On Wednesday, 29 November 2017 12:28:44 UTC, Martin Brown wrote:
On 29/11/2017 10:48, whisky-dave wrote:

Anyone with any insight would have known that 202/240 should give you the fraction that you use to multiply by so :-

No it doesn't.

Yes it does.


A heater of 2KW at 240V will consume 2000W
A heater of 2KW at 202V will consume 1683W

So if it's consuming 700W already we have 1683-700= 983W NOT 1300W that you expected.

He is more nearly right that you are.

No he is NOT because those are the real world results I get.
This is ONE of the reasons why we run both theory and practicals.



Anyone that understands the basics
of Ohms law will know that the power dissipation in a resistor R is I*V

= V^2/R

OK here's the resitances at cold of the relivant parts.
at 240V at 230V (the speicifed operational volatge)
I 66.4R 867W 797W
II 44.9R 1283W 1178W
III 27.1R 2125W 1952W


The power dissipated in a fixed resistor R scales as the *square* of the
applied voltage V all other things being equal.

Nominal 2kW at 240V = R = 29
So on a 202V supply it will consume 202^2/29 ~ 1.4kW

except it is wrong. Tim said the 1300W for just ONE of the 'resistors'
What I get 202^2 = 40804/44.9=908W NOT 1300W

But normmaly we have a voltage of about 215-220
217.5^2 = 47306/44.9=1054W NOT 1300W




R won't quite be constant but it won't vary by all that much either.
R might be a percent or so lower at 1.4kW since the element is cooler.

which element ? element I or II



Cheap and nasty resistive electric heaters are usually configured
series, single and parallel to present loads of V^2/2R, V^2/R and 2V^2/R
to the mains being low, medium and high heat settings respectively.

Yep and this is what I first suspected of this ~£25 heater.


You perhaps ought to worry if your lab mains voltage is as low as you
say it is something somewhere must be dropping nearly 30V.

I think it's the heaters as the lab was NEVER designed to be heated by heaters connected to the labs mains supply.

It really, is that simple.


Oh and I've run anothe rtest from cold.

with the heater switch to maximium and consuming about 1700W at 212V
I left it on and after 27mins 21 secs it went down to 685W.
Ambient temperature of room 19C .