Well spotted, it doesn't work that way. Power is the current multiplied by
the amps. And current is the voltage divided by the resistance. If you alter
the voltage you alter the current too so the power output will also change.
It follows then that A 230 V linear appliance used on a 240 V supply will
take 4.3% more current and will consume almost 9% more energy. A 230 V rated
3 kW immersion heater, for example, will actually provide almost 3.27kw when
fed at 240 V (nobody google that last sentence, I've plagiarised it :-)
Lets call old voltages and powers V1, P1, and new voltages and powers
V2, P2. Using P = VI and V = IR for both pairs, and eliminating R, we get:
P2 = (V2/V1)**2 * P1
So if we switch a shower operating at 9500W (P1) and 240V (V1) to a
230V(V2) supply, we can find the new power (P2):
P2 = (230/240)**2 * 9500 = 8700W (2SF)
So a shower that operates at 9.5kW on 240V power will only operate at
8.7kW on 230V power.
The only question that is left to be asked is how the current is affected.
Introducing I2 and I1 as the two currents, and using P = VI for each
with the previous result, we get:
I2 / I1 = V2 / V1 = 230 / 240 = 0.96
I2 = 0.96 x I1
So we can see that the new current is 4% lower than the old current.
In general, the power consumed decreases with the voltage.
In general, the current decreases with the voltage.
(Which is how most of us would guess that it worked in any case).
I'm not clear what you mean by your double*. I'm also unclear when you refer
to power if you mean the actual power consumed in the circuit or the power
figure quoted by the manufacturer. The former is only variable by altering
the voltage the latter is assuming a specified voltage.
Sorry if I'm being thick John but I'm struggling to see what you either
asking or telling us. Is your question rhetorical? If not then let me just
say this. Power is determined by the voltage applied to the load and the
resistance and or reactance of the load whether that be capacitive or
inductive. In a resistive load such as the shower the reactance is
negligible so the only consideration for the power consumed is P = (V/R) * V
where (V/R) is the current and V is the voltage. End of story.
The manufacturer works out what resistance the heating element should
have at a specified supply voltage, to give the required output power.
And then fits a heater with that resistance. If he is designing for a
lower voltage then yes, he will use a lower resistance element and have
to allow for the higher current needed to provide the same power.
Usually though, we see it from the user, rather than designer viewpoint.
The element resistance has already been fixed by the manufacturer. If
the supply voltage is lower than the specified voltage, then the current
and the power will be less too. If the supply voltage is higher than the
specified voltage, then the current and the power will be more.
Only if the element resistance could be changed could the same power be
given out for a range of input voltages. If the voltage was low, then
the element resistance would have to be reduced and the load current
increased to compensate.
I'm fairly sure they changed it. Or at leas, they changed the spec but
left the existing value within tolerances.
I've just gone and measured mine with two different DVMs - both reading
247 or 248 volts.
Make of that what you will. Perhaps being only a couple of miles from
Buildwas power station has something to do with it.
Roland Butter :- There\'s nothing like a knob of butter.
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