Loads composed of purely resistive elements (resistive loads) can be powered by DC, so there is no need to use an inverter. However, due to the large power consumption of resistive loads, the use of such loads should be limited in independent power supply systems, including electric heaters, kettles, water heaters, electric irons, and hair dryers. Resistance wires in incandescent light bulbs are not within this scope. You may wish to opt for DC lighting circuits (such as halogen lamps), which require dedicated lighting inverters (usually installed in the lampshade), and fluorescent lamps that can be powered by AC or DC (no inverter required).
Most household appliances are equipped with DC motors, but their prices are generally higher due to the small production scale. The use of ultra-low voltage direct current is also limited due to the issue of voltage losses. Therefore, for most households, it is more cost-effective and more convenient to use AC household appliances through inverter conversion than directly using DC household appliances.
If you have a mix of DC and AC circuits in a home, wiring can get very complicated, so generally the easiest way to do this is to use an inverter to provide AC power for the entire home (including lighting). Due to the efficiency loss of the inverter and the energy storage battery, the AC power supply scheme may increase the load of the entire system, so whether to adopt this scheme should be decided after evaluating all the alternatives. If a high-efficiency DC cooling system is used, then connecting the DC lighting circuit will not cause too much additional complexity. Remember that the DC and AC circuits must be physically separated, and that the DC wiring must use polarized two-pin plugs instead of 240V three-pin plugs and sockets. DC circuits can also increase the problem of radio wave interference unless the necessary precautions are taken.
Note: With the large-scale application of energy-saving fluorescent lamps, the wiring method of independent houses in Australia is usually the same as that of ordinary houses connected to the grid, that is, 240V AC is always used, and the whole system is connected to the main switchboard.
1 The advantages of direct current and alternating current
The biggest advantage of alternating current is that it can be stepped up or down directly through a transformer. The principle of the transformer is to use the generated alternating magnetic field to induce an alternating current in the conductor on the other side. In addition, the cable loss and cost of 240V AC system is much lower than that of 12~48V DC system.
In the past, it was difficult to directly transform direct current. To achieve the transformation of DC power, it needs to be converted into AC power, then converted into high voltage AC power transformation, and then converted back to DC power. The above DC transformation process can complete the conversion with an efficiency similar to that of a DC-AC inverter.
Note: With the advancement of technology, DC-DC converters have been able to easily realize the transformation of direct current. This also promotes the development of transformerless inverters.
2 AC measurement
The voltage, current, and power of alternating current are calculated differently than direct current. Since the instantaneous voltage of alternating current is constantly changing, in order to accurately describe these variables, it is necessary to calculate some average value of its waveform. There are two kinds of average values commonly used in AC circuits: the first is the root mean square (effective value, RMS) voltage. Considering that the electrical power consumed by the resistive load is proportional to the square of the instantaneous voltage, it can be found that for the same resistance circuit, The root mean square voltage of alternating current is its equivalent direct current voltage; the second average voltage is the time average of the voltage, which is mainly used to determine the excitation characteristics of transformers and motors. For different voltage waveforms, the time-averaged voltage may be greater than or less than the rms voltage.
The third important feature of the AC waveform is its peak value. The peak value of the sinusoidal voltage waveform of a 240V AC power supply system is about 340V.
The relationship between the rms voltage and the peak voltage is expressed as
URMS=Up/√2=0.707Up
Taking AC sine wave and square wave as an example, the relationship between the root mean square value and the peak value is shown in Figure 1 and Figure 2.
When measuring voltage and current from a non-sinusoidal AC source, a rms meter must be used.