About Power Factor
What is Power Factor
To understand power factor, we’ll first start with the definition of some basic
terms:
KW is Working Power (also called Actual Power or Active Power or Real Power).
It is the power that actually powers the equipment and performs useful work.
KVAR is Reactive Power.
It is the power that magnetic equipment (transformer, motor and relay) needs to
produce the magnetizing flux.
KVA is Apparent Power.
It is the “vectorial summation” of KVAR and KW.
Let’s look at a simple analogy in order to better understand these terms….
Let’s say you are at the DELHI, and it is a really hot day. You order up among of
your favorite beer. The thirst-quenching portion of your beer is represented by
KW (Figure 1).
Unfortunately, life isn’t perfect. Along with your ale comes a little bit of foam.
(And let’s face it…that foam just doesn’t quench your thirst.) This foam is represented
by KVAR.
The total content of your mug, KVA, is this summation of KW (the beer) and KVAR
(the foam).
So, now that we understand some basic terms, we are ready to learn about power factor:
Power Factor (P.F.) is the ratio of Working Power to Apparent Power.
P.F. = KW
KVA
Looking at our beer mug analogy above, power factor would be the ratio of beer (KW)
to beer plus foam (KVA).
P.F. = KW
KW + KVAR
P.F. = Beer
Beer + Foam
Thus, for a given KVA:-
• The more foam you have (the higher the percentage of KVAR), the lower your ratio
of KW (beer) to KVA (beer plus foam). Thus, the lower your power factor.
• The less foam you have (the lower the percentage of KVAR), the higher your ratio
of KW (beer) to KVA (beer plus foam). In fact, as your foam (or KVAR) approaches
zero, your power factor approaches 1.0.
Our beer mug analogy is a bit simplistic. In reality, when we calculate KVA, we
must determine the “vectorial summation” of KVAR and KW. Therefore, we must go one
step further and look at the angle between these vectors.
The Power Triangle
P.F. = KW = COS θ
KVA
KVAR = SIN θ
KVA
KVA = /KW2 + KVAR2 =
KV * I * _ / 3
Note that…in an ideal world…looking at the beer mug analogy:
• KVAR would be very small (foam would be approaching zeo)
• KW and KVA would be almost equal (more beer; less foam)
So….
In order to have an “efficient” system (whether it is the beer mug or Mac dragging
a heavy load), we want power factor to be as close to 1.0 as possible.
Sometimes, however, our electrical distribution has a power factor much less than
1.0. Next, we’ll see what causes this.
What Causes Low Power Factor?
Since power factor is defined as the ratio of KW to KVA, we see that low power factor
results when KW is small in relation to KVA. Remembering our beer mug analogy, this
would occur when KVAR (foam, or Mac’s shoulder height) is large.
What causes a large KVAR in a system? The answer is…inductive loads.
Inductive loads (which are sources of Reactive Power) include:
• Transformers
• Induction motors
• Induction generators (wind mill generators)
• High intensity discharge (HID) lighting
These inductive loads constitute a major portion of the power consumed in industrial
complexes.
Reactive power (KVAR) required by inductive loads increases the amount of apparent
power (KVA) in your distribution system. This increase in reactive and apparent
power results in a larger angle θ (measured between KW and KVA). Recall that, as
θ increases, cosine θ (or power factor) decreases.
So, inductive loads (with large KVAR) result in low power factor.
Why Should we Improve My Power Factor ?
You want to improve your power factor for several different reasons. Some of the
benefits of improving your power factor include:
1. Lower utility fees by:
A. Reducing peak KW billing demand
Recall that inductive loads, which require reactive power, caused your low power
factor. This increase in required reactive power (KVAR) causes an increase in required
apparent power (KVA), which is what the utility is supplying.
So, a facility’s low power factor causes the utility to have to increase its generation
and transmission capacity in order to handle this extra demand.
By raising your power factor, you use less KVAR. This results in less KW, which
equates to a dollar savings from the utility.
B. Eliminating the power factor penalty
Utilities usually charge customers an additional fee when their power factor is
less than 0.95. (In fact, some utilities are not obligated to deliver electricity
to their customer at any time the customer’s power factor falls below 0.85.) Thus,
you can avoid this additional fee by increasing your power factor.
2. Increased system capacity and reduced system losses in your electrical system
By adding capacitors (KVAR generators) to the system, the power factor is improved
and the KW capacity of the system is increased.
For example, a 1,000 KVA transformer with an 80% power factor provides 800 KW (600
KVAR) of power to the main bus.
1000 KVA = / (800 KW)2 + ( ? KVAR)2
KVAR = 600
By increasing the power factor to 90%, more KW can be supplied for the same amount
of KVA.
1000 KVA = / (900 KW)2 + ( ? KVAR)2
KVAR = 436
The KW capacity of the system increases to 900 KW and the utility supplies only
436 KVAR.
Uncorrected power factor causes power system losses in your distribution system.
By improving your power factor, these losses can be reduced. With the current rise
in the cost of energy, increased facility efficiency is very desirable. And with
lower system losses, you are also able to add additional load to your system.
3) Increased voltage level in your electrical system and cooler, more efficient
motors
As mentioned above, uncorrected power factor causes power system losses in your
distribution system. As power losses increase, you may experience voltage drops.
Excessive voltage drops can cause overheating and premature failure of motors and
other inductive equipment.
So, by raising your power factor, you will minimize these voltage drops along feeder
cables and avoid related problems. Your motors will run cooler and be more efficient,
with a slight increase in capacity and starting torque.