APPLICATIONS Reference: Textbook-Chapter 6,8 & 9 'Power Electronics',C APPLICATIONS Reference: Textbook-Chapter 6,8 & 9 'Power Electronics',C.W Lander, 3rd Edition,1993

DC DRIVE Figure 1: DC motor Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

DC DRIVE Figure 2: Modes of operation of DC motor Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

DC DRIVE DC MOTOR DRIVE USING CONTROLLED RECTIFIER Figure3 Basic variable-speed drives. Voltage adjustment by controlled rectifier Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

DC DRIVE DC MOTOR DRIVE USING CONTROLLED RECTIFIER The speed of a d.c. motor could be adjusted by variation of the armature voltage. Figure a shows one layout where a diode rectifier can be used in conjunction with a voltage regulator. The layout shown in Fig.3 is the more usual, where a controlled rectifier is used to supply the motor armature. The speed of the motor is determined by its mean armature voltage, any oscillating torque produced by the harmonic voltage (current) components being heavily damped by the motor inertia. Hence, the motor speed is dependent on the firing delay angle of the rectifier, if required; the field can be supplied via a controlled rectifier. Where the machine is required to generate, (say) if rapid braking is required, then the converter must be capable of operation in the inverting mode. Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

DC MOTOR DRIVE CIRCUIT USING HALF- CONTROLLED RECTIFIER Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

AC DRIVE INDUCTION MOTOR SPEED CONTROL Figure 4: Voltage adjustment with quasi square wave inverter (a) by controlled rectifier (b) by chopper regulator Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

AC DRIVE INDUCTION MOTOR SPEED CONTROL With the quasi-square wave inverter the voltage adjustment can be achieved by varying the level of the D.C. source voltage to the inverter. Two common arrangements for obtaining the necessary voltage adjustment are shown in Fig 4. The section between the rectifier and the inverter is known as the D. C. link. A controlled rectifier can be used as it gives a fast response to any control demand but it suffers from the major disadvantage of all controlled rectifiers of lagging power factor on the a.c. supply. However, regeneration into the supply mains is possible with the fully-controlled rectifier and the arrangement is relatively cheap. The other arrangement involves a diode rectifier giving a fixed d.c. busbar voltage, which is then fed to a chopper such as shown in Fig.4 b. The chopper on/off action adjusts the mean level of the inverter input voltage, the chopper output being filtered before reaching the inverter. An advantage of the diode rectifier is that a battery can be placed across the fixed busbar to maintain the inverter supply in the event of a mains failure. Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

STANDBY INVERTER Figure 5:Standby Inverter Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

STANDBY INVERTER Standby inverters are used mostly to provide an emergency supply at mains frequency (50/60 Hz), in the event of a mains failure. The form of standby may demand an uninterruptible supply that is in the event of mains failure the inverter immediately operates without loss of waveform. A typical uninterruptible system is shown in Fig5. Here the load can be permanently fed from the inverter, the inverter d.c. source being obtained by rectifying the a.c. mains. In the event of a mains failure, the inverter will take its power supply from the battery, thus avoiding any interruption to the load. Loads which typically demand an uninterruptible supply are computers, communication links, and essential instrumentation in certain processes. The battery will have a limited capacity, and will be constantly trickle -charged to maintain its float voltage. Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

STANDBY INVERTER A change-over from mains to inverter supply will demand that the inverter be synchronized with the mains to avoid waveform distortion. In those situations where a short interruption in supply can be tolerated, the inverter would only be brought into operation and connected to the load after mains failure. Such circumstances might include emergency lighting where a loss of supply for (say) 0.2 second could be tolerated. Pulse-width modulation techniques can be used very much reducing the filtering requirements as low-order harmonics are not present in such waveforms Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

HVDC (high voltage DC transmission) Systems An application which requires strings of high-voltage thyristors in the converters is that of high-voltage, direct-current (HVDC) transmission. Direct-voltage transmission lines are much more economical than alternating-voltage transmission lines for power transmission above 500km. But the ease with which alternating voltages can be altered in level by transformers, coupled with generator and motor considerations, makes the three-phase a.c. system the best overall, both economically and technically. However, for long overland or underwater power transmission, it is economical to link two a.c. systems by an HVDC transmission line. Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

HVDC (high voltage DC transmission) Systems Figure 6: HVDC system Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

HVDC (high voltage DC transmission) Systems A simple representation of a HVDC interconnection is shown in Figure 6. AC power is fed to a converter operating as a rectifier. The output of this rectifier is DC power, which is independent of the AC supply frequency and phase. The DC power is transmitted through a conduction medium; be it an overhead line, a cable or a short length of busbar and applied to the DC terminals of a second converter. This second converter is operated as a line-commutated inverter and allows the DC power to flow into the receiving AC network. Conventional HVDC transmission utilises line-commutated thyristor technology.

HEATING LOAD CONTROL USING TRIAC(AC-AC CONTROLLER) Figure8: a) Power control in heating load using TRIAC b) Phase Angle control c) Integral Control Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993

HEATING LOAD CONTROL USING TRIAC(AC-AC CONTROLLER) A heating load of the resistance type can be controlled to different power levels by the use of the triac connection shown in Fig8. The phase control can be used as shown in fig6 where the start of each cycle is delayed by an angle α The majority of heating loads have thermal time constants of several seconds or longer. In this case little variation of heater temperature will occur if control is achieved by allowing a number of cycles on with a number of cycles off as shown in the last waveform. This form of control is integral cycle control. Reference:Textbook-'Power Electronics',C.W Lander, 3rd Edition,1993