Three Phase Correction

In this article, we will discuss three phases. In the previous article, we found that the process of converting an AC input source to a fixed DC resource is called Correction, and the most popular circuits used to perform this correction process are based on solid-state semiconductor diodes.

In fact, the straightening of alternative voltages is one of the most popular applications of diodes, since diodes are inexpensive, small and robust, allowing us to create a large number of rectifier circuits using individually connected diodes or just a single integrated bridge rectifier module.

As in homes and offices, single-phase feeds are often called 120 Vrms or 240 Vrms phase-neutral, as well as line neutral (LN), and produce an alternating voltage or current at a nominally constant voltage and frequency. Sinusoidal waveform given the abbreviation "AC".

Three-phase straightening, also known as multi-phase straightening circuits, is similar to previous single-phase rectifiers, this time the difference is that we use three interconnected, single-phase welding produced by a single three-phase. generator.

The advantage here is that 3-phase straightening circuits can be used to power many industrial applications, such as engine control or battery charging, which require higher power requirements than a single-phase rectifier circuit can provide.

3-phase sources take this idea a step further by combining three AC voltages with the same frequency and amplitude as each AC voltage being called a "phase". These three phases are out of phase 120 degrees from each other and form a phase sequence or 360o ÷ 3 = 120o phase rotation as shown.

Three-phase WaveForm

Three Phases

The advantage here is that a three-phase alternating current (AC) source can be used to provide direct electrical power to balanced loads and rectifiers. Since a 3-phase feed has a constant voltage and frequency, it can be used by a straightening circuit to produce a dc power with constant voltage, which can then be filtered and results in an output DC voltage with less fluctuation than a single-phase straightening circuit.

Three Phase Correction

Once we have seen that a 3-phase resource consists of combining only three single phases, we can use this multi-phase feature to create 3-phase rectifier circuits.

As with single-phase straightening, three-phase straightening uses diodes, thyristors, transistors or transducers to create half-wave, full wave, uncontrolled and fully controlled rectifier circuits that convert a specific three-phase source to a fixed DC output level. . In most applications, a three-phase rectifier is fed directly from the mains power grid or from a three-phase transformer, if different DC output levels are required by the connected load.

As with the previous single-phase rectifier, the most basic three-phase rectifier circuit is an uncontrolled half-wave rectifier circuit that uses three semiconductor diodes, one diode per phase as shown.

Half-wave Three-Phase Straightening

Three Phases

So how does this three-phase half-wave rectifier circuit work? The anode of each diode, the cathodes of the three diodes are connected to a phase of the voltage source depending on each other to the same positive point, effectively creating a diode-"OR" type arrangement. This common point is the positive (+) terminal of the load, while the negative (-) terminal of the load is connected to the neutral (N) of the feed.

Assuming that the Red-Yellow-Blue (VA – VB – VC) phase rotation and the red phase (VA) begin at 0o. The first diode to be communicated will be diode 1 (D1), as it will have a more positive voltage in the anode than the D2 or D3 diodes. Thus, the D1 diode is transmitted for the positive half cycle of the VA, while D2 and D3 are in reverse state. The neutral wire provides a way for the load current to return to the source.

After 120 electrical degrees, diode 2 (D2) begins to transmit for the positive semi-cycle of VB (yellow phase). Now the anodes become more positive than the D1 and D3 diodes, both of which are "OFF" because they are inverted. Similarly, after 120o, the VC (blue phase) begins to flip "ON" diode 3 (D3) as the anodynaus becomes more positive, thereby making the D1 and D2 diodes "OFF".

Then, for three-phase straightening, we can see that whatever diode has the more positive voltage in the anode compared to the other two diodes, it will start transmitting automatically, thereby giving the D1 D2 D3 a transmission model, as shown.

Half-wave Three-Phase Rectifier Transmission WaveForm

Three Phases

From the above waveforms for a resistant load, we can see that each diode for a half-wave rectifier passes current for a third of each cycle, with the output waveform three times the input frequency of the AC source. Therefore, there are three voltage peaks in a given cycle, so by increasing the number of phases from a single-phase phase to a three-phase resource, the correction of the source is improved, that is, the output DC voltage becomes smoother.

For a three-phase half-wave rectifier, VA VB and VC supply voltages are balanced, but with a phase difference of 120o:

  • VA = VP*sin(ωt – 0o)
  • VB = VP*sin(ωt – 120o)
  • VC = VP*sin(ωt – 240o)

Thus, the average DC value of the 3-phase half-wave rectifier output voltage waveform is given as follows:

Three Phases

Since voltage provides peak voltage, VPvrms equals 1,414, so it concludes that VRMS is equal to VP/1,414 or 0.707VP as 1/1,414 = 0.707. The average DC output voltage of the rectifier can then be expressed as follows in terms of the average square root (RMS) phase voltage:

Three Phases

Full wave Three Phase Straightening

The full wave three-phase uncontrolled bridge rectifier circuit uses six diodes, two per phase, similar to a single-phase bridge rectifier. A 3-phase full wave rectifier is obtained using two half-wave rectifier circuits. The advantage here is that the circuit produces a lower surge output than the previous half-wave 3-phase rectifier, since it has a frequency six times that of the input AC waveform.

In addition, the full wave rectifier can be fed from a balanced 3-phase 3-wire delta-connected feed, as no fourth neutral (N) cable is required. Consider the full wave 3-phase rectifier circuit below.

Full wave Three Phase Straightening

Three Phases
Full wave Three Phase Straightening

As before, assuming that the Red-Yellow-Blue (VA – VB – VC) phase rotation and the red phase (VA) begin at 0o. Each phase is connected between a pair of diodes, as shown. One diode of the conductive pair powers the positive (+) side of the load, while the other diode powers the negative (-) side of the load.

The D1 D3 D2 and D4 diodes form a bridge rectifier network between phases A and B, similarly the D3 D5 D4 and D6 diodes are between phases B and C, and phases D5 D1 D6 and D2 are between phases C and A.

Thus, the D1 D3 and D5 diodes feed the positive rail and transmit it depending on which one has the more positive voltage in the anode terminal. Similarly, the D2 D4 and D6 diodes feed the negative rail and transmit whatever diode has more negative voltage in the cathode terminal.

Next, we can see that the diodes for three-phase straightening move in pairs that match, giving a transmission model for the following load current: D1-2 D1-6 D3-6 D3-6 D3-4 D5-4 D5-2 and D1 -2 as shown.

Full Wave Three Phase Rectifier Transmission WaveForm

Three Phases
Full Wave Three Phase Rectifier Transmission WaveForm

In 3-phase power rectifiers, transmission always occurs in the most positive diode and the corresponding most negative diode. Thus, as the three phases rotate along the rectifier terminals, the transmission passes from the diode to the diode.

Each diode then transmits 120o (one-third) in each feeding cycle, but since two diodes must be transmitted in pairs, each pair of diodes will transmit only 60o (one-sixth) of a cycle at any time, as shown above. .

Therefore, we can say correctly that each phase will be separated by 360o/3 for a 3-phase rectifier fed with transformer secondarys "3" and therefore requires 2*3 diodes.

Also note that unlike the previous half-wave rectifier, there is no common connection between the rectifier input and output terminals. Therefore, it can be fed with a star-linked or triangular connected transformer source.

Therefore, the average DC value of the 3-phase full wave rectifier output voltage waveform is given as follows:

Three Phases

Where: VS equals (VL(PEAK) ÷ √3), where VL(PEAK) is the maximum interline voltage (VL*1.414).

Fully controlled 3-phase Bridge Rectifier

Three Phases