Load Voltage Current Load (a) For Resistive Load Load voltage b) For Resistive-Inductive Load (with continuous current conduction) FIGURE 30.10 Single-Phase full-wave converter with transformer. FIGURE 30.11 Single-Phase bridge converter. In Fig. 30.12(a), the top or bottom thyristors could be replaced by diodes. The resulting topology is called thyristor semiconverter With this configuration, the input power factor is improved, but the regeneration is Cycloconverters Cycloconverters are direct ac-to-ac frequency changers. The term direct conversion means that the energy does not appear in any form other than the ac input or ac output. The output frequency is lower than the input frequency and is generally an integral multiple of the input frequency. a cycloconverter permits energy to be fed back into the utility network without any additional measures. Also, the phase sequence of the output voltage can be easily reversed by the control system. Cycloconverters have found applications in aircraft systems and industrial drives. These cycloconverters are suitable for synchronous and induction motor control. The operation of the cycloconverter is illustrated in Section 30.4 of this chapter. DC-to-AC Converters The dc-to-ac converters are generally called inverters. The ac supply is first converted to dc, which is ther converted to a variable-voltage and variable-frequency power supply. This generally consists of a three-phase bridge connected to the ac power source, a dc link with a filter, and the three-phase inverter bridge connected c 2000 by CRC Press LLC
© 2000 by CRC Press LLC In Fig. 30.12(a), the top or bottom thyristors could be replaced by diodes. The resulting topology is called a thyristor semiconverter. With this configuration, the input power factor is improved, but the regeneration is not possible. Cycloconverters Cycloconverters are direct ac-to-ac frequency changers. The term direct conversion means that the energy does not appear in any form other than the ac input or ac output. The output frequency is lower than the input frequency and is generally an integral multiple of the input frequency. A cycloconverter permits energy to be fed back into the utility network without any additional measures. Also, the phase sequence of the output voltage can be easily reversed by the control system. Cycloconverters have found applications in aircraft systems and industrial drives. These cycloconverters are suitable for synchronous and induction motor control. The operation of the cycloconverter is illustrated in Section 30.4 of this chapter. DC-to-AC Converters The dc-to-ac converters are generally called inverters. The ac supply is first converted to dc, which is then converted to a variable-voltage and variable-frequency power supply. This generally consists of a three-phase bridge connected to the ac power source, a dc link with a filter, and the three-phase inverter bridge connected FIGURE 30.10 Single-phase full-wave converter with transformer. FIGURE 30.11 Single-phase bridge converter. (a) For Resistive Load a Load Voltage & Current wt (b) For Resistive-Inductive Load (with continuous current conduction) a Load Voltage wt T1 Load Em Sin wt T2
T2 TI T6TI FIGURE 30.12 (a) Three-Phase thyristor full bridge configuration;(b )output voltage and current waveforms to the load. In the case of battery-operated systems, there is no intermediate dc link Inverters can be classifie as voltage source inverters(VSIs)and current source inverters(CSIs). A voltage source inverter is fed by a stiff dc voltage, whereas a current source inverter is fed by a stiff current source. A voltage source can be converted to a current source by connecting a series inductance and then varying the voltage to obtain the desired current c 2000 by CRC Press LLC
© 2000 by CRC Press LLC to the load. In the case of battery-operated systems, there is no intermediate dc link. Inverters can be classified as voltage source inverters (VSIs) and current source inverters (CSIs). A voltage source inverter is fed by a stiff dc voltage, whereas a current source inverter is fed by a stiff current source. A voltage source can be converted to a current source by connecting a series inductance and then varying the voltage to obtain the desired current. FIGURE 30.12 (a) Three-phase thyristor full bridge configuration; (b) output voltage and current waveforms. T1 i A vAN vBN vCN T3 T5 T4 T6 T2 R L + i 0 vO (a) T4 T6T2 T1 T3 T5 aaaa vAN vBN vCN a a a T6 wt vAB vO vAC vBC 60° i O i TI i A T1 T6 T1 T2 T1 T1 T1 T4 wt wt wt wt (b)
21 3. Phase FIGURE 30. 13 (a) Three-phase converter and voltage source inverter configuration;(b) three-phase square-wave inverter A VSI can also be operated in current-controlled mode, and similarly a CSI can also be operated in the voltage control mode. The inverters are used in variable frequency ac motor drives, uninterrupted power supplies, induction heating, static VAR compe Voltage Source Inverter A three-phase voltage source inverter configuration is shown in Fig. 30.13(a). The VSIs are controlled either in square-wave mode or in pulsewidth-modulated(PWM)mode. In square-wave mode, the frequency of the output voltage is controlled within the inverter, the devices being used to switch the output circuit between the plus and minus bus. Each device conducts for 180 degrees, and each of the outputs is displaced 120 degree to generate a six-step waveform, as shown in Fig. 30.13(b). The amplitude of the output voltage is controlled by varying the dc link voltage. This is done by varying the firing angle of the thyristors of the three-phase bridge converter at the input. The square-wave-type VSI is not suitable if the dc source is a battery. The six-step output voltage is rich in harmonics and thus needs heavy filtering. In PWM inverters, the output voltage and frequency are controlled within the inverter by varying the width of the output pulses. Hence at the front end, instead of a phase-controlled thyristor converter, a diode bridge rectifier can be used. A very popular method of controlling the voltage and frequency is by sinusoidal pulsewidth waveform, as shown in Fig. 30. 14. The power devices in each phase are switched on at the intersection of sine c 2000 by CRC Press LLC
© 2000 by CRC Press LLC A VSI can also be operated in current-controlled mode, and similarly a CSI can also be operated in the voltagecontrol mode. The inverters are used in variable frequency ac motor drives, uninterrupted power supplies, induction heating, static VAR compensators, etc. Voltage Source Inverter A three-phase voltage source inverter configuration is shown in Fig. 30.13(a). The VSIs are controlled either in square-wave mode or in pulsewidth-modulated (PWM) mode. In square-wave mode, the frequency of the output voltage is controlled within the inverter, the devices being used to switch the output circuit between the plus and minus bus. Each device conducts for 180 degrees, and each of the outputs is displaced 120 degrees to generate a six-step waveform, as shown in Fig. 30.13(b). The amplitude of the output voltage is controlled by varying the dc link voltage. This is done by varying the firing angle of the thyristors of the three-phase bridge converter at the input. The square-wave-type VSI is not suitable if the dc source is a battery. The six-step output voltage is rich in harmonics and thus needs heavy filtering. In PWM inverters, the output voltage and frequency are controlled within the inverter by varying the width of the output pulses. Hence at the front end, instead of a phase-controlled thyristor converter, a diode bridge rectifier can be used.A very popular method of controlling the voltage and frequency is by sinusoidal pulsewidth modulation. In this method, a high-frequency triangle carrier wave is compared with a three-phase sinusoidal waveform, as shown in Fig. 30.14. The power devices in each phase are switched on at the intersection of sine FIGURE 30.13 (a) Three-phase converter and voltage source inverter configuration; (b) three-phase square-wave inverter waveforms. 3 - Phase + V T1 i A O – A T4 T3 B T6 T5 C T2 N K Inverter (a) LF vAB vBC vCA vNO vAN i A (b) wt wt wt wt wt V/3 V/6 -V V V -V V -V -V 2/3 V v i AN A
*IIIIIIL InLLInEInIL FIGURE 30.14 Three- phase sinusoidal PwM inverter waveform and triangle waves. The amplitude and frequency of the output voltage are varied, respectively, by varying the amplitude and frequency of the reference sine waves. The ratio of the amplitude of the sine wave to the amplitude of the carrier wave is called the modulation index The harmonic nts in a PWM wave are easily filtered because they are shifted to a higher-frequency region. It is desirable to have a high ratio of carrier frequency to fundamental frequency to reduce the harmonics of lower-frequency components. There are several other PWM techniques mentioned in the literature. The most notable ones are selected harmonic elimination, hysteresis controller, and space vector PWM technique In inverters, if SCRs are used as power switching devices, an external forced commutation circuit has to be used to turn off the devices. Now, with the availability of IGBTs above 1000-A, 1000-V ratings, they are being used in applications up to 300-kW motor drives. Above this power GTOs are generally used. Power Darlington transistors, which are available up to 800 A, 1200 V, could used for inverter applications. Current source lnverter Contrary to the voltage source inverter where the voltage of the dc link is imposed on the motor windings, in the current source inverter the current is imposed into the motor. Here the amplitude and phase angle of the motor voltage depend on the load conditions of the motor. The current source inverter is described in detail in Section 30.4 c 2000 by CRC Press LLC
© 2000 by CRC Press LLC and triangle waves. The amplitude and frequency of the output voltage are varied, respectively, by varying the amplitude and frequency of the reference sine waves. The ratio of the amplitude of the sine wave to the amplitude of the carrier wave is called the modulation index. The harmonic components in a PWM wave are easily filtered because they are shifted to a higher-frequency region. It is desirable to have a high ratio of carrier frequency to fundamental frequency to reduce the harmonics of lower-frequency components. There are several other PWM techniques mentioned in the literature. The most notable ones are selected harmonic elimination, hysteresis controller, and space vector PWM technique. In inverters, if SCRs are used as power switching devices, an external forced commutation circuit has to be used to turn off the devices. Now, with the availability of IGBTs above 1000-A, 1000-V ratings, they are being used in applications up to 300-kW motor drives. Above this power rating, GTOs are generally used. Power Darlington transistors, which are available up to 800 A, 1200 V, could also be used for inverter applications. Current Source Inverter Contrary to the voltage source inverter where the voltage of the dc link is imposed on the motor windings, in the current source inverter the current is imposed into the motor. Here the amplitude and phase angle of the motor voltage depend on the load conditions of the motor. The current source inverter is described in detail in Section 30.4. FIGURE 30.14 Three-phase sinusoidal PWM inverter waveforms
Q (K-IV.C Cr tVa FIGURE 30.15 Resonant dc-link inverter system with active voltage clamping. FIGURE 30 16 Resonant ac-link converter system showing configuration of ac switches. ResonantLink Inverters The use of resonant switching techniques can be applied to inverter topologies to reduce the switching losses in the power devices. They also permit high switching frequency operation to reduce the size of the magneti components in the inverter unit. In the resonant dc-link inverter shown in Fig. 30.15, a resonant circuit is added at the inverter input to convert a fixed dc voltage to a pulsating dc voltage. This resonant circuit enables ne devices to be turned on and turned off during the zero voltage interval. Zero voltage or zero current switching is often termed soft switching. Under soft switching, the switching losses in the power devices are almost eliminated. The electromagnetic interference(EMI) problem is less severe because resonant voltage pulses have lower dv/dt compared to those of hard-switched PWM inverters. Also, the machine insulation is less stretched because of lower dv/dt resonant voltage pulses In Fig. 30.15, all the inverter devices are turned on simultaneously to initiate a resonant cycle. The commutation from one device to another is initiated at the zero dc-link voltage. The inverter output voltage is formed by the integral numbers of quasi-sinusoidal pulses The circuit consisting of devices Q, D, and the capacitor C acts as an active clamp to limit the dc voltage to about 1.4 times the diode rectifier voltage V There are several other topologies of resonant link inverters mentioned in the literature. There are also resonant link ac-ac converters based on bidirectional ac switches, as shown in Fig. 30.16. These resonant link converters find applications in ac machine control and uninterrupted power supplies, induction heating, etc. The resonant link inverter technology is still in the development stage for industrial applications c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Resonant-Link Inverters The use of resonant switching techniques can be applied to inverter topologies to reduce the switching losses in the power devices. They also permit high switching frequency operation to reduce the size of the magnetic components in the inverter unit. In the resonant dc-link inverter shown in Fig. 30.15, a resonant circuit is added at the inverter input to convert a fixed dc voltage to a pulsating dc voltage. This resonant circuit enables the devices to be turned on and turned off during the zero voltage interval. Zero voltage or zero current switching is often termed soft switching. Under soft switching, the switching losses in the power devices are almost eliminated. The electromagnetic interference (EMI) problem is less severe because resonant voltage pulses have lower dv/dt compared to those of hard-switched PWM inverters. Also, the machine insulation is less stretched because of lower dv/dt resonant voltage pulses. In Fig. 30.15, all the inverter devices are turned on simultaneously to initiate a resonant cycle. The commutation from one device to another is initiated at the zero dc-link voltage. The inverter output voltage is formed by the integral numbers of quasi-sinusoidal pulses. The circuit consisting of devices Q, D, and the capacitor C acts as an active clamp to limit the dc voltage to about 1.4 times the diode rectifier voltage Vs . There are several other topologies of resonant link inverters mentioned in the literature. There are also resonant link ac-ac converters based on bidirectional ac switches, as shown in Fig. 30.16. These resonant link converters find applications in ac machine control and uninterrupted power supplies, induction heating, etc. The resonant link inverter technology is still in the development stage for industrial applications. FIGURE 30.15 Resonant dc-link inverter system with active voltage clamping. FIGURE 30.16 Resonant ac-link converter system showing configuration of ac switches