Three Phase Semi Converter feeding Separately Excited DC Motor:

Large-kW motors are fed from 3-phase supply through three Phase Converters. In a three Phase Semi Converter, the ripple frequency of the motor terminal voltage is higher than that in the single-phase converter, as a result the filtering requirements for smoothing out the motor current are less and the current is mostly continuous. Three Phase Semi Converter could be half-wave, semi-converter, full-converter and dual-converter. The 3-phase half-wave converter is useless for most practical purposes because the supply currents contain dc component. Semi- and full-converters are most common in use. A dual-converter is employed for reversible drives with a power rating up to several MW. For want of space only the three Phase Semi Converter feeding Separately Excited DC Motor will be described here, as shown in the circuit diagram of Fig. 11.25.

Figure 11.26(a) shows the phase voltage waveforms applied to the converter. Convenient references for the thyristor firing angles are ωt₁, ωt₂, ωt₃ … spaced 120° apart. Let the firing angle be α. The three thyristors fire in sequence with a time spacing of 120° as shown in the figure. After Th₁ fires, diodes D₂ or D₃ conducts depending upon which voltage is more positive of V_AB and V_AC. For the value of α chosen, it may be immediately seen from Fig. 11.26(b) that diode D₂ conducts up to 90° beyond which D₃ conducts till Th₂ fires. Then D₃ and D₁ conduct till Th₃ fires followed by D₁, D₂ conducting in sequence. This process then repeats resulting in the voltage waveform of v_a as shown in Fig. 11.26(b). For this value of α, the free­wheeling diode is not called upon to conduct the armature current. The armature current i_a and line current i_A are shown in Figs. 11.26(c) and 11.26(d) respectively. The fundamental i_A1 of the line current lags by a small angle Φ₁ resulting in less than unity power factor.

For a firing angle α = 60°, only one diode conducts during the conduction period of a thyristor. The conduction sequence is now Th₁D₃, Th₂D1, Th₃D₂. For larger values of α, V_AC becomes negative before Th₂ fires resulting in interruption of line current so that i_a flows through D_FW, the free-wheeling diode.

If the free-wheeling diode is not used, free-wheeling takes place through the diode and thyristor of a given branch, e.g. after D₃ stops conducting, free-wheeling occurs through Th₁D₁.

The average value of armature voltage V_a reduces continuously as α is increased. Simultaneously, the lag angle Φ₁ of i_A1 increases resulting in deterioration of the line power factor.

The average of ν_a, the armature terminal voltage is derived below:

Let the supply voltages be

For the case of continuous armature current:

or

It must be noted that Eqs (11.12a) and (11.12b) are identical expressions.

The expression of Eq. (11.10) for average motor speed now modifies to