Power factor

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Besoeker

Senior Member
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UK
Offset in the sense the mean is not zero?

Are you (also?) saying there cannot be a DC component to an AC-supply-power waveform?
Back to this. Supply power generally comes from a power transformer. DC in that would be unusual. Saturated transformers and all that.

I have seen power circuits containing both DC and AC components but this has been the DC output of rectifier circuits. I was involved in the design of 6, 12, and 24-pulse rectifiers up to several MW.

6Prectifier04_zps5e3a326b.jpg


As shown, there is an AC component in the DC at six times supply frequency but the supply side of the bridge doesn't see a mixture of AC and DC.
 

Ingenieur

Senior Member
Location
Earth
It's a power waveform. If the is no direct current in the power transfer, there can be no DC offset in the power waveform. It is simply a shifted or offset waveform... no DC offset.

it has a DC component
v = sinwt, note no dc
i = sinwt, note no dc
assume pf=1
v x i = sinwt^2 = 1/2 (1 - cos2wt), note now there is dc
the 1/2 x 1 is a DC offset component
 

Ingenieur

Senior Member
Location
Earth
6Prectifier04_zps5e3a326b.jpg


As shown, there is an AC component in the DC at six times supply frequency but the supply side of the bridge doesn't see a mixture of AC and DC.


Yes it does
you can measure the ripple
but you are mixing apples and oranges, rectifiers with none

we had a scenario where a ac relay was applied to detect a vfd dc bus fault
fault was limited to (1.35 x 480) /2 / 18.4 Ohm = 17.6 A by a NGR
if the relay (Bender 423 series) was turned down to 1 A it would trip
it was rated for > 30 Hz
it would not trip at >1 A
we scoped it
it was seeing the 4-6% ac ripple
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
yep
it represents the positive power flow
it is why the Parks dq transformation is useful
negates the cancelling ac component and leaves the dc
makes the math much simpler

Total signal = DC + sin(wt + ph ang)

The DC part has me now confused.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
You guys are using different terms to refer to the same math.

When the I and V waveforms have 0 average value (usual AC, no "DC" components), then the product I*V waveform typically will not have 0 average value. It will be, e.g., a sinewave with an offset, i.e. sin(a*t) + b. The disagreement is whether to call that offset a "DC" components to the power waveform or not.

Cheers, Wayne
 

Ingenieur

Senior Member
Location
Earth
How though?

assume
pf = 1
all pu quantities
single phase
v = sinwt
i = sinwt
w = 2 Pi f

Power = v x i = sinwt x sinwt = sinwt^2 = 1/2(1- cos2wt)
simple trig identity

you now have a power signal:
Whose frequency is 2 x f
whose phase is shifted by 90 deg from v and i (sin vs cos)
whose AC component magnitude is 1/2 of either v or i (1/2cos2wt bs 1sinwt)
And who has a DC component offset of 1/2
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
assume
pf = 1
all pu quantities
single phase
v = sinwt
i = sinwt
w = 2 Pi f

Power = v x i = sinwt x sinwt = sinwt^2 = 1/2(1- cos2wt)
simple trig identity

you now have a power signal:
Whose frequency is 2 x f
whose phase is shifted by 90 deg from v and i (sin vs cos)
whose AC component magnitude is 1/2 of either v or i (1/2cos2wt bs 1sinwt)
And who has a DC component offset of 1/2

Would this not be RMS power instead of DC?
 

Smart $

Esteemed Member
Location
Ohio
it has a DC component
v = sinwt, note no dc
i = sinwt, note no dc
assume pf=1
v x i = sinwt^2 = 1/2 (1 - cos2wt), note now there is dc
the 1/2 x 1 is a DC offset component
No DC...

Yes, the sinusoidal waveform is offset (i.e. the mean value is not zero), but it is not "DC offset".
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
the waveform has an ac and dc component
I think we can agree it has a constant component and a sinusoidal component. Apparently many find it objectionable to refer the the constant component of a power waveform as a DC component, since it is not a current waveform, and the current waveform in question has no DC component.

Cheers, Wayne
 

Ingenieur

Senior Member
Location
Earth
I think we can agree it has a constant component and a sinusoidal component. Apparently many find it objectionable to refer the the constant component of a power waveform as a DC component, since it is not a current waveform, and the current waveform in question has no DC component.

Cheers, Wayne

but power, the product of v and i does
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
but power, the product of v and i does
Yes, it has a constant component, the disagreement is in terminology, should that constant component be called a "DC" component, when it arises strictly from AC? I would just call it a constant component.

Cheers, Wayne
 
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