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.

Figure 6: A general rectifier 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.

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 E_B. If α is large, say 75°, voltage E₀ is very small and current will only flow if voltage E_B is near zero.

Figure 9: Waveform for single-phase bridge in rectifier mode (α = 0°).

Figure 10: Waveforms for single-phase bridge in rectifier mode (α = 30°)

If the firing angle α is greater than 90°, E_D becomes negative. In this case, current will only flow if the polarity of the battery voltage is reversed, so that E_D is still higher than E_B. 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. 

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.

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