Effective value of an ac sine wave ?

This topic has 6 replies, 5 voices, and was last updated 16 years, 9 months ago by Andy Birchall.

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October 27, 2006 at 2:47 pm #314

Jason Graham
Participant

When you have the RMS (240v) to get the peak value you x by 1.41
240v x 1.41 = 338.4v

but when you have peak value (338.4v) you x it by 0.707
338.4v x 0.707 = 239.2488v (and not 240v

why not divide by the same amount (ie 1.41)??????????????

as in http://www.sea.siemens.com/step/downloads.html
part 2/2 page 50

many thanks paddy graham

October 27, 2006 at 3:06 pm #8645

truered
Participant

try this for good explanation.

http://www.ultracad.com/articles/rms.pdf

October 27, 2006 at 3:30 pm #8646

Jason Graham
Participant

Thanks truered
and now my brain hurts even more I’m going to go for a little
sleep sleep now, waken me up when spring is here.

Thanks chap all the best
paddy

October 27, 2006 at 4:36 pm #8647

Rons_ROV_Links
Participant

Hello Paddy,

You shouldn’t use 1.41 but the root of 2 and not 0.707 but 0.5* root of 2.

See also: http://en.wikipedia.org/wiki/Alternating_current

March 7, 2008 at 7:12 pm #8648

Andy Birchall
Participant

You absolutely CAN divide by the same amount, because mathematically it is the same thing.

I hope this isn’t too patronising. Apologies if it is. Either way, take a look:

1.414 ia a 3dp rounding of root2. 0.707 is a 3dp rounding of 1/root2.

Peak V = RMS x 1.414 1.414 is roughly the same as root2 so:
Peak V = RMS x root2

Likewise –

RMS = PeakV x 0.707 0.707 is roughly the same as 1/root2
RMS = PeakV x 1/root2 Therefore:
RMS = PeakV/root2 Which is roughly the same as:
RMS = PeakV/1.414

Hope this helps.

Regards,

Andy.

March 8, 2008 at 12:58 am #8649

deevalue
Participant

Sorry for moaning tonight – just new to the forum and nothing positive to say.

Is thsi for real?
surely rounding up / down was somewhere in your education??

This is like the joke my neice loves – 11 Fingers??

holds up both hands counts down from 10 and adds 5 to the 6 that she has reached. She is 7 – your age is??

March 8, 2008 at 11:06 am #8650

Andy Birchall
Participant

Lol. Yeah, rounding was in my GCSE Maths education.
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