PE UNIT IV - INVERTERS.pptx

INVERTERS

1. UNIT – IV INVERTERS POWER ELECTRONICS

2. Inverters A circuit that converts dc power into ac power at desired output voltage and frequency is called an inverter. Some industrial applications of inverters are for adjustable-speed ac drives, induction heating, stand-by air-craft power supplies, UPS (uninterruptible power supplies) for computers, h.v.dc transmission lines etc. Phase-controlled converters, when operated in the inverter mode, are called line-commutated inverters. But line-commutated inverters require at the output terminals an existing ac supply which is used for their line commutation. This means that line- commutated inverters cannot function as isolated ac voltage sources or as variable frequency generators with dc power at the input. Therefore, voltage level, frequency and waveform on the ac side of line-commutated inverters cannot be changed.

3. Classification of inverters

4. Half-bridge VSI

5. Full-Bridge VSI

6. 3 phase bridge inverter for 180ᵒ mode of operation

7. In the three-phase inverter of each switch conducts for 180° of a cycle. Switch pair in each arm, i.e., S1, S4; S3, S6 and S5, S2 are turned on with a time interval of 180°. It means that S1 conducts for 180° and S4 for the next 180° of a cycle. Switches in the upper group i.e., S1, S3, S5 conduct at an interval of 120°. It implies that if S1 is gated at wt = 0°, then S3 must be gated at ωt = 120° and S5 at ωt =240°. Same is true for lower group of switching devices. On the basis of this gating scheme, a table is prepared as shown at the top of Fig. In this table, first row shows that S1 from upper group conducts for 180°, S4 for the next 180° and then again S1 for 180° and so on. In the second row, S3 from the upper group is shown to start conducting 120° after S1 starts conducting. After S3 conduction for 180°, S6 conducts for the next 180° and again S3 for the next 180° and so on. Further, in the third row, S5 from the upper group starts conducting 120° after S3, or 240° after S1. After S5 conduction for 180°, S2 conducts for the next 180°, S5 for the next 180°and so on.

8. 3 phase bridge inverter for 120ᵒ

9. 3 phase bridge inverter for 120ᵒ For the 120-degree mode VSI, each switch conducts for 120° of a cycle. Like 180° mode, 120° mode inverter also requires six steps, each of 60° duration, for completing one cycle of the output ac voltage. For this inverter too, a table giving the sequence of firing the six switches is prepared as shown in the top of Fig. In this table, first row shows that S1 conducts for 120° and for the next 60°, neither S1 nor S4 conducts. Now S4 is turned on at ωt = 180° and it further conducts for 120°, i.e., from ωt = 180° to ωt = 300°. This means that for 60° interval from ωt = 120° to ωt = 180°, series connected switches S1, S4 do not conduct. At ωt = 300°, S4 is turned off, then 60° interval elapses before S1 is turned on again at ωt = 360°. In the second row, S3 is turned on at ωt = 120° as in 180° mode inverter. Now S3 conducts for 120°, then 60° interval elapses during which neither S3 nor S6 conducts. At ωt = 300°, S6 is turned on, it conducts for 120° and then 60° interval elapses after which S3 is turned on again. The third row is also completed similarly. This table shows that S6, S1 should be gated for step I; S1, S2 for step II; S2, S3 for step III and so on. During each step, only two switches conduct for this inverter-one from the upper group and one from the lower group; but in 180° mode inverter, three switches conduct in each step

10. Modified McMurray half bridge inverter

11. Modified McMurray half bridge inverter It consists of main thyristors T1, T2 and main diodes D1, D2. The commutation circuit consists of auxiliary thyristors TA1, TA2, auxiliary diodes DA1, DA2; damping resistor Rd, inductor L and capacitor C. Three-wire dc supply is required and ac load is connected between terminals A and B as shown in Fig. The function of capacitor is to provide the energy required for commutating the main thyristors. Inductance L₁ is for limiting di / dt to a safe value in main and auxiliary thyristors. As an auxiliary thyristor is used for commutating the main thyristor, this inverter is also known as auxiliary-commutated inverter. In the original inverter circuit given by McMurray, elements DA1, DA2 and Ra were not present, hence the circuit of Fig is now commonly known as the modified McMurray inverter. The following simplifying assumptions are made for this inverter : ( i ) Load current remains constant during the commutation interval. (ii) SCRS and diodes are ideal switches. (iii) Inductor L and capacitor C are ideal in that they have no resistance.

12. Modified McMurray half bridge inverter

13. Modified McMurray half bridge inverter

14. Single-phase auto-sequential commutated inverter

15. Single-phase auto-sequential commutated inverter Out of the force-commutated current source inverters, auto-sequential commutated inverter is the most popular. Though three-phase ASCI is the universal choice in industrial applications. A constant current source I feeds the load which is assumed here an inductance L for simplicity. Thyristor pairs T1, T2 and T3, T4 are alternatively switches to obtain a nearly square wave load current. Two commutating capacitors, one C1 in the upper half and the other C2 in the lower half are connected as shown in Fig.(a), diodes D1 to D4 are connected in series with each SCR to prevent the commutation capacitors from discharging into the load. The inverter output frequency is controlled by adjusting the period T through the triggering circuits of thyristors. The operation of this inverter can be explained in two modes as under. Mode I: In the beginning i.e., before t = 0, assume that T3, T4 are conducting and a steady current I flows through the path T3, D3, L, D4, T4 and source I as shown in Fig.(a). The commutating capacitors are assumed to be initially charged equally with the polarity as shown in Fig. (a), i.e. vc 1 =vc 2 =- V co . This means that both capacitors C1 and C2 have right hand plate positive and left hand plate negative

16. Single-phase auto-sequential commutated inverter

17. INVERTERS

18. Half Bridge Inverter with R Load

19. Half Bridge Inverter with RL Load

20. Single Phase Full Bridge Inverter

21. Operation with Resistive load

22. Operation with Inductive Load

23. Three Phase Inverter

24. 180 Degree Mode of conduction

25. 120 Degree Mode of conduction

26. Comparison of 180 and 120 degree mode of operation

27. PWM TECHNIQUES

28. Multiple Pulse width Modulation

29. Sinusoidal PWM

30. MODIFIED SINUSOIDAL PWM The width of the pulses near peak of the sine wave do not change much when modulation index is increased. PWM has less no of pulses RMS value is more for the same modulation index Harmonic content is reduced Switching losses also reduced Relative complex than sine PWM

31. Current source inverter Input and output current remains constant Output voltage depends upon the load

32. Current Source Inverter