power factor
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Ed MacLaren
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Re: power factor
This is a difficult topic to explain in this kind of forum. To help answer the question I will use a page from one of my tutorials.
"Power Factor is most easily explained using an AC motor as an example circuit, and we will use a single phase circuit for simplicity. Think of the motor winding as if it were a resistor and a coil in parallel, because the motor winding has both resistance and inductive reactance. (Fig. 1)
There are three "different kinds of power" in AC circuits, three values that are called power, but strictly speaking only the True Power (watts) is a measure of energy converted into work, the definition of power. It is calculated by multiplying the volts x amps in the circuit component where the voltage and current are in phase, the resistor, which is the only component in the circuit that can convert energy.
Many industrial facilities electrical systems operate at a low power factor because the majority of their load consists of motors. Motor loads require Reactive Power (vars), often referred to as magnetizing vars because it sustains the motor's magnetic field. This means some of the current that the motor draws is "out-of-phase" current(shown in blue). This is in addition to the "in-phase" current (shown in red), that delivers the true power that represents energy converted into mechanical motion.
Reactive power is calculated by multiplying the volts x amps in the circuit component where the voltage and current are out-of-phase, the coil, which as a circuit component, can only store energy, not convert it.
Apparent Power (volt-amps) is a measure of what the actual power would be if the line voltage and current were perfectly in phase. It is calculated by multiplying the line (supply) volts x amps, or by adding the watts and vars vectorially.
The term Power Factor actually means - the percentage of the apparent power that is true power. It can be calculated by a number of methods.
PF = W/VA
You can also think of it as - what percentage of the total current is in-phase current?
PF = In-phase current/total current
The reactive "power" is not a useful energy conversion. It represents the energy that flows in to the motor to build up the magnetic field each cycle, and when the field collapses one half cycle later this energy is regenerated back to the power plant.
This represents current (out of phase) flowing in the service equipment, transformers, and circuit wiring, that does no useful work. If a device were connected to the circuit that could intercept and store this energy, and return it to the motor winding for the next magnetic field buildup, instead of requiring it to come from the power plant each cycle, the motor could do it's job with less current.
A capacitor can perform this function when connected in parallel with the motor, and this is called power factor correction.
With power factor correction the motor still draws the magnetizing (out of phase) current, however it now comes from the capacitor. Power factor correction is normally only installed in large facilities where most of the load is motors. Individual capacitors can be installed at each motor or a large capacitor bank can serve a feeder or motor control center. The capacitor bank ratings are normally calculated in Kilovars (kvar)."
Ed
[ May 21, 2003, 07:35 AM: Message edited by: Ed MacLaren ]
This is a difficult topic to explain in this kind of forum. To help answer the question I will use a page from one of my tutorials.
"Power Factor is most easily explained using an AC motor as an example circuit, and we will use a single phase circuit for simplicity. Think of the motor winding as if it were a resistor and a coil in parallel, because the motor winding has both resistance and inductive reactance. (Fig. 1)
There are three "different kinds of power" in AC circuits, three values that are called power, but strictly speaking only the True Power (watts) is a measure of energy converted into work, the definition of power. It is calculated by multiplying the volts x amps in the circuit component where the voltage and current are in phase, the resistor, which is the only component in the circuit that can convert energy.
Many industrial facilities electrical systems operate at a low power factor because the majority of their load consists of motors. Motor loads require Reactive Power (vars), often referred to as magnetizing vars because it sustains the motor's magnetic field. This means some of the current that the motor draws is "out-of-phase" current(shown in blue). This is in addition to the "in-phase" current (shown in red), that delivers the true power that represents energy converted into mechanical motion.
Reactive power is calculated by multiplying the volts x amps in the circuit component where the voltage and current are out-of-phase, the coil, which as a circuit component, can only store energy, not convert it.
Apparent Power (volt-amps) is a measure of what the actual power would be if the line voltage and current were perfectly in phase. It is calculated by multiplying the line (supply) volts x amps, or by adding the watts and vars vectorially.
The term Power Factor actually means - the percentage of the apparent power that is true power. It can be calculated by a number of methods.
PF = W/VA
You can also think of it as - what percentage of the total current is in-phase current?
PF = In-phase current/total current
The reactive "power" is not a useful energy conversion. It represents the energy that flows in to the motor to build up the magnetic field each cycle, and when the field collapses one half cycle later this energy is regenerated back to the power plant.
This represents current (out of phase) flowing in the service equipment, transformers, and circuit wiring, that does no useful work. If a device were connected to the circuit that could intercept and store this energy, and return it to the motor winding for the next magnetic field buildup, instead of requiring it to come from the power plant each cycle, the motor could do it's job with less current.
A capacitor can perform this function when connected in parallel with the motor, and this is called power factor correction.
With power factor correction the motor still draws the magnetizing (out of phase) current, however it now comes from the capacitor. Power factor correction is normally only installed in large facilities where most of the load is motors. Individual capacitors can be installed at each motor or a large capacitor bank can serve a feeder or motor control center. The capacitor bank ratings are normally calculated in Kilovars (kvar)."
Ed
[ May 21, 2003, 07:35 AM: Message edited by: Ed MacLaren ]
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: power factor
Here's another way of explaining apparent power. I have posted this before, so it may look familiar to some forum members.
- - - - - - - - - - - - - - - - - - - - -
Wrap a wire around a nail and connect it to a battery. What you get is not just a magnet, it is also a magnetic field. A generator and a motor have this same type of magnetic field. So does any load that is ?inductive? in nature.
Place two metal plates close together and connect them to a battery. What you get is not just a capacitor, it is also an electric field.
When you connect a motor to a generator (or to a transformer), two things happen. The second thing is that the motor goes roundy-roundy. That is its job; that is the ?real power? being put to use. But the first thing that happens is that the magnetic field of the generator exchanges energy with the magnetic field of the motor. It is in one sense a useless expenditure of energy, in that it doesn?t make the motor go any faster or drive a larger load. But the motor could not work at all, if it were not for that exchange of energy. So it is a necessary useless expenditure of energy. This is what is called the ?Reactive Power.?
You get the ?apparent power? by adding the real power to the reactive power. But you add them by treating each as one leg of a right triangle, with the apparent power being the hypotenuse (i.e., use the Pythagorean Theorem).
What happens when you add the capacitor to the load? Now the magnetic field of the motor will exchange energy with the electric field of the capacitor. It takes the magnetic field of the generator out of the picture. Note that this takes place close to the motor, and that the circuit breaker does not see the current associated with this energy exchange. Result: the current seen by the circuit breaker goes down.
This phenomenon is commonly described by saying that the capacitor is canceling out the inductor. More precisely, it is said that the ?leading? reactive power added by the capacitor is opposite to the ?lagging? reactive power of the motor. It can be pictured by taking one leg of a right triangle and shrinking its length to nearly zero. What you will see is that the hypotenuse (apparent power) starts looking very much like the other leg (real power).
Here's another way of explaining apparent power. I have posted this before, so it may look familiar to some forum members.
- - - - - - - - - - - - - - - - - - - - -
Wrap a wire around a nail and connect it to a battery. What you get is not just a magnet, it is also a magnetic field. A generator and a motor have this same type of magnetic field. So does any load that is ?inductive? in nature.
Place two metal plates close together and connect them to a battery. What you get is not just a capacitor, it is also an electric field.
When you connect a motor to a generator (or to a transformer), two things happen. The second thing is that the motor goes roundy-roundy. That is its job; that is the ?real power? being put to use. But the first thing that happens is that the magnetic field of the generator exchanges energy with the magnetic field of the motor. It is in one sense a useless expenditure of energy, in that it doesn?t make the motor go any faster or drive a larger load. But the motor could not work at all, if it were not for that exchange of energy. So it is a necessary useless expenditure of energy. This is what is called the ?Reactive Power.?
You get the ?apparent power? by adding the real power to the reactive power. But you add them by treating each as one leg of a right triangle, with the apparent power being the hypotenuse (i.e., use the Pythagorean Theorem).
What happens when you add the capacitor to the load? Now the magnetic field of the motor will exchange energy with the electric field of the capacitor. It takes the magnetic field of the generator out of the picture. Note that this takes place close to the motor, and that the circuit breaker does not see the current associated with this energy exchange. Result: the current seen by the circuit breaker goes down.
This phenomenon is commonly described by saying that the capacitor is canceling out the inductor. More precisely, it is said that the ?leading? reactive power added by the capacitor is opposite to the ?lagging? reactive power of the motor. It can be pictured by taking one leg of a right triangle and shrinking its length to nearly zero. What you will see is that the hypotenuse (apparent power) starts looking very much like the other leg (real power).
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Re: power factor
KVA (thousand volt-amperes) is a measurement of total work (power) available. Transformers are sized/rated in KVA.
KW (thousand watts) is a measurement of the amount of work performed (power used). Utility bills are based on KW.
KVAR (thousand volt-amperes reactive) is a measurement of the difference between the work capable of being performed and the actual work done (KVA and KW). Utilities may actually measure this on large services.
PF (power factor) is a measurement of the ratio between KW and KVA. Some utilities measure the PF and others calculate it using KW and KVAR.
Now, for a more non-technical explanation. Think of:
KVA as the size of a beer mug.
KW as the amount of beer in the mug.
KVAR as the amount of foam in the mug.
PF, then is the percentage of beer in the mug.
A bad power factor is too much foam and not enough beer. While, an ideal PF would be lot's of beer with a small "head" on it.
[ May 21, 2003, 12:10 PM: Message edited by: jim dungar ]
KVA (thousand volt-amperes) is a measurement of total work (power) available. Transformers are sized/rated in KVA.
KW (thousand watts) is a measurement of the amount of work performed (power used). Utility bills are based on KW.
KVAR (thousand volt-amperes reactive) is a measurement of the difference between the work capable of being performed and the actual work done (KVA and KW). Utilities may actually measure this on large services.
PF (power factor) is a measurement of the ratio between KW and KVA. Some utilities measure the PF and others calculate it using KW and KVAR.
Now, for a more non-technical explanation. Think of:
KVA as the size of a beer mug.
KW as the amount of beer in the mug.
KVAR as the amount of foam in the mug.
PF, then is the percentage of beer in the mug.
A bad power factor is too much foam and not enough beer. While, an ideal PF would be lot's of beer with a small "head" on it.
[ May 21, 2003, 12:10 PM: Message edited by: jim dungar ]
Re: power factor
I would say the #1 concern with low power factor is the penalties $$$ the utility company imposes on a service that has very low power factor. Like previously stated, you will normally only see this in large industrial facilities with a lot of motor loads. The stiff penalty is incentive enough for the bean counters to appropriate funds to do something about it.
#2 is the fact you can free up a lot of system capacity by improving power factor.
BTW: The utilities impose penalties because even though the KVAR?s are being returned to their system (the end user is not using or being charged for the power) the utilities have to maintain generating, transmission and distribution capacity to supply apparent power.
Jim, great analogy!
I would say the #1 concern with low power factor is the penalties $$$ the utility company imposes on a service that has very low power factor. Like previously stated, you will normally only see this in large industrial facilities with a lot of motor loads. The stiff penalty is incentive enough for the bean counters to appropriate funds to do something about it.
#2 is the fact you can free up a lot of system capacity by improving power factor.
BTW: The utilities impose penalties because even though the KVAR?s are being returned to their system (the end user is not using or being charged for the power) the utilities have to maintain generating, transmission and distribution capacity to supply apparent power.
Jim, great analogy!
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: power factor
Actually, it does affect the system. The voltage drop from the utility?s substation to your service entrance is a function of circuit impedance (which you cannot control) and the total current (which you can control). The higher the KVARs, the higher the apparent power, and the higher the current, and therefore the greater the voltage drop. This is also true internal to your own distribution system. High KVARs result in unnecessarily high voltage drop, power losses, and heat generation within your system.
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