Rectifier and inverter modes

The function of the rectifying circuits seen so far is to convert ac current to dc current. The process is shown in Figure 6. Since the ideal rectifies does not consume power, the active power supplied by the source must be absorbed by the load. Note that the term load is used in its most general sense. A load can even be a voltage source, as in the case of a battery charger circuit.
An inverter is a circuit which performs the opposite function of a rectifier; it converts dc current to ac current. Figure 7 shows a general inverter system.
The active power supplied by the source is consumed by the load, as the ideal inverter does not consume power.
Inverters fall into two categories:
  1. the self-commutated or autonomous inverter in which the frequency of the output ac current is proportional to the triggering rate of the thyristors;
  2. the line-commutated or non-autonomous inverter in which the frequency of the output ac current is imposed by the ac network to which the inverter is connected.





Rectifier: A general rectifier system Figure 6: A general rectifier system.
Rectifier: A general inverter system Figure 7: A general inverter system.
The frequency of self-commutated inverters is fixed. You will use the line-commutated inverter in this exercise.
The single-phase thyristor bridge can be used both as a rectifier and as an inverter. The inductor is large enough to ensure continuous conduction over a wide range of firing angles. The power supplied to the battery is P0 = I × E0, where E0 is the average dc voltage at the output of the bridge.
Rectifier: Single-phase bridge with active dc load Figure 8: Single-phase bridge with active dc load.
The average dc voltage E0 decreases as the firing angle α is increased, as shown in Figure 9 and 10. However, current flows only as long as voltage E0 is more positive than the battery voltage EB. If α is large, say 75°, voltage E0 is very small and current will only flow if voltage EB is near zero.
Rectifier: Waveform for single-phase bridge in rectifier mode (α = 0°) Figure 9: Waveform for single-phase bridge in rectifier mode (α = 0°).
Rectifier: Waveforms for single-phase bridge in rectifier mode (α = 30° Figure 10: Waveforms for single-phase bridge in rectifier mode (α = 30°)
If the firing angle α is greater than 90°, ED becomes negative. In this case, current will only flow if the polarity of the battery voltage is reversed, so that ED is still higher than EB. This is illustrated in Figure 11. The power supplied to the battery is still P0 = I × E0. However, since the voltage E0 is negative, this power is negative, showing that power is actually transferred from the the battery to the ac source. The bridge is now operating as a line-commutated inverter, converting dc power to ac. 
Rectifier: Waveform for single-phase bridge in inverter mode (α = 150° Figure 11: Waveform for single-phase bridge in inverter mode (α = 150°)
Procedure summary
In the first part of the exercise, you will set up the equipment.
In the second part, you will observe the operation f a controlled bridge supplying a passive load, with both a resistive and an inductive load.
In the third part, you will set up two different bridge configurations having two thyristors and two diodes.
In the fourth part, you will operate a thyristor bridge in both the rectifier and the inverter modes. The reversible dc power supply circuit (see Familiarization with the Reversible DC Power Supply) is used to simulate a battery.
previous Bridge rectifier with two thyristors and two diodes
next Thyristor Single-Phase Bridge Rectifier/Inverter (PROCEDURE)

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