Not clear on power factor

I understand that the lagging current does not do useful work and reduces
the supply line capacity to supply power. Adding PF correction capacitors of
the right amount balances out the inductive reactance by capacitive
Does the effect of poor power factor show at the generator? I would say yes
because of the extra heat generated in the wires (conservation of energy).
If so, is that the only extra torque the generator needs is to over come the
extra heat loss in the lines from low power factor?
Although the transformer's capacity is also reduced by the effects of
lagging current, does the lagging current pass though the transformer to the
supply side?
I see some devices, such a equipment with a diode bridge and capacitor to
rectify and filter the AC supply to DC. These are rated to have low power
factor because the asymmetrical current draw due to very high crest factor
from the charching of the capacitor near the peak voltage. The current isn't
lagging, just distorted. Are they generalizing terms by calling devices with
high crest factor to be of low power factor? (i.e. It draws more VA than
real power consumed so it has a lower than 1.0 power factor)
In an experiment, I added 2uf of capacitance to a 9 watt magnetically
ballasted fluorescent lamp. This brought the measured VA of 18 down to 11
which seems like the power drawn should be (9 watts plus 2 watts lost in the
ballast). When I scoped the current wave form it had a distinct double hump
(not unlike the top part of the heart symbol). Wouldn't this asymmetry still
cause issues with utility equipment (transformers and such)?
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------- Essentially so- indirectly there will be some other losses as well. -------
--------- Yes -------
----- There is such a generalisation ----------------
---------- It can and that is why utilities do set allowable harmonic limits for industrial loads. -- Don Kelly remove the urine to answer
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Don Kelly
Yes, it shows at the generator. Although the I^2R in the wiring/transformers/etc must be supplied by the generator (as extra torque on the constant speed shaft as you say), there is another concern.
Generator armature windings are limited in the total kVA /MVA they can carry. Capability curves for generators have three distinct regions, and one of them is strict kVA/MVA loading. If a poor power factor is allowed, then the generator can not be operated to full *real* power capabilities because you reach the apparent power limit of the windings.
A second region of generator capability curves is maximum *field* current. Running with a severe *lagging* power factor will require more field current to maintain terminal voltage than would be required with unity power factor and the same kVA/MVA load. To avoid burning up the field windings, allowable kVA/MVA loading with a lagging power factor is lower than the armature winding limit.
So you want as close to unity on the generator as you can get to avoid having to reduce the *real* power component to stay inside the capabilities curve. Thus you get the most kW/MW from your unit and create the most revenue (assuming your prime-mover is well matched).
The strict definition of 'power factor' is W/VA. In the 'olden' days, this was affected most by phase shift between voltage and current by inductive loads (motors transformers and the like). Nowadays, with more non-linear loads, the harmonics can cause poor power factor as much as inductive loads.
Yes, if allowed in large magnitude. Large industrial customers with non-linear loads must purchase harmonic-filtering equipment and/or pay an additional fee to the utility for such problems. IIRC, DC arc furnaces are one such culprit.
HVDC transmission lines use SCRs for the AC-DC converter/inverter section. These can create a lot of harmonics, so the installations also have some *serious* harmonic filtering.
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Thank you for the detailed response.
I did not realize poor power was such a problem at the generator! John
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Back in thoe old days, when ac/dc tube radios were the norm, I would ocassionly let my mind wander to worry about dc flowing in a transformer. If all the radios pere plugged in correctly, the half-wave rectifier used would draw a dc current component. This would tend to saturate the transformer.
IIRC a typical radio of this kind consumed no more than 30 W. 18 of these would be to run the heaters. Must used simple capacitor input folowed by an RC stage. This meant that rectifier current was a series of fairly narrow spikes at 60 (not 120) Hz.
While I doubt that these small radiow posed a problem for the power companies, larger power draws using half-wave rectifiers, especially with capacitor input filters could present a problem.
Can anyone enlighten me on how much power companies worried about such things?
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Repeating Decimal
The plug on these were not polarized or grounded in nearly every case. I would think that in all the homes with the radio on the chances that the power for rectifier would be drawn off the positive half of the cycle or the negative half would average out to 50/50. I scoped the current wave form from a transformerless tube radio and, yes the wave for is asymmetrical, but the spikes are rather smooth looking tops to a regular sine wave. The current limiting characteristics of the rectifier tube kept the crest factor low relative to the solid state rectifier that lets "anything go".
Several years back, halogen flood lights had a diode in the base because it was easier to design a 82 volt halogen bulb than a 120 volt one of the needed characteristics. Apparently these weren't an issue either. John
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In the old days- not at all. Non-linear loads were such a small part of the total load that the effects were lost in the background "fuzz". For larger loads where it would make a difference, half wave rectifiers weren't used. The number of such large non-linear loads has increased rapidly in the last 25 years, to the point where harmonic content and interaction between harmonic sources is a real problem. -- Don Kelly remove the urine to answer
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Don Kelly
in article HMEnb.18980$, jriegle at wrote on 10/28/03 5:32 PM:
There usually were instructions to revrse the plug if you got hum. That is, one position would give better performance than the other.
Some of the sets were death traps. One side of the line may have been connected directly to the chassis. Correct polarization was a safety factor. In some cases, even if plugged in optimally, the chassis could be hot via the string of heaters if the set were not turned on.
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Repeating Decimal
My brother (an old radio fan) tells me that one side of the plug was tied to the chassis inside the wooden case. So if it was the side that went to 'neutral' you effectively grounded the chassis through the service panel. If the other side, then you had the chassis 'floating' at 120V above ground and picking up all the noise in the neighborhood.
As far as all the radios in the neighborhood giving a DC component, I think they would all have to be plugged in 'right' and all on the same 'hot' phase in every home. This would be quite a fluke to happen ;-)
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