There are mainly two different criteria for distinguishing the types of inverters.
1. The way of converting direct current to alternating current
1) Rotary inverters (motor inverters) Before the advent of solid-state inverters, 240V alternators driven by DC motors were widely used to convert between DC and AC. This conversion method benefits from the mechanical inertia of the rotating equipment, so that the DC input voltage can still operate normally when the power generation temporarily drops under low-load conditions.
2) Basic solid-state inverter
The conversion of ultra-low voltage DC power and output of 240V AC power is divided into two different steps. In the process, the amplitude of the voltage must be increased, and the output voltage frequency is 50Hz. For many years, all inverters first converted DC to 50Hz AC, then went through a transformer to boost the AC voltage to 240V. This inverter is relatively simple and effective, and is the mainstream inverter in many remote power systems. Currently the capacity can reach several kVA, but since the transformer for 50Hz AC is larger, the inverter is also quite heavy.
3) Switching inverter
With the development of technology, some inverters first use high-frequency DC-DC switching converters to boost the DC voltage to 338V, and then convert the DC to AC. This type of voltage converter works on the same principle as modern switching power supplies and has led to a dramatic reduction in the weight of many powered devices (eg televisions, battery chargers) over the past decade.
Although the small size of the inverter is an advantage, it is not an absolute advantage. Compared with the low-frequency inverter of the same rated power, the high-frequency inverter has a smaller heat dissipation area and a higher body temperature when the equipment is working under the same heat dissipation. This shortcoming can be overcome to a certain extent by increasing the size of the inverter as a whole, but due to the small thermal capacity of the high-frequency inverter, short-term overload can still cause it to overheat rapidly. Therefore, according to the rated capacity of the inverter, not all switching inverters are suitable for those electrical equipment with large starting current motors.
2. output waveforms
Inverters can also be classified according to the output waveform. Transistor switches can also be used to generate a sine wave, using the same principle as a stereo amplifier, again by controlling the voltage drop across the transistor junction in its linear region. However, transistors cannot operate efficiently unless they can quickly switch between fully off and fully on. For all points in between these two states, the transistor consumes a lot of energy. Therefore, it is impractical for transistors to directly generate a sine wave output in an inverter.
To overcome this limitation, various approximations are used on the set waveforms. The easiest way to do this is to generate square waves that will operate in a wide variety of powered devices, many devices (such as incandescent light bulbs and resistive loads) typically operate just fine with square waves.
1) Square wave inverter
The simplest form of square wave inverter is to provide a square wave power supply with full pulse width, and the peak and average value of the waveform are 240V. Most power-type devices can be directly connected to this type of inverter (Figure 1). Since these inverters do not have output voltage regulation, the inverter output voltage fluctuates greatly when there are changes in the input power and load. Some unstable loads will cause the inverter terminal voltage to exceed 300V, which is likely to seriously damage sensitive equipment.
If the peak voltage is lower than 300V, some loads will not work properly even when the average and rms values of the voltage are at normal levels. The equipment most affected are switching power supplies used in televisions, computers and monitors. These devices rectify the input and store energy in capacitors, which are then passed through a voltage converter to generate the voltages required by various system circuits. If the peak voltage is too low, the capacitor will not charge sufficiently. However, most voltage converters are designed with a wide input voltage range and it is easier to compensate for low input voltages. In Australia, as 240V is at the high end of all designed input voltage ranges, undervoltage is not a problem for powered equipment.
However, a slightly higher output peak voltage is likely to damage equipment or cause a power failure.
With all non-sinusoidal inverters and switching power supply units brought into operation, further consideration needs to be given to the fast rise time of the waveform, which will cause very high peak currents in the input components of the rectifier and consumers, causing these devices to burn out . As for which devices will be affected, it’s hard to make reliable predictions. Equipment manufacturers do not preset waveforms other than sine waves, so performance does not necessarily correlate with quality. Many equipment switching power supplies are designed for 90~260V input, because when operating at a low voltage input (such as 110V AC), a higher input current will be designed to ensure reliable power supply.
Interestingly, when the load is sufficient, since the square wave inverter rectified voltage is almost constant, there is little chance of problems. A stepped square wave inverter can discharge the storage capacitor in a switching power supply, causing a fast inrush current at the beginning of the next pulse.
The voltage of most full-pulse square wave inverters is difficult to control, so it is advisable to take special care before using sensitive equipment such as computers and televisions.
There is no feedback in the output control circuit, so there is no voltage regulation.
2) Corrected (ladder) square wave inverter
To control the AC output voltage, a “pulse width modulation” (PWM) technique can be used. The rms of the output voltage can be reduced by reducing the duration of the pulses (Figure 2). If the output voltage is fed back to the control circuit, a stable output voltage will be produced. This technique can be used to compensate for battery voltage and load variations.
Using narrow pulses can bring the peak voltage closer to a true sine wave, and the output will also carry less energy and more harmonic currents. However, the peak voltage is not stable and varies proportionally with the input voltage. This is simply a function of transformer turns ratio and peak voltage, and varies with input voltage and load.
This type of inverter is often referred to as “modified square wave”, “quasi-sine wave” or “modified sine wave” (not quite exactly). Note, however, that in most cases their performance reverts to a square wave at full load. The schematic diagram of the modified square wave inverter is shown in Figure 3.
Feedback in the output control circuit allows voltage regulation by changing the output pulse width.
The modified square wave inverter in its original form had serious limitations. The wave region between alternating pulses can simply find its own level, any inductance present in the load circuit or the inductance of the power transformer itself will cause the voltage between pulses to become non-zero. In effect, the resulting wave would carry small wide pulses between regular output pulses, a condition that would destroy mains-powered equipment, causing the clock to run at twice the design speed; it could also cause induction motors to run at double the speed Run so that the induction motor hums and vibrates. The solution to this problem is to control the “dead-band” voltage (Figure 2).
Different design ideas will be realized in different forms and will be marketed under different names. The basic idea is to short-circuit the load within the “dead zone” and allow the induced current to decay naturally through the inverter without generating false pulses. Many manufacturers short-circuit the transformer windings, not the load itself. The same circuit is often used to suppress high voltage spikes that occur when the load is inductive.
It is worth noting that this circuit tends to be less powerful than the main inverter, and has the potential to overload the inverter’s inductive control unit at less than full power, such as the start of a large induction motor. The motor behaves almost purely inductively due to stall, and the inverter generates a large output inrush current which is then fed back to the smaller inductive control circuit, creating a potential overload. These circuits are usually thermally protected, but in extreme cases, the inrush current can be too high to control, causing damage to the circuit before the temperature sensor overheats.
Such inverters can often be paralleled with a dedicated synchronous output to double the peak power.
3) Sine wave inverter
Some devices, such as the growing number of “smart appliances,” only work well with low-distortion sine waves. In the 1990s, inverter manufacturers began to focus on developing such high-quality sine wave inverters, which are currently very dominant.
As shown in Figure 4, the waveforms in these “synthesized” sine wave inverters are built up from short pulses. By switching sufficiently, the transistor can be made to function efficiently. The transistor waveform is a high frequency square wave whose pulse width is modulated with a sine wave at a frequency of 50Hz. The filter is used to smooth the waveform to a pure 50Hz sine wave. Capacitors can also be used to store charge.
The choice of power frequency is arbitrary. Using more pulses reduces the size of the filter components, but also results in more switching losses.
In such inverters, noise (both acoustic and electrical) caused by harmonics can be a significant problem.
The simplest form of a sine wave inverter is shown in Figure 5, with two pulses per cycle, and the filter is a complex resonant type with capacitors. Its resonance types include series resonance and parallel resonance, which have been widely used. The large circulating resonant current makes parallel resonance very inefficient when operating at part load. In series resonance, the load current is resonant and the losses are proportional to the load. Such inverters are not common anymore.
Synthetic sine wave inverters can be adjusted to compensate for larger changes in input voltage than square wave inverters without changing their peak voltage, and some types can work fine with 12 or 24V input voltage.
Sine wave inverters are best managed using a microprocessor, and many additional functions can be added, such as user modification of control parameters, data logging (eg, battery charge monitoring), and charging power control. In many cases only a signal needs to be provided to start the auxiliary generator when the battery becomes low, while the use of a microprocessor also allows the user to set control parameters.
By adding a modem to the system, information can be sent to and from the device, and performance monitoring can be performed, a very useful feature in large or remote systems.
4) Bidirectional inverter (battery charging inverter)
Some inverters have additional advantages, such as when properly programmed on the microprocessor, the inverter can also act as a battery charger, so when the battery needs to be charged from an AC source (generator), current can be fed into the inverter converter, convert it to DC, and then use it to charge the battery. This saves costs, reduces the number of system components, and eliminates the need to add an external AC transfer switch between the inverter and generator. The only downside might be that the battery charger won’t run when the inverter needs service and won’t run.
An extension of this versatility is the interactive inverter, which can be synchronized with an external source.
The generator’s interactive inverter/charger typically starts the generator when the battery needs to be charged or when the AC load is greater than the inverter can supply. For the latter (the AC load is greater than the inverter’s supply capacity), the inverter will supply power in parallel with the generator. For example, if the inverter is 2kVA and the generator is 4kVA, then there is a total of 6kVA supplied to the AC load.
The ability of the inverter to synchronize with a power source that deviates from the design frequency is a major factor that differentiates the performance of different interactive inverters. Sometimes generators and inverters are too unstable to function properly. When installing an interactive system, the generator recommended by the inverter manufacturer is usually used.