Thyristor Single-Phase Bridge Rectifier/Inverter

Objectives

1. To demonstrate the operation of a thyristor single-phase bridge in both rectifier and inverter modes.
2. To demonstrate the operation of bridge formed with thyristors and two diodes.

Discussion

Thyristor single-phase bridge (Figure 1) operates on the same principle as the diode single-phase bridge rectifier, except that each thyristor begins to conduct only when a current pulse is injected into the gate (providing that the thyristor is forward biased). Once a thyristor begins to conduct, it continues to conduct until the current flowing through it becomes zero. With a resistive load, the current becomes zero the instant the ac source voltage E_s passes through zero volts. Therefore, the output is a full-wave rectified voltage which is always positive (Figure 2(b)).

Figure 1: Thyristor single-phase bridge.

Since conduction can be initiated at any angle in the waveform between 0° and almost 180°, the average output voltage E₀, and therefore the average current, can be varied between 0 and 100%.

Figure 2: Output waveforms for a thyristor bridge.

The following equation gives the value of the average output voltage E₀ as a function of the firing angle. this equation is only when conduction is continuous, that is, when the on-time of the thyristors corresponds to 180°.

E₀ = 0.9 E_s cos α
where E_s is the voltage of the source [V ac]
         α is the firing angle in degrees.

When the load is inductive, the output voltage can be negative for part of the cycle, as shown in figure 2(c). This is because an inductor stores energy in its magnetic field which is later released. Current continuus to flow, and the same thyristors continue to conduct, until all the stored energy is released. Since this occurs some time after the ac source voltage passes through zero, the output voltage becomes negative for part of cycle.

The negative part of the output voltage waveform reduces the average output voltage E₀. As seen the previous exercise, a free-wheeling diode can be placed in the circuit to prevent the output voltage from going negative (Figure: 3). When the output voltage begins to go negative, the free-wheeling diode conducts. This maintains the output voltage at approximately zero while the energy stored in the inductor is released. The output voltage waveform is the same as for a purely resistive load (Figure: 2(b)), and the average output voltage is therefore greater than it would be without the free-wheeling diode. The addition of a free-wheeling diode makes the output current waveform smoother.

Figure 3: Thyristor single-phase bridge with free-wheeling diode.

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