Rectifiers convert AC power to DC power. In most rectifiers the resulting output current is not constant but pulse current. Different filters are used for smoothing the pulsations. There are many types of rectifiers. Here are some popular classifications:
According to the type of the device – mechanical, thyristor, transistor, diode rectifiers;
According to the power – high-power and low-power;
According to the number of the rectified half periods – one half-period rectifiers (letting only one half-wave of the sinusoid), two half-period rectifiers (letting both half-periods), half wave rectifiers (using only part of the sinusoidal half-wave of the current), full wave rectifiers (using the full half-wave of the current);
According to the magnitude of the rectified voltage – low voltage (up to 100V), medium voltage (100 to 1000V), high voltage (above 1000V);
According to the number of the phases that are used – single-phase, two-phase, three-phase and polyphase;
According to the scheme – bridge rectifiers, rectifiers multiplying the voltage, transformer rectifiers, rectifiers with galvanic separation;
According to their manageability – uncontrolled (diode) and controlled (thyristor);
We will have a look at the most common rectifiers: the Graetz circuit and the Illarionov circuit.
Graetz circuit. The Graetz circuit is a diode two half-period bridge converting alternating current into pulsed current. It was invented by the German physicist Leo Graetz. AC voltage, which doesn’t have to be sinusoidal, is supplied at the input. In each of the half-cycles the current flows only through the two diodes.
The other two are blocked. As a result, pulsed voltage with frequency two times greater than the supplying voltage is generated at the output.
In order to have rectified, rather than pulsed voltage, a capacitor, throttle or voltage stabilizer is needed.
The advantages of two half-period (two-way) voltage rectification compared to one half-period voltage rectification are obvious: the resulting output voltage has double frequency, and then it can easily be “smoothed” by a capacitor filter, permanent magnetizing of the secondary winding of the transformer is avoided and the efficiency is higher.
The Graetz circuit can be built by individual diodes or can be encapsulated in a unit. The latter type of rectifier is cheaper and more compact. Moreover the installation of such a module is easy. The diodes which are selected in the component have the same characteristics and are in constant thermal regime. The disadvantage of the unit is that if one of the diodes burns out, the whole construction must be thrown away.
The Graetz circuit is often placed in a common housing with a transformer, and this device is referred to as an adapter or power supply. The parameters that fully define an adapter are: nominal DC at the output of the adapter and the range of its alternation, power, nominal input voltage and the range of its alternation. When designing or selecting a two half-period rectifier, you need to consider that the input AC voltage must be measured based on its effective value and the rectified voltage after the capacitor, in the absence of load – based on its amplitude value and it is 1.41 times greater than the effective one.
For example: if the measured single-phase voltage before the bridge is 12VAC, then after the bridge and the capacitor it will be 17VDC. When there is load, the voltage at the output of the rectifier will drop, but not lower than the value of the effective voltage. This decrease depends on the internal resistance of the transformer and the capacity of the filter capacitor. Hence the voltage of the secondary winding of the transformer must be selected based on the maximum permitted value of the input voltage and the capacity of the smoothing capacitor must be large enough so that the output voltage, when there is load, doesn’t drop below the minimum threshold for the circuit, which it will power. Of course for greater accuracy we must take into account the resistance of the connecting wires and the greatest possible amplitude of the input voltage.
The most common three-phase rectifiers are based on the Mitkevich circuit
– they have 3 LEDs, where each valve (diode) is uncapped during part of one half-period of the phase
Less known is the circuit with 3 parallel bridges with 12 diodes as well as the circuit with three consecutive bridges with 12 diodes. Depending on the connection of the three-phase transformer or the three-phase generator (star or triangle) there are two types of rectifiers – Illarionov triangle and Illarionov star. In the rectifier of the type Illarionov triangle the losses of copper are greater, therefore the more commonly used circuit is the Illarionov star.
The primary windings of the three-phase transformer are three and can be coupled as a star or a triangle, the secondary windings are also three or multiples of three. In the operation of the Illarionov triangle curcuit there are two periods, the large period equals 360° (2), the small period equals 60° and is repeated more than 6 times. The small period is composed of two small half-periods of 30°, which are mirror-symmetrical.
rectifier is used in power supply generators of almost all vehicles (auto-tractor vehicles, water, underwater and air transport, etc.) In diesel-electric vehicles (locomotives and ships) almost all the power is derived from the Illarionov star rectifier. The average electromotive force in the Illarionov star circuit is greater than that in the Illarionov triangle circuit.
Other circuits that are used are the 12 pulse static three-phase rectifier representing two connected Illarionov rectifiers with phase shift of the input three-phase current. Thus the number of the rectified half-periods is doubled, compared to conventional Illarionov bridges. The relative amplitude of the pulsations is reduced, when the frequency of the pulsations of the rectified current is doubled.
Even less known is the three-phase rectifier circuit “six bridges” – in series and parallel. It consists of two three-phase transformers, whose primary windings are connected – one in a star, and the other in a triangle, which shifts the phase to 30°.
The six secondary windings are connected with six bridges (24 diodes), where the bridges may be coupled in various ways, for example, all six bridges can be connected in parallel.
This rectifier, just like the rectifier with three parallel bridges, excels the bridges of Illarionov and the bridges of Mitkevich in many parameters. Moreover it uses diodes which are much smaller by current, i.e. we obtain a high power rectifier which consists of low power components.