Rectifiers convert DC to AC. Inverters convert AC to DC.

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1 DT23-3 Inverter Ntes 3 January 23. The difference between Rectifiers and Inverters Rectifiers cnvert DC t AC. Inverters cnvert AC t DC. 2. Uses f Inverters Battery Backup. Batteries stre DC. Many appliances use AC. An inverter can be used t generate an AC supply frm a battery. Fuel Cell pwer. Fuel cells are a relatively new frm f pwer surce which cnvert fuel (eg Hydrgen and Oxygen directly int d.c. electricity. A fuel cell plus inverter can be used t supply a hme r small business. The heat given ff by the fuel cell can be used t heat the building. This is a frm f CHP (cmbined heat and pwer. A DC link may be used t cnnect the electricity grids f tw cuntries. A Rectifier cnverts AC t DC at the sending end and an inverter cnverts back t DC at the receiving end. This allws the tw cuntries t exprt and imprt electricity even if they have different frequencies. It als allws the use f lng undersea cables. Yu cannt use lng undersea cables fr AC because the capacitance is t great. Example Myle intercnnectr frm Nrthern Ireland t UK. ariable Speed Mtr Drives. AC mtrs (inductin mtrs and synchrnus mtrs are cst effective, rbust and reliable but the speed f the mtr is linked very much t the supply frequency. T get gd variable speed peratin yu need a variable frequency variable vltage supply. An inverter allws yu t generate any frequency and vltage yu want. A typical variable speed drive has an AC input, a rectifier generating a DC link and an inverter, which generates the variable frequency variable vltage AC t drive the mtr. Islated DC t DC cnverters. Buck, bst and fly back cnverters can change frm ne DC value t anther but there is n islatin between input and utput. Islatin may be may be required fr safety reasns. A transfrmer can give islatin but yu cannt pass DC thrugh a transfrmer. Slutin: Use an inverter t chp up the DC int AC, pass the AC thrugh a transfrmer and rectify it again n the ther side.

2 Fur Quadrant Operatin An inverter has t be able t utput psitive and negative current and psitive and negative vltage. If the vltage and current are ppsite plarity then pwer is actually been fed back frm the AC side t the DC side. Mst inverters can d this and are in fact bi-directinal. They can be used as rectifiers r as inverters. Any lad with inductance r capacitance will absrb pwer frm the ac supply at sme pints f the cycle and return it t the supply at thers s 4 quadrant peratin is essential fr this type f lad. This feature is handy in sme applicatins e.g in an electric vehicle the drive runs as an inverter t accelerate the mtr and runs as a rectifier t brake the mtr and recver sme f the kinetic energy by pumping it back int the battery.in a DC link between tw cuntries bi-directinal inverters allws pwer flw in bth directins depending n wh is buying and wh is selling. I ut DC Input Inverter ut ut Negative I ut Psitive Pwer Flwing frm AC t DC => Rectificatin I ut ut Psitive I ut Psitive Pwer Flwing frm DC t AC => Inversin ut Negative I ut Negative Pwer Flwing frm DC t AC => Inversin ut Psitive I ut Negative Pwer Flwing frm AC t DC => Rectificatin ut

3 Cmmn Types f Inverter. Older inverters used SCRs in a phase cntrlled inverter. These are n lnger very cmmn but may still be fund at very high pwer levels. 2. Transistrs may be used t make a square wave r quasi-square wave inverter. The switches turn n and ff nce per cycle. Cheap lw pwer, lw cst inverters may use this frmat. 3. Transistrs may als be used with pulse width mdulatin t prduce a sine wave mdulated inverter. The switches turn n and ff at the pwm frequency which is many time higher than the ac line frequency. and may be khz r mre. This is the mst cmmn type f inverter used fr variable speed drives and almst always uses IGBTs

4 The Single Phase Full Bridge Inverter Square Wave peratin. v Square Wave Operatin Output ltage v (t SA+, SB- + d Parameter Peak ltage RMS ltage Fundamental Ttal Harmnic Distrtin SA-, SB+ Ideal Sine Wave = 2 ( = - d time Square Wave Inverter = d T 2 v ( t. dt = = T 4 ( =. d = π ( THD = ( dis = 2 ( ( 2 = 48%

5 Significance f the peak vltage: Any equipment with a rectifier will respnd t the peak vltage (eg mst electrni equipment Significance f the rms vltage: Any device using electricity t generate heat will respnd t the rms vltage (eg ckers, light bulbs Significance f the fundamental vltage Any device that has lw pass characteristics which includes mtrs and almst any electrmechanical device will respnd t the fundamental cmpnent f the vltage. In a square wave inverter it will nt be pssible t get the crrect peak, rms and fundamental all at the same time.

6 Square Wave Inverter with Dead Time (Quasi Square Wave v (t β SA+, SB- + d ωt Parameter Peak ltage RMS ltage Fundamental Ttal Harmnic Distrtin Ideal Sine Wave SA-, SB+ Square Wave Inverter = d T 2 v ( t. dt 2β = =. = T π 2 ( = 2π 2 2 ( =. v ( t.sin( ωt. dωt =.sin( β. 2. π π 2 2 ( ( dis THD = = ( ( - d Special Case: If β=π/4 then the wavefrm has the same peak t rms ratin as a pure sinewave. Cheap lw cst lw pwer inverters emply this type f wavefrm t generate an utput f crrect peak and crrect rms vltage. The fundamental is still incrrect hwever and this culd cause prblems fr mtr lads. Exercise. Calculate the fundamental and ttal harmnic distrtin fr this special case.

7 Inverter with an Inductive Lad i v AB v AB X i X ωt X In the Regins marked X the vltage and current are ppsite plarity (Rectifying Quadrant. This means that the current is flwing backwards thrugh the switches (Frm Negative t Psitive. Mst transistrs (Biplars, IGBTS cannt handle reverse currents s reverse dide are needed acrss the transistrs t take the reverse currents. Nte that Msfets have an intrinsic reverse dide. Be careful hwever ften the intrinsic bdy dide is much slwer and has a lwer current rating than the msfet itself.

8 Full Bridge Circuit with IGBTs The reverse dides are essential t allw reverse currents. In fact the dides cnduct whenever the bridge is perating in a rectifying quadrant. The DC bus capacitance absrbs ripple currents assciated with switching. In thery if the lad is purely resistive and full square wave peratin is used then the current ut f the supply is cnstant dc, In reality even in this case dc bus capacitance is needed (as clse t the IGBTs as pssible t cmpensate fr stray inductance in teh circuit and stabilise the vltage at the IGBTs.