Phase angle firing is a term that usually refers to controlling the
point in a cycle when a thyristor is turned on. If firing is very
early, near 0 degrees, then the thyristor conducts for most of the
half-cycle and current can flow to the load for most of the cycle. If
firing is delayed to nearly 180 degrees, then current only flows to the
load during the very late portion of the half-cycle.
In a resistive only load, the current is in phase with the voltage when
the thyristor is conducting, but effectively shut off during the first
part of the half-cycle before the thyristor is fired.
Oh ok. that's what I am looking for. I think you answered my question. I am
trying to settle
a dispute. I have Phase angle fired controllers
on a purely resisitve load only and they are Porprtional
Zero cross triacs and they have 100% power factor
acording to specs. That tells me there is no shift. I am being told my power
meaurment is off becuse he thinnks the controller is changing the phase
angle of current to voltage. I don't see how anything else could cause
this. there are no caps or inductors in the circut.
When I told him this he says hes not talking about
Inductance and phase angle relationship between current voltage. So I have
no clue what he means. To me that is the only thing that could cause a power
The description of what you are using isn't clear enough to me to
identify what it is. On one hand, you refer to phase angle firing
which is like a standard light dimmer. On the other hand you make
reference to Zero cross which might be referring to zero crossing
switching, which is a different technique for heating loads, where
on/off switching is always done at the zero crossing point (no phase
control) and the proportional control is done by varying the ratio
of on half-cycles to off half-cycles. You didn't say what the
resistive load was either, so I can't guess from that.
I have put together an example of a phase control dimmer waveform
where you can change the level and see what happens...
http://www.cucumber.demon.co.uk/PowerFactor/ it's the second example
down the page, "Phase Control (Light Dimmer)".
For a phase control dimmer, there is scope to easily mis-measure
the power consumed, depending on the type/quality of the power
metering device used. Your main electricity meter will get it right,
but some of the devices you may have bought to measure power
consumption might not. If you play with the Phase Control applet on
the web page above, you will see that the power factor drops below
1 when you start dimming, and the failure to take this into account
with some measuring devices will lead to an over-estimation of the
power consumed as you start dimming the light.
[email address is not usable -- followup in the newsgroup]
Thanks for the help.
Here is what I am using.
They say power factor is 100%.
There is indeed a mis-reporting of power at low levels but for a different
reason I think. I have an uneven heating load using these controllers on
both sides of a 240 volt genset to the N phase The current sensors are
taking average current, not peak. If they were peak sensors that would
indeed show a gross over reporting at the load decreases beacuse the current
Here is where things take a bizzare turn. From 30% power on up the readings
appear correct usning 1.111 X average correction.
At about 25% power on down going lower let's say at around 10% Things get
ugly, the atcual heating load using true rms voltage measurements at the
load and known resistance it appears to be 250 watts when controls are at
this low setting. I belive that. I should be using RMS sensors I know this
but the error using average sensors is only 1.111 X avaerage.
. Well guess what happens when I hook up true RMS current meters? It reads
Higher! It says there is 4 amps on L1 and so does my average sensor using
1.111 correction. The current on L2 reads 6.5 but the average sensors
says 3.4! SO the RMS reading is double on this side! In other words it
agrees with one but not the other.
. This tells me that we have a total of 10.5 amps in the system. Well
that's about 1250 watts. There is no way that load is on the engine. If you
bypass the controllers with a known 1200 watt load you can clearly here the
engine groan. So where is the other 1050 watts of power going or is not
really there? I think it's either there and being stored in one side of the
stator or it's apparent or reactive power becuase there is a phase
Do you see what I see? Here is what I think is going on. As the voltage is
off longer there is more energy stored and there is an imbalance of load. It
appears the stored enegery is being pushed back to the side with lower load
on it and the system is now has inductance or reactive power in it. I think
my rms current measurments prove it.
It is a resisitive load but the generator is bascilly an electric motor and
this is a closed system. Once you start turning off the voltage longer
things go haywire.
Keep in mind that we want the actuaual power to the shaft. If the power
factor was alway's 100 then not an issue. I am no expert but it sure looks
to me like we do have a big power factor problem. I think balancing the load
would help. I guess my question is, just WTF is going on?
I think the best way to deal with this is to correct the power factor if
that is the problem, not try to measure it.
Well this is 'mostly' right. When the triac is conducting, the current
and voltage are in phase. If you go by the older idea of power factor,
(pf=cos(theta)), then with 0 degree phase shift you would conclude that
When you're firing the triacs very early (nearly 0 degree firing delay),
this all works out pretty much okay.
But power factor is *really* defines as pf=W/VA. And measuring the
current when you use this type of controller is a bit tricky. For
example, when you fire the triacs at 90 degrees, there are some
harmonics to the fundamental frequency current so a typical ammeter
won't give accurate results.
On top of this, is the issue of actually measuring the current. A
typical D'Arsonval movement ammeter (an 'analog meter') responds to
*average* current, not RMS. The meter face is re-calibrated by a factor
of 1.1 so that it reads out RMS if the current happens to be sine wave.
So determining the current is not so easy.
A few meters are 'true RMS' and will read a different value than the
typical D'Arsonval meter when used on a waveform like the current that
flows in this case (90 degrees firing delay).
He's wrong here. The triacs are not changing the phase relationship
The current measurement can have errors in it as I mentioned above, so
that can introduce error in power measurement.
If you had a meter that digitally sampled the current very fast and gave
you the 'true RMS' reading, you won't have any error.
Or a power meter that digitally samples the instantaneous voltage and
multiplies by the instantaneous current, that will give an accurate
reading as well.
Thought I would follow up. I installed a balanced load
and it's like magic. Everything works fine even at late firing
of the controllers. All measurnets are correct. One side still wants to
drift a bit but I think the simple answer is a load balance capacitor. Any
I think this kinds proves my case that the controllers are making the system
Get a good power electronics book. You will find that when using a phase
angle fired controller, at any power level other than 100% on (0 degrees)
you get BOTH a reduction is true power factor (W / (Vrms*Irms)) and
displacement power factor (angle between voltage and current). Yes, Yes,
the current is in phase with the voltage when during the portion of the half
cycle after the firing BUT the fact that you are not getting a full half
cycle of current causes a phase shift between the voltage and current at the
fundamental frequency. This is explained in many power electronics books.
When you do an FFT on the resulting waveforms (say at 50% power) you will
see that the fundamental component of the current is shifted (lagging).
What this means is that if you use this type of controller on a purely
resistive load, at other than 100% power, your displacement power factor
will be less than unity.
It is quite easy to test with a light dimmer, an incandescent bulb, and a
good PQ meter like the Fluke 43B. The Fluke will show both true power
factor and displacement power factor. Changing the dimmer from full on
through the range to zero on you will see the displacement power factor
If you had attended EPRI's Power Quality Assurance conference in New York
last year you could have seen a paper presented on this very topic. It is
not much of a problem when you are dimming a light bulb, BUT when you want
to control a 4kV, 2 MW resistive heater....then it can have a negative
impact on your bill (power factor penalties are not uncommon).
Charles Perry P.E.
Charles that was most informative. Thank you.
I agree the power factor should change off 100% on but it's not untill You
get below 50 or 40% load.
The total load is 6600 watts of heaters. With one controler or 2 in line
There is no error in Power factor untill 40 or 50 % power reduction if there
is then it must be very small
This could be becuase the controllers are not standard light dimer. They
cost 7 bucks and these cost 100 Each.
Or perhaps the genset has some built in PF correction and we don't see the
Anyway, thanks for help. At least we know just what is going on. Just
but based on readings it looks the PF is dropping from to about .08 at the
low end of power on.
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