Why is power draw measured in WATTS ( which is volts multiplied by amperage ) but power production or power sources are measured in VA ( volt amps ). Are they not identical? What’s the reason for the difference.
In: Engineering
If you are familiar with a sine wave, picture in your mind that the current and voltage can each have their own sine wave in the same circuit. So when you have a resistive circuit (space heater, soldering gun, etc.), the voltage and current sine waves are both peaking at the same time, so power in this case is watts = volts x amps since both are sine waves are maximum simultaneously. It can be thought of as the voltage and current being “in phase” with each other, and the only thing impeding current flow is the resistance of the conductor.
In an inductive circuit (motor, coil, or anything with a winding), you have what is called inductive reactance. When current flows and changes (as it is constantly doing in a sine wave) the expanding and contracting magnetic field around the wire is inducing a voltage (called CEMF or counter electro magnetic force) in the wire itself that opposes the current flow creating it. So now you have not only the resistance of the conductor opposing the flow of current, you have the CEMF. The two together would be the total “impedance” of the circuit.
(Quick off-topic analogy. Think of resistance in a water pipe being the constrictive size of the pipe diameter. Now you come to a hill and the pump has to overcome gravity (inductance) as well as the resistance. They both oppose the flow of water, but differently).
If you’ve wrapped your head around that, then think that this CEMF is “slowing” the rise of current in the circuit, so that the current sine wave is lagging the voltage sine wave. In other words, the voltage sine wave peaks and starts decaying before the current sine wave peaks. Now you cannot simply multiply the volts x amps because they are not peaking at the same time, so you have to take into consideration how much gap, or “phase shift” in the sine waves you have.
Read this sentence then forget it if it’s too confusing… if portrayed vectorially, the amps would be lagging the voltage by a certain number of degrees. The numerical “cosine” of that angle is the determining factor in the efficiency of the circuit.
So to calculate power now, you would multiply the volts x the amps x how much out of phase they are (the cosine of the angle) to get power. Since the cosine is always 1 (for a resistive circuit with no shift) or less than one for an inductive circuit, VA will only equal W on a resistive circuit and VA will be greater than watts on any other type of circuit.
So…resistive circuit, volts x amps = watts
Inductive circuit, volts x amps x cosine of the phase shift = watts
The same is true of a capacitive load except that the current leads the voltage instead of lagging it.
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