Heating Effect - DC vs AC
Heating Effect - DC vs AC
(OP)
Hi,
Although it is known that the heating effect of DC is equal to AC at the same RMS, can anyone point me to any publications or references that substantiate this?
Thanks in advance.
Although it is known that the heating effect of DC is equal to AC at the same RMS, can anyone point me to any publications or references that substantiate this?
Thanks in advance.





RE: Heating Effect - DC vs AC
<P(t)> = <I^2(t)>*R where < > denotes average over time.
For dc current I(t)=I0, <I^2(t)> = I0.
For ac sinusoidal, <I^2(t)> = Irms = Ipeak/sqrt(2)
For any general signal nonsinudsoial or otherwise, <I^2(t)> = Irms (definition of rms).
Irms determines heating and plays the same role as I0 does for dc
Aside - If the frequency becomes high and/or conductor is large, skin effect may play a role to increase the effective resistance.
RE: Heating Effect - DC vs AC
1974-75 Radio Shack dictionary of electronics
"rms amplitude"
"The value assigned to an alternating current or voltage that results in the same power dissipation in a given resistance as dc current or voltage of the same numerical value."
RE: Heating Effect - DC vs AC
For dc current I(t)=I0, sqrt(<I^2(t)>) = I0.
For ac sinusoidal, sqrt(<I^2(t)>) = Irms = Ipeak/sqrt(2)
For any general signal nonsinudsoial or otherwise, sqrt( <I^2(t)>) = Irms (definition of rms).
Irms determines heating and plays the same role as I0 does for dc. It comes in straightfoward manner from the definition of rms.
RE: Heating Effect - DC vs AC
RE: Heating Effect - DC vs AC
RE: Heating Effect - DC vs AC
RE: Heating Effect - DC vs AC
I think what your trying to say is if we have a signal that contains DC and AC then the current that will cause heating is the TRMS of the AC portion plus the DC portion. If we simply have an AC waveform with no DC (average voltage = 0) then the TRMS value of this AC waveform by itself, has the same heating effect of just a DC signal with the DC=AC (TRMS).
If there is some new revelation into this age old information then I would like to know some sources of your information.
RE: Heating Effect - DC vs AC
"10A of battery DC current or rectified and smoothed full wave sinusoidal current will produce more heat than 10A RMS sinusoidal current"
RE: Heating Effect - DC vs AC
FYI
The test:
Thermocouples were inserted into a device at selected points. The device passed 200A AC (60Hz) for 2 hours until thermal stabilisation was achieved. Then the supply was switched to batteries (DC 500 Ah).
Result:
Alas, it was observed that the temperatures initially rose by several degrees C before the battery current began to drop off.
I had two significant problems with this test; firstly, maintaining a constant battery current, and secondly controlling the airflow and ambient temp around the device.
RE: Heating Effect - DC vs AC
If you believe conservation of energy and ohm's law, that leads to p(t)=i(t)^2*R
<p(t)>= <i(t)^2>*R
Definition of Irms is sqrt(<i(t)^2>)
therefore substitute Irms^2 = <i(t)^2>
<p(t)>= Irms^2*R
Assuming that R is constant there is nothing left to prove. What is the purpose of your test?
RE: Heating Effect - DC vs AC
You must understand that some people/customers are not always convinced with the math, they like to see the physical evidence - that's all!
RE: Heating Effect - DC vs AC
RE: Heating Effect - DC vs AC
RE: Heating Effect - DC vs AC
Also, how/why do you believe that the dc current created by the battery was equal to the rms current during the ac part of the test... what measurements did you take? And as buzz says when you measured ac quantities did you really measure true rms?
RE: Heating Effect - DC vs AC
RE: Heating Effect - DC vs AC
Well known Form Factor F is:
F=Irms/Iav=Irms/Idc=(1/0.9)=1.1111...,
for the sinusoidal wave.
RE: Heating Effect - DC vs AC
Cheers.