdenum disilicide would be used in an element for a domestic oven

 

[JohnW2]

At JohnW2 Yes it took about 2:30 minutes to warm up , then off for around 50s, well, only using 40 Watts. Then on for 50s, and so on. The on of periods were so similar it was if it was timed rather than a temperature sensor.

Those are strange observations, particularly if the "and so on" goes on indefinitely.

It would make no sense for an oven element to be controlled by a timer (rather than thermostat) in the manner you suggest. It would probably result in the oven getting far too hot, possibly dangerously so.

I've posted the graph below in the past. As I've previously explained, I don't have an electric oven, but I do have a (thermostatically-controlled) 1,700 W fat fryer. As you can see, after initially being 'on' for about 8 minutes, until it gets 'up to temperature', the element is just occasionally 'on' again for 1-2 minutes, to to 'top up' the heat to the required temp as it cools down. I would expect an electric oven (or a hob ring etc.) to behave, conceptually, in much the same way.

Kind Regards, John

 

[JohnW2]

I've posted the graph below in the past. As I've previously explained, I don't have an electric oven, but I do have a (thermostatically-controlled) 1,700 W fat fryer. As you can see, after initially being 'on' for about 8 minutes, until it gets 'up to temperature', the element is just occasionally 'on' again for 1-2 minutes, to to 'top up' the heat to the required temp as it cools down. I would expect an electric oven (or a hob ring etc.) to behave, conceptually, in much the same way.

I should perhaps have added ...... Although the peak power drawn was 1,700 W (about 7.4 A at 230V), the average over the first 60 minutes was only about 482 W (about 2.1 A) - i.e. about 28% of the maximum. In other words, during the first hour (from cold) the fryer consumed only about 0.48 kWh ('units') of electricity, as compared with the 1.7 kWh it would have been had the element been 'on' continuously.

Kind Regards, John