PHASE SHIFT CALCULATIONS
- Thread starter elect1
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: PHASE SHIFT CALCULATIONS
I see that we?re working in ohms and not in watts. I don?t know your level of mathematical skill, but I will take a stab at it. You also need to know the inductive reactance (if any), and the resistance. Subtract the capacitive reactance from the inductive reactance. If the result is negative, meaning that the circuit is dominantly capacitive, then the current will lead the voltage. If the result is positive, meaning that the circuit is dominantly inductive, then the current will lag the voltage. The angle by which one of those two things will happen can be found dividing the net reactance by the resistance, and taking the inverse tangent of the result.
I do not know how to draw pictures and post them on this forum. But imagine a horizontal line, the length of which represents resistance. From the right hand end of this line, add a vertical line upwards, with the length representing inductive reactance. From the top point of this vertical line, add another vertical line, this one going downwards, with the length representing capacitive reactance. Draw a line from the endpoint of this second line to the left end point of the original horizontal line. You should now have a triangle, and the angle at the left end is the phase angle that you wanted to find.
I see that we?re working in ohms and not in watts. I don?t know your level of mathematical skill, but I will take a stab at it. You also need to know the inductive reactance (if any), and the resistance. Subtract the capacitive reactance from the inductive reactance. If the result is negative, meaning that the circuit is dominantly capacitive, then the current will lead the voltage. If the result is positive, meaning that the circuit is dominantly inductive, then the current will lag the voltage. The angle by which one of those two things will happen can be found dividing the net reactance by the resistance, and taking the inverse tangent of the result.
I do not know how to draw pictures and post them on this forum. But imagine a horizontal line, the length of which represents resistance. From the right hand end of this line, add a vertical line upwards, with the length representing inductive reactance. From the top point of this vertical line, add another vertical line, this one going downwards, with the length representing capacitive reactance. Draw a line from the endpoint of this second line to the left end point of the original horizontal line. You should now have a triangle, and the angle at the left end is the phase angle that you wanted to find.
bphgravity
Senior Member
- Location
- Florida
Re: PHASE SHIFT CALCULATIONS
The mathematical expresion that can be used to find exactly what Charlie has decribed is:
The angle of LEAD:
Tangent of angle lead = Xc/R
The angle of LAG:
Tangent of angle lag = XL/R
This angle is the power factor angle which the ratio of resistance to impedance. That angle is called the angle THETA(cosine of theta) This formula can be solved with a scientific calculator.
You can also find the ratio of current in phase to the line current to obtain the power factor, then multilpy the voltage, line current, and power factor to find true power. Now you can take the ratio of the true pwer to the reactive power and perform the cosine of the angle to obtain the pahse angle.
The mathematical expresion that can be used to find exactly what Charlie has decribed is:
The angle of LEAD:
Tangent of angle lead = Xc/R
The angle of LAG:
Tangent of angle lag = XL/R
This angle is the power factor angle which the ratio of resistance to impedance. That angle is called the angle THETA(cosine of theta) This formula can be solved with a scientific calculator.
You can also find the ratio of current in phase to the line current to obtain the power factor, then multilpy the voltage, line current, and power factor to find true power. Now you can take the ratio of the true pwer to the reactive power and perform the cosine of the angle to obtain the pahse angle.
Ed MacLaren
Senior Member
Re: PHASE SHIFT CALCULATIONS
The explanations above are correct for loads connected in series.
For loads in parallel, the phase relationship of the currents in the branches would determine the phase angle between the line voltage and current (power factor).
The sketch below illustrates the phase relationships for an example circuit with a resistance load (R), an inductive load (XL), and a capacitive load (XC) in series.
Ed
[ March 31, 2003, 04:33 PM: Message edited by: Ed MacLaren ]
The explanations above are correct for loads connected in series.
For loads in parallel, the phase relationship of the currents in the branches would determine the phase angle between the line voltage and current (power factor).
The sketch below illustrates the phase relationships for an example circuit with a resistance load (R), an inductive load (XL), and a capacitive load (XC) in series.
Ed
[ March 31, 2003, 04:33 PM: Message edited by: Ed MacLaren ]
Ed MacLaren
Senior Member
Re: PHASE SHIFT CALCULATIONS
When loads in connected in parallel, the phase relationship between the currents in the branches determine the phase angle between the line voltage and line (total) current. The circuit power factor is the cosine of that phase angle.
The sketch below illustrates the phase relationships for an example circuit with a resistance load (R), an inductive load (XL), and a capacitive load (XC) connected in parallel.
It is just a coincidence that the power factor (and phase angle) are the same as in the series circuit example above.
Ed
When loads in connected in parallel, the phase relationship between the currents in the branches determine the phase angle between the line voltage and line (total) current. The circuit power factor is the cosine of that phase angle.
The sketch below illustrates the phase relationships for an example circuit with a resistance load (R), an inductive load (XL), and a capacitive load (XC) connected in parallel.
It is just a coincidence that the power factor (and phase angle) are the same as in the series circuit example above.
Ed
- Location
- Massachusetts
Re: PHASE SHIFT CALCULATIONS
I was looking through some old posts and came across this one.
What is phase shift?
I was looking through some old posts and came across this one.
What is phase shift?
Ed MacLaren
Senior Member
Re: PHASE SHIFT CALCULATIONS
We could go into the details if anyone wants to.
Ed
[ February 14, 2004, 09:30 AM: Message edited by: Ed MacLaren ]
The short answer is - When the current is caused to be "out of phase", that is, to lead or lag the voltage, in a circuit, generally due to the effects of inductive or capacitive reactance in the circuit.
We could go into the details if anyone wants to.
Ed
[ February 14, 2004, 09:30 AM: Message edited by: Ed MacLaren ]
- Location
- Massachusetts
Re: PHASE SHIFT CALCULATIONS
Thanks Ed we can try going into details but some of the math may escape me.
Is phase shift something an installer like myself should be aware of or is it something that the engineers deal with when sizing transformers and switchgear?
What are the effects of phase shift?
Thanks, Bob
Thanks Ed we can try going into details but some of the math may escape me.
Is phase shift something an installer like myself should be aware of or is it something that the engineers deal with when sizing transformers and switchgear?
What are the effects of phase shift?
Thanks, Bob
Ed MacLaren
Senior Member
Re: PHASE SHIFT CALCULATIONS
Although the current and voltage is "out of phase" in any system that has inductive loads(motors, fluorescent lighting, etc.), the term "phase shift" would not normally be used at all by anyone in the electrical wiring industry.
It is more of an electronics term.
We would refer to such loads as having a "power factor" of less than unity (100%).
What are the effects of low power factor on building wiring systems?
The load devices that have low power factor, generally motors, draw more current to deliver the same horsepower than they would if the power factor was corrected.
This causes increased voltage drop and line losses in services, feeders, and branch circuits.
I think the originating poster's question may have been referring to a homework assignment or exam question.
Ed
[ February 14, 2004, 02:27 PM: Message edited by: Ed MacLaren ]
Although the current and voltage is "out of phase" in any system that has inductive loads(motors, fluorescent lighting, etc.), the term "phase shift" would not normally be used at all by anyone in the electrical wiring industry.
It is more of an electronics term.
We would refer to such loads as having a "power factor" of less than unity (100%).
What are the effects of low power factor on building wiring systems?
The load devices that have low power factor, generally motors, draw more current to deliver the same horsepower than they would if the power factor was corrected.
This causes increased voltage drop and line losses in services, feeders, and branch circuits.
I think the originating poster's question may have been referring to a homework assignment or exam question.
Ed
[ February 14, 2004, 02:27 PM: Message edited by: Ed MacLaren ]
dellphinus
Member
- Location
- Southern Illinois, USA
Re: PHASE SHIFT CALCULATIONS
Thanks,
DP
Ed, this is kinda OT, but what software do you use to produce those fabulous diagrams you use to illustrate your posts with?
Thanks,
DP
Ed MacLaren
Senior Member
Re: PHASE SHIFT CALCULATIONS
dellphinus,
In answer to your question, I used AutoSketch for the original drawings, and Paint Shop Pro, which is a sort of "poor man's Photoshop", to convert the files to the gif format for the web.
Ed
dellphinus,
In answer to your question, I used AutoSketch for the original drawings, and Paint Shop Pro, which is a sort of "poor man's Photoshop", to convert the files to the gif format for the web.
Ed
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