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Knowhow 12 1 2021 Powered by Committed to Excellence www rutronik com Drive technology Precharging capacitors using pulsewithstanding resistors Electric motors are usually controlled using converters Electrolytic capacitors are frequently used to stabilize and buffer the DC voltage in the DC link There are a number of issues to consider with switching these capacitors in and out of the circuit By Bert Weiss Technical Support at Rutronik and Roger Renfordt Key Account Manager at Krah When a capacitor is charged through a resistor the increase in voltage follows a curve in the form of a natural exponential function Figure 1 blue line The relevant charging current of the capacitor green on the other hand takes the form of a decaying exponential function The maximum current flows right at the start of the charging process at which point it is 100 V 50 Ω = 2 A After around 1 5 seconds the capacitor reaches a voltage of close to 100 Vwhile the current is close to 0 A Lets say this capacitor is charged without using a resistor so that when the voltage is hard switched there is only a very low specific resistance in the supply lineestimated to be 10 mΩin which case there is theoretically a current of up to 10 000 Aat the first moment of charging! However in reality in addition to the ohmic resistance of the connecting wire there are other elements providing resistance •the ohmic resistance of the capacitor estimated at around 25 mΩ •the internal resistance of the voltage source estimated at around 20 mΩ •the transfer resistance of the connecting terminals switching contacts etc of max 5 mΩ This means that a realistic total resistance would be around 50 mΩ With this there is still a peak current of over 2 000 A 100 V 0 05 Ω = 2 000 A While this very high current would only be flowing for a very brief moment you can still imagine what impact these current surges can have on other componentscertainly so when you remember that DC voltages of 800 Vare commonplace in DC link solutions for converters nowadays Switchon processes in consideration of wire inductance If we consider not only the capacitance but also any wire inductance we observe behavior such as that seen in Figure 3 Figure 1 The voltage and charging current of a capacitor follow opposing natural exponential functions Figure 2 Charging a capacitor at 5 000 µF to a voltage of 100 Vthrough a resistor of 50 Ω Figures Krah