Basics of AC power calculation
Since watts
is volts times amps, what is VA? VA (or volt-amps) is also volts times amps,
the concept however has been expanded and extended cover some of the subleties
of AC power.
For DC
current:
VA = Watts (DC current).
In AC if the
volts and amps are in phase (for example a resistive load) then the equation is
also
VA=Watts (resistive load)
where V is
the RMS voltage and A the RMS amperage.
But here is
the difference. In AC, the volts and amps are not always in phase (meaning that
the peak of the voltage curve is does not happen at the peak of the current
curve). Think of pushing a swing. The greatest force is applied when the swing
stops at the peak, but the greatest velocity is at the bottom of the arc. When
you have reactive circuit elements the current can't keep up with the voltage,
but lags behind. So in AC, if the volts and amps are not precisely in phase you
have to calculate the watts by multiplying the volts times the amps at each
moment in time and take the average over time, i.e. the integral of V*A dt over
a full cycle. The ratio between the VA (i.e. rms volts time rms amps) and Watts
is called the power factor PF.
VA·PF = Watts (for any load,
including inductive loads)
In other words, volt-amps x
power factor = watts. Similarly, KVA*PF = KW,
Or kilovolt-amps times power
factor equals kilowatts.
When you want
to know how much the electricity is costing you, you use watts. When you are
specifying equipment loads, fuses, and wiring sizes you use the VA, or the rms
voltage and rms amperage. This is because VA considers the peak of both current
and voltage, without taking into account if they happen at the same time or
not.
Finding the Power Factor
How do you
find the power factor? This isn’t easy. The idea is that you multiply the
amps times the volts for every increment of time and take the sum and average.
You then divide this by the power you would have calculated if the amps and
volts are exactly in phase. Since the voltage and current can be out of phase
(which is the whole point of power factor) the power factor can be anywhere
between zero and one. For computer power supplies and other supplies that are
power factor corrected the power factor is usually over 90%. For high
power motors under heavy load the power factor can be as low as 35%.
Industry
standard rule-of-thumb is that you plan for a power factor of 60%, which
somebody came up with as a kind of average conservative power
factor.
Converting VA to Amps
How to
convert VA to amps? Use the following formula: A = (Va * Pf)/V
Where A
stands for the RMS amps, VA stands for volt-amps, V stands for RMS volts and PF
stands for the power factor.
Converting VA to Volts
How to
convert VA to volts? Use the following formula: V=(Va*Pf)/A
Where V
stands for RMS volts, A stands for the RMS amps, VA stands for volt-amps, and
PF stands for the power factor.
What is KVA?
KVA is just
kilovolt-amps, or volts times amps divided by 1000:
KVA·PF = KW (any load, including
inductive loads)
Where KVA
stands for kilovolt-amps, KW stands for kilowatts, and PF stands for the power
factor.
Keep the factor of 1000 straight when dealing with mixed
units:
KVA·PF = W/1000 (any load,
including inductive loads)
VA·PF = 1000·KW (Kilowatts
to VA)
The Following equations can be used to convert
between amps, volts, and VA. To convert between kilovolt-amps, kilowatts, and
kiloamps, keep track of the factor of 1000.
Convert VA to Amps (at a fixed voltage)
Convert KVA to KW (kilovolt-amps to kilowatts)
Convert KVA to Amps (at a fixed voltage)
Converting Watts to KVA (watts to kilovolt-amps)
Convert Amps to VA (at a fixed voltage)
Convert VA to Volts (at a fixed current)
Convert Volts to VA (at a fixed current)
Convert Volts to Amps (at a fixed VA)
Convert Amps to Volts (at a fixed VA)
The conversion of VA to Amps is governed by the equation Amps =
VA·PF/Volts)
For example 12 VA·0.6/(12 volts) = 0.6
amp
The conversion of KVA to KW is governed by the equation KVA
= KW/PF)
For example, if the power factor is 0.6
120 KVA·0.6
= 72 Kilowatts
The conversion of W to KVA is governed by the equation
KVA=W/(1000*PF)
For example 1500W/(1000*0.83) = 1.8 kVA (assuming a
power factor of 0.83)
Converting Amps to VA (voltage fixed)
The
conversion of Amps to VA is governed by the equation VA = Amps ·
Volts/PF
For example 1 amp * 110 volts/0.6 = 183 VA
The conversion of Amps to KVA is governed by the equation KVA =
Amps · Volts/(1000·PF)
For example 100 amp * 110
volts/(1000*0.6) = 18.3 KVA
The conversion of VA to Volts is governed by the equation Volts
= VA·PF/Amps
For example 100 VA · 0.6/10 amps = 6
volts
The conversion of Volts to VA is governed by the equation VA =
Amps · Volts/PF
For example 1.5 amps * 12 volts/0.6 = 30
VA
The conversion of Volts to Amps is governed by the equation Amps =
VA·PF/Volts
For example 120 VA* 0.6 /110 volts = 0.65
amps
The conversion of Amps to Volts is governed by the
equation Volts = VA·PF/Amps
For Example, 48 VA · 0.6 / 12
Amps = 2.4 Volts
Converting Amps to Watts and VA in 3-Phase
Power
In three phase power, the basic equation is multiplied by the
square root of 3, or 1.732
So VA = V * I * 1.732
W = V * I * 1.732 *
PF
Thus a 3-phase motor running at 6 amps and 460 volts would require
5000VA
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