AC Recovery

To obtain the necessary DC power electronic circuits, the AC power available from the network using specialized circuits- rectifiers or diodes, in association with auxiliary circuits which ensure the required characteristics. The rectifier is an electronic circuit able to convert AC electricity into DC electricity. The most commonly used are half-wave rectifiers and bi-alternance recitifiers.

The recovery phenomenon consists of transforming a current which changes its directions, in time, in a current that has the same direction. In particular, this power may be sinusoidal (a current that periodically changes its direction);

The recovery plants include, usually, a power transformer (which has the role to adapt the input voltage to the output voltage value and to reduce short circuit currents) Recovery Elements (with unilateral conductive) High-speed protection elements (fuses, circuit breakers, disconnectors); Filter Elements

The semiconductor diode is an electronic device that, (due to the diffusion phenomena from the semiconductor junction area) has the property to allow the passage of electric current in one direction (forward bias, Fig. 1). Semiconductor diode At reverse bias, the diode’s resistance is high, the current being negligible. The current-voltage characteristic, nonlinear, is qualitatively shown in Fig. 2. This behavior allows the use of semiconductor diodes for the recovery of the alternating current.

The rectifier diodes operate due to the property of behaving differently to stress biased and reverse bias voltages. Thus, at direct bias voltages, the direct resistance is very low and at reverse polarization the reverse resistance is very high. Due to this property – like when applying a work alternative, they function on a positive alternation, leading a high (of order mA or A). On negative alternation they will block, permiting to pass to a very small A mA or order, which can be neglected. This process of transforming an alternating signal into a continuous signal is called RECOVERY. These diodes are used in the construction rectifiers that work with large signals and low frequencies (50Hz). You can achieve They can be realised both from germanium and silicon – the silicon ones have the have the following advantages over the germanium ones: 1. The reverse current is much smaller; 2. The breakdown voltage is much higher; 3. The maximum working temperature is of 190 degrees, versus 90 degrees at germanium. Disadvantage – the opening voltage is considered to be slightly higher.

In Fig. 3 it is shown a scheme of a simple recovery installation in which, between the AC source (S) and the resistive receptor (R) a rectifier is inserted. (D.R., recovery mechanism). In Fig. 4 it is shown qualitatively the shape variation of the AC voltage provided by the S source (sinusoidal) and of the recovery current at one or both alterations. Proper use of rectifier diodes involves keeping the limit values (Catalog data) that need not to be overcome, such as: - the continuous direct current (IF), the continuous converse tension (VR), the average rectified current (I0), the repetitive peak direct current (IFRM), working peak reverse voltage (VRWM). It is noted that the rectified signal has only one direction, but it is pulsating. If roller devices are used, namely stabilization, we can greatly reduce these pulsations, and the signal is approaching a continuous one.

Network NETWORK TRANSFORMER RECTIFIER ELEMENT (RECOVERY SYSTEMS) Load resistance Smoothing filter Typical block diagram of a single phase rectifier:

Making alternating single rectifier The engine is connected to the AC power supply through rectifying diode as shown. The diode allows the passage of a single half of the alternating current from the source to the load (the alternation of positive or negative, depending on connection of the diode in the circuit). In other words, only a semi-period of the A.C. signal that will pass from the source to the load. The result is that the engine "sees" a uni- directional, pulsatory current, which allows its rotation in a single direction.

As one can see, the DC motors with permanent magnets do not work very well in AC. The conductor is temporarily removed and diode’s position in the circuit is inverted. It can be observed the effect on th engine operation. The DC voltage is measured on the motor terminals, as follows:

DIODE RECTIFIERS BEFORE RECOVERY Half-wave rectifier TRANSFORMER AFTER RECOVERY

Explanation of operation During the positive alternations of the tension, the diode leads (being directly biased, with plus on the anode and minus on the cathode); During the negative semialternants, the diode polarization is reversed, so that it can not driveanymore (it is blocked).

The efficiency of an reseter is only 40 %)

Double alternation in bridge rectifier This rectifier circuit is called a double alternation, because it uses the entire alternative waveform, both positive and negative semi- alternation, for the supply of the load. The result is a much smaller ripple voltage on the load. The actual value of the output voltage is also higher for this type of alternating single rectifier versus the mono-alternance rectifier.

TRANSFORMER BRIDGE RECTIFIERS BEFORE RECOVERY AFTER RECOVERY DOUBLE ALTERNATOR rectifier

Explanation of operation During the positive alternations, the diodes come into conduction one at a time, depending on the polarization.

The advantage of this waveform recovery is is the wave shape - as close to the continuous one. The pusatory signal frequency, obtained from the load terminals, is 100 Hz. The efficiency will double (80%) Advantages:

Disadvantages: The scheme is more complicated. The scheme is more expensive ( secondary to the median plug, four rectifying diodes)

Presentation by: Chitu Madalina Master teacher: Dena Georgeta