The AC power grid creates a sine wave voltage which is the yellow line on the picture. If you connect a resistive load like an incandescent ligh,t or a heating element, then the current drawn from the grid will also be a sine wave as is shown with the blue line. This is the most efficient possible use of both the ac power source (the grid) and the transmission lines and transformers that connect your house to the grid. Inductive motors also take current in a sine wave but it is delayed from the voltage so that there is a phase difference. The greater this phase angle, the less real power is drawn from the grid. If the phase angle were to be 90° then there would be no actual power produced by the motor even though it had a significant current. Because inductive motors were commonly used from the very beginning of the grid a term was created to describe this phenomenon, Power Factor. This is simply the real power drawn from the grid divided by the volts (rms) times the amps (rms).
When you have an energy source, like a coal fired turbine, then the amount of coal you use is determined by the amount of real power being drawn. However, the limit of how much current the wires in your generator, transmission lines and the wires in the transformers can carry is based on current, not power. For this reason many generators, transformers and UPS systems are rated in volt-amps rather than watts. Also, the amount of power that is wasted in all of these wires is dependent on the amps, not the watts. Inductive motor loads can have their lagging current balanced out by connecting the right amount of capacitors across the line. The grid companies have very large banks of capacitors that they switch on and off to help balance out the power factor of the inductive loads. These capacitor banks are generally on the grid side of the transformers so they only give their benefits for the high voltage transmission lines and the generators windings but provide no benefits for the power factor induced losses in the transformers and lines to the houses. View more detailed paper.
The New Age of Electronic Loads
Practically all electronic loads need to use DC voltages so they run the current through a diode bridge into a capacitor. The problem with this is that it just takes current from the top part of the sine wave. In this picture the blue line is the current of compact florescent lights (CFL) and it has a power factor of about .5.
This means that a generator with transmission lines and transformers that could support 100 kW could only support 50 kW of these loads. On top of that it will double the amount of power that is wasted with transmission losses. In most grid systems the total transmission loss is around 12% so these .5 power factor loads would be actually wasting an additional 36% power.
This means that the grid power plant would actually have to use an additional 36% of coal for these loads. Now consider the impact of the entire country switching from incandescent lights (PF = 1.0) to CFL lights (PF = .5). View more detailed paper.
Computers and Consumer Electronics
It doesn’t have to be this way. Modern electronics can provide power factor corrected electronic power supplies which totally eliminate this problem but they are much more expensive and in this competitive price world, PF corrected supplies are almost never used. Electronic power supplies in computers and similar electronics have the same problem as CFL but not quite as bad. Their power factor varies between .6 and .75 and this next picture shows the current wave shape which is typical for computers. At a power factor of .75 the 100 kW plant could only power 75 kW and would have to burn an additional 9.3% of coal.
The Transverter power modules switch at very high frequencies and use logic to force their current wave shape to be identical to the voltage wave shape so all the power they take from the grid for DC loads and battery charging is at unity power factor (PF = 1.0). In addition they can compensate for all of the loads in their system to cancel out the power factor problems of CFL lights and computers so that all the grid sees is unity power factor. A typical home would have 4 kW of Transverter power modules which is more than enough to compensate the power factor for all the CFL lights and computers in the home. Since this compensation is done in the house, very close to the loads, then its compensation benefits the entire transmission system including the transformers and the wires to the house.
This power factor issue is so critical that the Transverter remote data panel continuously displays power factor and the included software uses the internal data acquisition to display real time pictures of the wave shapes. In fact, the oscilloscope pictures shown here were actually taken from the software on a live Transverter system. The hope is that this will increase consumer awareness of power factor and aid them in selecting new equipment.
Inductive Motor Magic
It is interesting to watch the power factor of an inductive motor change with the load on the motor. When the motor is lightly loaded it takes pretty much current but has a very low power factor so there isn’t much real power consumed. As the load increases the current doesn’t change so much but power factor increases so the real power increases. You could think of the volt-amps that are there when the power factor is low as kind of a reserve power that is ready to be applied any time by simply raising the power factor. When the power factor gets very close to 1.0 this means that the motor powering its maximum possible load and additional loading will stall the motor. Monitoring power factor in real time can be used for everything from protecting motors, determining pressure and gallons per minute on a pump to determining if there is a freon leak in a refrigeration system.
THIGH high voltage power module with automatic power factor correction. View more detailed paper.
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