Hi, I am a final year electrical engineering student at NMU and I am designing and building a vfd for my capstone project. I'm having trouble understanding how to calculate the ripple current rating for my capacitor. My input is from the mains (230Vac, 50Hz), and I am aiming for a 5% voltage ripple, I've calcuated my the cap i need to be 2700uF at 450Vdc (rounded and derated), but I'm unsure about the ripple current. I keep on recalculating it as there are so many different methods on the internet to calculate it , some consider the high frequency and low frequency, some only consider the charging and discharging. Please assist me.

 

A typical VFD has 3ph input rectifier followed by the capacitor bank, Is yoursas intended for 3ph? And also what HP motor?

 

Hi. It's intended for a 3 phase induction motor rated at 220V, 3.3A and 750W. I don't know the efficiency of the induction motor so I'm designing for 1kW. My input is a single phase (from the mains),230Vac,50Hz(South Africa).

 

DC bus will always have large ripple. It is just not practical to keep it low.
DC bus voltage is 320V, 5% of that is 16Vpp. with 2700uF you get 9.5Vpp.


as expected, increasing capacitance reduces ripple.

 

Hi, thank you for your response. Understood. My main concern is how to take into account the ripple from the switching of the frequency (inverter). The above simulation only takes into account the low frequency ripple (100Hz).

 

same way... modify circuit model to match what you have. it can be as simple or as complex as needed. typically that would work at much higher frequency (8-16kHz for example) so in addition to electrolytic caps you may add in parallel ceramic caps and for both enter actual ESR values. ripple is normally greatest when duty is 50%. and your load is inductive...

 

Okay. Thank you. I'll let you know how it goes.