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For recreational vehicles & farms A 2kW 24V/240VAC This high power inverter can deliver 2000 watts (continuous) at 240VAC. It operates from a 24V battery supply to produce a sinewave output - the same as from the 50Hz AC mains supply. The availability of 240VAC mains power is almosta prerequisite for modern living. We rely on it for powering such items as refrigerators, washing machines, microwave ovens, power tools and hifi and video equipment. In fact, just about all domestic appliances are designed to operate from mains power. It is only when mains power is unavailable that its importance is fully realised. At remote building sites, mains power is required for circular saws, drills and sanders, while many farm houses and sheds are unserviced by mains power simply because the cost of bringing the power in from the national grid can be prohibitive. The only practical alternatives to mains power are either motor generators or a solid state inverter. Motor generators work well but they are expensive to run, cause air pollution and can be noisy. By contrast, an in- verter is completely silent and causes no pollution. It can operate from a battery bank which is charged by a solar cell array. Up till now though, there have been few high power inverters available and most have not had sinewave output. To our knowledge, this is the first really high power inverter to be described in any magazine throughout the world and it is certainly the first do-it-yourself design to produce a genuine sinewave output. Basic waveforms Fig.l(a) shows the sinewave output of the new inverter. It has a peak output of close to 340V, similar to that from a normal 50Hz mains supply. Most commercial and kit inverter designs on the market provide only a square wave or a modified square wave output (Fig. lb & Fig. le). This type of waveform may not be suitable for powering all appliances and may result in the appliance overheating or malfunctioning. To understand why this can happen we need to examine the differences between a sinewave with an RMS value of 240VAC and a square wave with the same RMS value of 240VAC. For the sinewave, the peak of the waveform is just under 340V while for the square wave it is only around 240V. As an improvement on this, the modified square wave is often used to provide a higher peak voltage. To maintain the 240VAC output, the duty cycle of the waveform is decreased. The lower peak voltage from the square wave inverter can be a disadvantage when powering appliances which rely on the peak voltage being at 340V. Any appliance which uses a rectifier and filter to obtain a DC voltage for its internal power supply usually depends on the peak voltage for correct operation. Examples of these appliances are hifi equipment, TVs, VCRs and microwave ovens. On the other hand, many TV sets and most computers use switchmode power supplies and most of these can function satisfactorily with the lower peak voltages provided by square wave inverters. The reason they do so is simply because they are designed to +300V +240v- -240 V (b) SQUAREWAVE -300V (a) SINEWAVE Fig.I: many DC to AC inverters produce a square wave (b) or a modified square wave (c) instead of a sinewave as in (a). This can lead to problems when driving some types of equipment, such as electric motors. · 16 SILICON CHIP (c) MODIF IED SQUAREWAVE By JOHN CLARKE Sinewave Inverter This project sponsored by Rod Irving Electronics, 1992. The 2kW sinewave inverter will power fridges, washing machines, microwave ovens, power tools, lights & video/hifi equipment. It runs from 24V DC&, used in conjunction with a solar panel array, could form the basis of an electricity supply system on farms & in other remote locations where no mains supply system is available. function over a large range of mains voltages. Electric motors can also present problems when driven by square wave inverters. The high harmonic content can lead to higher power dissipation in the motor and more buzz from the windings and laminations. In commutator motors, the low peak voltage can result in a lower top speed, while the lagging power factor can cause problems for induction motors be- cause the motor will be drawing substantial current when the drive tran~ sistors are being switched. While lagging power factor in induction motors still presents problems for sinewave inverters, this new design has enough power to start and run just about any domestic appliance using an electric motor (apart from airconditioners). In normal use, we expect that this inverter will be used to power fridges, washing machines, vacuum cleaners, TVs, VCRs, lights , microwave ovens, power tools and video and hifi equipment. We would not normally expect it to be used to power appliances such as electric jugs, frypans or radiators since it would be far more efficient to use nat)-lral gas or LPG for heating and cooking. Physical dimensions The new sinewave inverter is not a small package and nor could you expect it to be since it delivers such high power. It is large and bulky. It measures 452mm wide x 170mm high x 400mm deep and weighs 12kg. On OCT0BER1992 17 the lefthand side of the front panel is the DC power switch, LED power indicator and fuseholder while at the right is the double general purpose output (GPO) socket. At the rear of the fabricated aluminium enclosure are the heavy duty cables for connection to the 24V battery. These carry DC currents of more than 100 amps at full load. Both the left and righthand sides of the case carry large vertical heatsinks to dissipate the heat from the switching devices in the inverter circuitry. Not only is the inverter case large and bulky but the circuitry inside is heavy duty. As already noted, it draws input currents of more than 100A at 24V and this is used to generate voltages as high as 3 70V DC. This high voltage means that this inverter must be treated with the same caution and respect as the 240VAC mains supply. Fig.2 shows the basic arrangement of the inverter circuitry. 24V DC from the battery is stepped up to around 365V DC in a transformer driven DC to DC converter. This high voltage then feeds a switchmode sinewave converter which essentially chops up Specifications Input voltage ...... ............ .. ............... 22 - 28VDC (24V lead acid batteries) Output voltage ............... .. ...... .. ..... ... 240VAC sinewave (EMI suppressed) Power rating ............ ... .. .. ........ ................................ .... ...... 2kW continuous Peak power ... ... ...................... ... ... ... ................... ........... .. ... 3kW for 300ms Standby current ...... ..................... ....... ... .. ... .................... .............. . 3 amps Full load cu rrent ................................... ................ ... ...... .... .. . 114 amps DC Output regulation ...... ... .. ... .... ... .... .......... .. .................... .. ................... < 8% Efficiency .... ... ........... ... .. ... .. ......... .. .... ........................ ......... > 80% at 2kW Harmonic content ..... .. ................................ .................. .... < 10% distortion 50Hz accuracy ... ... .... ..... ............... ............ ................. .... ...... crystal locked 24V BATTERY - DC TO DC CONVERTER 24V TO 36SVOC - SWITCH MODE SINEWAVE CONVERTER - FILTER - 240VAC OUTPUT Fig.2: the block diagram of the 2kW sinewave inverter shows that it involves two processes; DC to DC conversion and DC to sinewave conversion. the 365V DC to form a pulse train with a duty cycle calculated to give the same RMS value as a 240VAC 50Hz sinewave. This varying duty cycle pulse train is then fed to a filter to remove all the high frequencies, leaving an essen- tially clean 50Hz sinewave. So in effect, there are two processes: DC to DC inversion and then DC to AC sinewave conversion. Because the DC to DC inverter runs at a high frequency, as does the sinewave conversion process, there is no MOSFET DRIVERS AND CONTROLLER 24V BATTERY STEP•UP 1--...........i TRANSFORMER x1B HIGH VOLTAGE FULL WAVE RECTIFIER HIGH VOLTAGE FILTER 1--CAPACITOR +36SV ---<t---------+--......- - - - - -- -~ ISOLATED VOLTAGE FEEDBACK SWITCH 1 A SWITCH 2 L1 SWITCH MODE SINEWAVE GENERATOR 0 y C SWITCH 3 0 DY Fig.3: this more detailed block diagram of the 2kW inverter illustrates the isolated voltage feedback system and the H-pack output drive which provides the sinewave conversion. 18 SJUCON CHIP L2 C1 X MAINS GPO This inside view of the prototype 2kW inverter was taken at a late s.tage of its development. In the lower section of the chassis is the large high-frequency transformer which is driven by the circuitry immediately below it. In the top lefthand corner is the high voltage rectifier & filter capacitor & the sinewave filter circuitry, while at top right is the PROM-based sinewave generator circuitry. Note the 100-amp cartridge fuse in the bottom lefthand corner of the chassis. The vertical board at the top of the photograph carries the H-pack switching devices. OCT0BER1992 19 vx DV 1Dms 1Dms VY OV t - - - -- - -- - --- +365V "------ 50Hz FILTERED SINEWAVE provides us with an accurate and precise 50Hz sinewave. The way in which points X and Y are switched is depicted in the waveforms of Fig.4. The top waveform shows the voltage at point X (Vxl while the second waveform shows the voltage at point Y (Vy). The difference between these two waveforms is the voltage between points X and Y and when this voltage (Vxy) is filtered by the output filter consisting of Ll, Cl and L2, the result is a 240VAC 50Hz sinewave. In effect, Fig.4 shows point Y being held high for the first 10ms while point X is rapidly switched between 365V and 0V. For the next 10ms, point X is held at 365V while point Y is rapidly switched between 365V and ov. Fig.4: the switching process used in the H-pack drive circuit. The switching sequence for half the sinewave is stored in memory and the resulting difference between Vx and Vy is Vxy which becomes a 50Hz sinewave after filtering. large and heavy 50Hz ZkW transformer employed. Such a transformer could be expected to weigh 25kg or more. The DC to DC inverter does employ a transformer but it is a high- frequency design weighing less than 3kg. Detailed block diagram Fig.3 shows a more detailed schematic arrangement of the inverter. Here the 24V battery voltage is fed to the just mentioned high frequency step-up transformer which is driven by power Mosfets in a push-pull configuration . The step-up ratio is x18 and the resulting AC voltage is rectified by a full-wave bridge and filtered with a high voltage capacitor. · The optically-isolated voltage feedback circuit adjusts the duty cycle of the Mosfet switching so that the DC voltage from the inverter remains more or less constant regardless of the load current. Readers may be wondering why the transformer step-up ratio is x18 when this multiplied by the 24VDC input will give 67V more than the required 365VDC. The extra leeway is needed to make up for losses in the inverter, the rectifier diodes and the filter so that we can still obtain around 365V at the full 2kW load. The 365V supply rail is floating with respect to the 24V battery termi20 SILICON CHIP nals to provide full high-voltage isolation. This isolation is provided by the insulation between the primary and secondary windings of the transformer and the optically coupled voltage feedback. This prevents the battery terminals from being at a high and lethal voltage above ground should a fault occur in any equipment powered by the inverter. Sinewave converter Across the 365V DC supply rail are connected four switches, in an H-pack configuration. Switch 1 is in series with switch 3 and switch 2 is in series with switch 4. The junction between switch 1 and switch 3 is point X and the junction between switch 2 and switch 4 is point Y. If switch 1 is turned on and switch 3 is off, point X is pulled up to 365V. Conversely, if switch 3 is on and switch 1 is off, then point X is pulled down to 0V. Similarly, point Y can be pulled up to 365V or down to 0V, depending on the closure of switch 2 or switch 4 respectively. The switchmode sinewave generator has four outputs which drive switches 1, 2, 3 and 4. The duration and sequence of switching are stored in a Programmable Read Only Memory (PROM) which is driven by counters clocked by a crystal oscillator. This Notice that the series of switchings between 365V and 0V varies from very narrow 0V going pulses at the start and end of the sinewave half cycle to being at 0V for almost all of the time at the peak of the sinewave. The switching sequence stored in the PROM actually only gives half the sinewave but when the difference is taken between the voltages at points X and Y we get the full sinewave. To make the waveform diagrams of Fig.4 easier to read, we have shown only 10 switchings for each 10ms period whereas there are actually 40 switchings per 10ms halfwave interval, or 80 switchings per 50Hz cycle. In other words, the four switches of the H-pack circuit are switched at 4kHz, with a constantly varying duty cycle. Full high-voltage isolation is provided between the sinewave generator outputs at A, B, C and D and switches 1, 2, 3 and 4 across the 365V DC supply. This is achieved by using fast optocouplers for the signals and miniature transformers to supply the necessary power required for each of the switches. Switches 1, 2, 3 and 4 are high power insulated-gate bipolar transistors (IGBTs). These have a very high voltage and current rating and are superior in this application to power Mosfets. Only four of these devices -are used in the H-pack circuit and they provide the full 2 kilowatt output from the inverter. Next month we shall continue the circuit description and feature the full parts list. SC