Features and working principle of three-phase rectifier, bridge rectifier circuit, single-phase device

The primary use of rectifiers is to lead a direct current (DC) source from an alternating current (AC) source. Almost all electronic devices require direct current, therefore three-phase rectifiers are used inside power supplies for a very wide range of electronic equipment.

Full cycle chain

It is a rectifier circuit that converts AC voltage to DC voltage. These circuits are called full wave rectifiers because they generate a full cycle output.

The advantages of three-phase rectifiers:

1. Due to their low cost compared to center push, they are widely used in the power supply chain.
2. This can be used to detect the amplitude of a modulated radio signal.
3. Bridge rectifiers can be used to supply polarized voltage for welding.

Three-phase rectifier circuit

Most industrial power supplies for electric motors and welding applications use three-phase AC voltage. This means that the device for these circuits must use a three-phase bridge that has six diodes to provide full-wave rectification (two diodes for each line of three phases). This figure shows an electrical three-phase rectifier bridge circuit.

In the diagram, the secondary winding of a three-phase transformer on the diode of the device. 1D, 3D, and 5D are connected together to provide a common point for negative DC output power. 2D, 4D and 6D are connected to provide a common point for a constant positive output power pin.

Electronic circuit of a three-phase bridge rectifier, where it is connected to the secondary winding of a three-phase transformer. Three-phase input sine waves (b). Six half-waves for DC output. A good rule of thumb for determining connections on diode devices is that the AC input voltage (U) will be connected to the bridge where the anode and cathode of any two diodes are connected.

Since this occurs at two points on the bridge, the input U does not have a defined polarity. The positive lead for the power supply will be connected to the bridge, where the two cathodes of the diodes are connected, and the negative lead will be connected to the bridge and the two anodes of the diodes are connected.

Since the six half-waves overlap, the DC voltage has no chance of reaching the zero point of the voltage, so the average DC output voltage is very high.

The three-phase full-wave bridge rectifier is used where the required amount of DC power is large and the efficiency of the transformer must be high. Since the half-wave outputs overlap, they provide a low ripple percentage.

In this circuit, the output ripple is six times the input frequency. Since the ripple percentage is low, the output U (DC) can be used without much filtering. This type of device is compatible with star or delta connected transformers.

Bridge type device

A three-phase bridge rectifier circuit uses six diodes (or thyristors if control is required). The output voltage is characterized by three values: minimum U, average U and peak voltage.

A full-wave three-phase rectifier is similar to a Heitz bridge.

Diagram of a full-wave three-phase device. Conventional three-phase rectifier does not use neutral. For 230 V / 400 V mains between two rectifier inputs. Indeed, there is always a composite voltage U (= 400 V) between the 2 inputs.

An unsupervised device means that the average output U for that input U cannot be adjusted. Uncontrolled rectification uses diodes.

The controlled rectifier allows you to regulate the average output voltage by acting on the thyristor response delays (used instead of diodes). This command requires a complex electronic circuit. The diode behaves like a thyristor, loaded without delay. The rectified voltage looks like this.

Output U three-phase output voltage. There are 7 curves in total: 6 sinusoids and a red curve connecting the top of the sinusoids ("sinusoidal caps"). 6 sinusoids represent 3 voltages that make up U between phases and 3 identical voltages, but with the opposite sign:

U31 = -U13U23 = -U32U21 = -U12.

The red curve represents U at the rectifier output, that is, at the resistive load terminals. This U does not apply to neutral. She swims. This U ranges between 1.5 V max and 1.732 V max (root of 3).

Umax is the peak value of one voltage and is 230 × 1.414 = 325 V.

Three-phase voltage properties

A curve acting only on a resistive load, uncontrolled rectification (with diodes), does not return to zero, unlike a mono-frequency device (Greitz bridge). Thus, the ripple is much lower and the dimensions of the inductor and / or smoothing capacitor are less restrictive than for the Heitz bridge.

To obtain a non-zero output U, at least two phases are required. Minimum, maximum and average voltage value. Numerically, for a 230 V / 400 V network, the rectified voltage fluctuates between the minimum voltage: 1.5 V min = 1.5 x (1.414 × 230) = 488 V, and the maximum: 1.732 Vmax = 1.732 x (1.414 × 230) = 563 V.

Average value of three-phase rectified voltage: avg = 1.654Vmax = 1.654 x (1.414 × 230) = 538 V.

Output voltage of three-phase output rectifier (zoom). 3-phase full wave rectifier MDS 130A 400V. 5 terminals: 3 phases, + and -. This rectifier contains 6 diodes.

Thus, the following points can be summarized:

• 6 diodes, 2 diodes per phase - weak ripple compared to a single-wave rectifier (Heitz bridge);
• average value of the rectified voltage: 538 V for a 230 V / 400 V network;
• the neutral is not used by the three-phase rectifier.

Single phase full wave device

The figure shows a single-phase full-flow controlled rectifier with load R.

The single-phase fully controlled rectifier allows the conversion of single-phase AC to DC. It is commonly used in various applications such as battery charging, motor speed control DC power supply and front of UPS (uninterruptible power supply) and SMPS (switchable power supply) mode).

All four devices used are thyristors. The switching times of these devices depend on the start signals. Shutdown occurs when the current through the device reaches zero and it is reverse biased for at least a duration equal to the shutdown time of the device indicated on the data sheet:

• In positive semi-cyclic thyristors T1 and T2, they fire at an angle α.
• When T1 & T2 conducts Vo = Vs IO = is = Vo / R = Vs / R.
• In the negative half cycle of the SC3 input voltage, T3 and T4 are triggered at an angle (π + α).
• Here, the output current and the supply current are in the opposite direction. T3 & T4 turns off at 2π.

Diode bridge operation

It consists of four diodes, and this configuration is connected through the load.

During the positive half cycle, the inputs of diodes D1 and D2 are forward biased and D3 and D4 are reverse biased. When a voltage that exceeds the threshold level of diodes D1 and D2 begins to conduct - current begins to flow through it, as shown in the figure below on the red line.

During the negative half cycle of the AC input, diodes D3 and D4 are forward biased and D1 and D2 are reversed. Load current begins to flow through diodes D3 and D4 when these diodes begin to conduct, as shown in the figure.

In both cases, the direction of the load current is the same, as shown in the figure one-sided, which means DC. Thus, when using a bridge rectifier, the AC input current is converted to DC. The output to the load with this bridge rectifier is pulsating, but pure DC requires an additional filter such as a capacitor. The same operation is applicable for various bridge rectifiers, but in the case of controlled rectifiers, a thyristor is triggered to control the current for the load.

Mode 1 (α to π). In the positive half cycle of the applied AC signal, T1 and T2 are forward biased and can be turned on at an angle α. The load voltage is equal to the positive instantaneous AC supply voltage.

Mode 2 (π toπ + α). When wt = π, the input power is zero, and after π it becomes negative. But the inductance counteracts any changes to keep the load DC in the same direction.

Because of this induced voltage of SC1, T1 and T2 are advanced despite the negative supply voltage. Thus, the load acts as a source and the stored energy in the inductor is returned back to the AC source.

Mode 3 (π + α up to 2π). At wt = π + α SCR T3 and T4 turn on and T1, T2 - reverse bias. Thus, the conduction process is transferred from T1, T2 to T3, T4. With a positive load voltage and energy consumption, the current is maintained.

Mode 4 (from 2π to 2π + α). At wt = 2π, the input voltage passes through zero.

Comparison of single-phase and three-phase devices

A single phase rectifier is generally less expensive than a three phase rectifier of the same power rating, but this cost advantage becomes less significant at higher loads. Larger rectifiers are used in large UPS systems, electroplating, electro-cleaning and anodizing plants, large DC motor controllers, etc.

Any device over 10 kW must have a three-phase input. In addition, variable frequency AC controllers that rectify the network directly without transformer, have a three-phase rectifier, although single-phase input is possible for motors less than 5 kW.

Below is a list of the advantages of three-phase and single-phase rectifiers with the same power output:

1. The mains input current is lower and balanced between the three phases. This balance is important if the rectifier load is a significant part of your plant's total load.
2. Input harmonic currents are smaller and more easily suppressed.
3. The output ripple is much less and the frequency is 3 times that of a single-phase rectifier. This makes smoothing much easier with smaller chokes and / or capacitors.

The average current of each is about 67% of the value for a single phase rectifier. Therefore, smaller devices can be used and are easier to distribute around the radiators. For small devices, these advantages are not so important. But for large rectifiers (over 10 kW), they become more significant.