Application Notes
1) DC LINK FILTER: HIGH CURRENTDESIGN & CAPACITANCE VALUE DESIGN 1b) Industrial motor drive, energy suppliedfrom supply network 1a) Energy supplied with batteries
Applications will be:electric carelectric
fork lift truck
In that case,
capacitor will be used as a decoupling capacitor.Film capacitor is particularly well adapted for this use,because main criteria for DC link capacitor will be rms.
current withstanding.It means that DC link capacitor can be designed on rms.current value.If we take an electric car in account as example:Requirement data:Working voltage:120Vdc
Ripple voltage allowed:4Vrms
Rms. current:80 Arms. @ 20kHzMinimum capacitance value will be:So, it will be easy to find a capacitance value close to thesevalues.Comparison with electrolytic capacitor.If we take in account 20mA per µF for example, in order tohandle 80 Arms, capacitance value minimum would be:
DC Link Filter DC link voltage waveform:
Capacitance value will be defined taken in account thatsupply frequency is lower than converter frequency.To determine needed capacitance, we can use the followingapproached equation:Irms. through capacitor will be (approached expression):Of course this current doesn’t take in account frequencyconverter current.So, with this approximation, Irms. through the capacitor willbe depending of the Power of load, Umax and U ripple.
UUU maxt C =P
load U U
ripple xU
max – ripple F 2x
rectifier C =I
rms U µ F
ripple x 2 x π x f= 159 I
ripple load x π
rms =x C x 2 x π x F
rectifier =U U U
max – ripple 22 x 22P
APP NOTES C =800.02= 4000 µ F
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