richxtlc
Senior Member
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- Tampa Florida
If I remember my theory 1 volt DC = 1 volt AC RMS, where did the 1.414 come from.
power factor questions
- Thread starter ohmhead
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If I remember my theory 1 volt DC = 1 volt AC RMS, where did the 1.414 come from.
ohmhead
Senior Member
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- ORLANDO FLA
Well yes i see my error its hard for me to relate sometimes when i post as iam not good at explaining my thoughts . But what i was trying to ask is which is which when we measure PF with a meter we use average values taken .
Can this be a true calculation of PF meaning at any frequency or i should say AC sine wave its peak voltage is not average as seen by a meter its higher in value .
Meaning its placement or cycle from zero to peak positive back thur zero to negative of ac cycle in deg .
Meaning its peak wave at positive or negative its not the average rms value that counts like effective value of dc the time in the cycle or place in time to leading or lagging voltage to current when we look for PF as a point of time in the cycle out of phase voltage with current at what degs in time so our we actually measuring correctly ?
DC 1 volt but AC 1 volt is really 1.414 volts to dc .
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richxtlc
Senior Member
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- Tampa Florida
No, 1.414 is the peak value of 1 volt RMS. Depending on the meter used, the average value is read, but the RMS value is calculated by multiplying the average value by 1.11, a true RMS meter will read the RMS value and will figure into that value any distortions caused by harmonics, whereas a standard meter, average reading, will only read a pure sine wave accurately.
If I remember correctly an analog powerfactor meter uses a resistor and an inductor placed 90 deg out of phase, so that if you measure a purely inductive or capacitive circuit it will read zero powerfactor. Also, if the frequency is different, other than 60 Hz, it will change the inductive reactance of the coil and introduce an error into a reading.
YES
NO. If the capacitance is infinite, then the dc component of the current will be a constant 1 amp because the capacitor will never get charged. The time constant is infinite.
ohmhead
Senior Member
- Location
- ORLANDO FLA
Well power factor is improved but not totally eliminated with capacitors meaning a inductive load is a displacement of current to voltage ?
Question harmonics like by example power electronics switching is it a distortion of the sine wave but not a direct displacement of current to voltage like a inductive load ?
To make this more clear is it just a chopping up of the sine wave at different points in the wave itself and it does create heat but can not be measured in watts .
Can a capacitor in the power factor correction process of improving inductive loads in a circuit make its own resonant circuit at some harmonic frequency and create more of a problem if we use these at one point of the incoming service .
Following this along can one use pf correction to improve one problem but create another problem ?
Question harmonics like by example power electronics switching is it a distortion of the sine wave but not a direct displacement of current to voltage like a inductive load ?
To make this more clear is it just a chopping up of the sine wave at different points in the wave itself and it does create heat but can not be measured in watts .
Can a capacitor in the power factor correction process of improving inductive loads in a circuit make its own resonant circuit at some harmonic frequency and create more of a problem if we use these at one point of the incoming service .
Following this along can one use pf correction to improve one problem but create another problem ?
mivey
Senior Member
absolutely
ohmhead
Senior Member
- Location
- ORLANDO FLA
Well Mivey if a capacitor of value could it become resonant at 3rd 4th 5th harmonics or is this not possible .
Harmonics are a frequency a multiple of fundamental ? yes /no tell me if iam way off base .
Harmonics are a frequency a multiple of fundamental ? yes /no tell me if iam way off base .
Harmonics are multiples of the fundamental. Even harmonics are uncommon.
In single phase non-linear loads, usually the lowest order is third. For three phase loads it is normally fifth and subsequent harmonics in the sequence 6n ? 1 (5th, 7th, 11th, 13th, etc).
Capacitors without chokes (I've seen them called naked capacitors) will attract all harmonics and, as I'm sure you know, the capacitor has reducing impedance with increasing frequency so, the higher the harmonic, the lower the impedance. This can lead to them being overloaded, sometimes to a degree that causes failure. In order to avoid this problem, it is becoming increasingly common to fit chokes (reactors) in series with the capacitor. On three phase systems I usually tune the PFC/choke resonant frequency to be 177Hz. Others use 225Hz. In either case, it is below 5th harmonic which is 250Hz for UK. The idea is that the capacitor circuit is now inductive above the tuned frequency and its impedance increases with frequency thus rejecting fifth and higher order harmonics.
To answer the first part of your question, yes with or without chokes, they can resonate with other connected equipment or with the supply impedance and that's something that needs to be taken account of, preferably at the design stage.
mivey
Senior Member
What he said.
I might add:
Resistive load adds dampening so a lightly loaded system is more likely to resonate.
Also, there are checks made for resonant harmonics. One is to calculate a resonant harmonics number h = sqrt(MVAs / MVARc) where MVAs is the short-circuit rating of the system and MVARc is the shunt capacitor rating.
You can also perform frequency scanning and measure the system response to identify possible resonant frequencies.
In some cases, you may have to use tuned capacitors to provide harmonics control and power factor correction at the same time.
I might add:
Resistive load adds dampening so a lightly loaded system is more likely to resonate.
Also, there are checks made for resonant harmonics. One is to calculate a resonant harmonics number h = sqrt(MVAs / MVARc) where MVAs is the short-circuit rating of the system and MVARc is the shunt capacitor rating.
You can also perform frequency scanning and measure the system response to identify possible resonant frequencies.
In some cases, you may have to use tuned capacitors to provide harmonics control and power factor correction at the same time.
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