Inductor Design for three phase inverter

[AMANORA]

Hey there, working through my capestone project and I have wasted nearly 1 month or so trying to design an inductor for a smps.

Topology is 2 level vsc and power rating is variable, I can go as low as 300watts up to 3k watts depending on the core, coilformers and magnet wire price.

I have a question in mind. I have been trying to design my inverter with a ferrite core since there is no way I can get a silicon or a higher permeability material. I have been designing an AC inductor since I am going to synthesize a three phase DC/AC inverter. Ferrites are not suitable for 50/60 Hz line frequency. But it occured to me that I might need to design a DC inductor since during almost all of the switching cycle, there will be an average flux in the inductor, hence a dc inductor is needed.

Can anyone shed the light on this ?. I have spent too much time trying to accomodate a ferrite core to 50HZ line frequency but I am too tired of trying at this point and time is running out.

Should I design a dc inductor and choose the core based on the switching frquency not the line frequency ?.

I have designed for a DC inductor with the following parameters.
0- Core :

ETD59/31/22-3C90

1- I max is around 6.3 Amps
2- BMax = 0.42
3- CoreLosses = 6 watts
4- Magnet wire = AWG25
5- V_dc bus = 80
6- Power/phase = 240
7- Number of turns = 77 turn
8- Air gap = 0.19 cm
9- accessories = etd59 coilformer and etd 59 clips
10 - L = 2e-3

 

[FvM]

Please clarify the function of the inductor in your inverter, e.g. by sketching the inverter topology. You didn't specify AC current and frequency, only peak current. Better give average DC current and AC peak-peak.

 

[AMANORA]

Please clarify the function of the inductor in your inverter, e.g. by sketching the inverter topology. You didn't specify AC current and frequency, only peak current. Better give average DC current and AC peak-peak.

 

[FvM]

Thanks for clarification. But where's the exact problem?

The inductor has to be designed for pwm ripple current and maximal bias of sine peak. The 50 Hz modulation affects the design in so far that the additional losses due to bias are averaged and you can allow a slightly higher peak current than for constant DC bias.

 

[AMANORA]

Thanks for clarification. But where's the exact problem?

The inductor has to be designed for pwm ripple current and maximal bias of sine peak. The 50 Hz modulation affects the design in so far that the additional losses due to bias are averaged and you can allow a slightly higher peak current than for constant DC bias.

Hi, there are no problems. This is my first time designing an inverter, with control and inductors so It's a lot to digest atm. And yes I have reached the same conclusion to design for maximum ac current + ripple. A question comes to mind about the coercive mmf and power loss due to it, I already calculated the power loss due to copper and pwm. But wondering about the 50Hz modulation losses since I am quite afraid to heat that inductor and reduce the max flux density.

 

[Easy peasy]

Switching frequency ? desired sw freq ripple - pk-pk in amps ? desired losses in choke ?

max LF AC current in amps rms, and peak ?

All these factor in to the choke design.

 

[AMANORA]

Switching frequency ? desired sw freq ripple - pk-pk in amps ? desired losses in choke ?

max LF AC current in amps rms, and peak ?

All these factor in to the choke design.

Any insights about what to use as an airgap material ?

 

[FvM]

Any insights about what to use as an airgap material ?

That's the lessest problem. You'll prefer core sets with dedicated center leg air gap, thus the air gap material is "air". If these special cores are not available for you, use polyester film or e.g. FR4 material to set a inner/outer air gap between core halves. Expect higher copper losses due to eddy currents with this air gap variant.

Your core calculation in post #1 doesn't seem to fit. 0.19 mm air gap neither corresponds to inductance nor saturation current values. Check with a calculation tool.

 

[AMANORA]

That's the lessest problem. You'll prefer core sets with dedicated center leg air gap, thus the air gap material is "air". If these special cores are not available for you, use polyester film or e.g. FR4 material to set a inner/outer air gap between core halves. Expect higher copper losses due to eddy currents with this air gap variant.

Your core calculation in post #1 doesn't seem to fit. 0.19 mm air gap neither corresponds to inductance nor saturation current values. Check with a calculation tool.

Oh, is there is a calculation tool for inductor designs ?. I made all calculations by hand.
Also 0.19cm. not mm. Thanks again )

 

[FvM]

O.k., 0.19 cm is in the right order of magnitude. There are calculation tools from TDK and Ferroxcube.

I think the inductance is set relative large for +/- 80V DC link voltage.

 

[KlausST]

Hi,

Oh, is there is a calculation tool for inductor designs ?. I made all calculations by hand.

Really?

My hints for your future:
* first do an internet search
* if you don´t find an online calculator: use Excel

Klaus

 

[AMANORA]

O.k., 0.19 cm is in the right order of magnitude. There are calculation tools from TDK and Ferroxcube.

I think the inductance is set relative large for +/- 80V DC link voltage.

Thanks for confirmation, I wanted my design to be conformant with IEEE power quality. while also reducing the cost of filter caps. Right now I am sitting at 2% THD which is still lower than 5%.
If I may ask, do you have any good resources for gate design?. thanks a bunch ))

 

[AMANORA]

Hi,


Really?

My hints for your future:
* first do an internet search
* if you don´t find an online calculator: use Excel

Klaus

Hi man. I did do an internet search but I couldnt find a tool for this type of core. Also I used matlab to ease off the process of calculations but firstly I wanted to get a feeling with the numbers and equations. I am still an undergrad and this project is/was too much specially the control/dsp aspect .

 

[BradtheRad]

Should I design a dc inductor and choose the core based on the switching frquency not the line frequency ?.

Side-by-side comparison showing LC values are selected chiefly based on switching frequency.
Select L value low enough so it passes your desired current.
Select C value to smooth the output waveform, as well as preserve optimum power factor.

Most likely your switching frequency is faster. This allows you to use lesser values for L & C.

Notice current through inductor should be maximum amplitude at same moment when spwm duty cycle is maximum. If they're misaligned then it's a power factor error, putting greater stress on your switching devices (half-bridge).

 

[AMANORA]

Side-by-side comparison showing LC values are selected chiefly based on switching frequency.
Select L value low enough so it passes your desired current.
Select C value to smooth the output waveform, as well as preserve optimum power factor.

Most likely your switching frequency is faster. This allows you to use lesser values for L & C.

Notice current through inductor should be maximum amplitude at same moment when spwm duty cycle is maximum. If they're misaligned then it's a power factor error, putting greater stress on your switching devices (half-bridge).

View attachment 179381

Yes, My inductor/Capacitor values are waaaay less than my main load resistor at 50Hz. So I didn't expect much of a power factor issue.