LC filter component selection

[xj25]

Hi,

I need a sinus-shaper filter for a VFD that supplies a fan motor. Vline=220V, Irms=0.7-0.8A (see thread237-300370).

Commercial solutions that I looked for are for at least 4A.

I tried a sample filter of 11A nominal current from a well known manufacturer, and voltage drop is about 100V! (i guess that is due to non saturation of the filter coils with so little current, any other idea about what could cause it?). Anyway it is really so big that we would have problems to get the space to install it.

I am trying to design a filter L (series) C(paralel) three phase to match the real need (both in space and power).

VFD Pulse freq is about 24kHz I did some simulations with spice and seems to work (on screen , see adjoint.

L=4.7mH (serial connected for each phase)
R=1K(parallel with L to add damping)
C=1.5uF (star connected between phases)

I have several doubts:
- is it better connect capacitors in star or wye?. If in star, i was thinking to connect central point to negative of VFD to reduce common mode voltage, does it make sense?.
- I am not familiar with this kind of app design, do I need any special features of capacitors/coils/resistor?

I am trying with adjoint L and C but don´t know really what should look for to assure a good realibility.

I selected a capacitor with high voltage rating and SR(the latter not sure if I need it really) and an inductor for 1A nominal current (1.2Ohms of serial resistance).

Any suggestions or ideas? I am worried specially about reliable component selection.

Regards

 

[hacksaw]

shouldn't you be using a smaller shunt resistor

 

[xj25]

Actually after real tests no resistor is needed.

I get a good sinewave but it occurrs a puzzling fact:

VFD output (in theory configured to 220V):
Vrms=185V (true rms Fluke and scope)
V1(first armonic measured with FFT)= 80Vrms (quite low!)
I see in scope that maximun duty cycle of PWM is just about 60% (?).

Filter output:
Vrms=120V (so 65V drop, true rms Fluke and scope)
V1(first armonic measured with FFT)= 117Vrms

I would need to get as near to Vin as possible (185Vrms) because if not I am supplying the motor with reduced voltage.

Also I don´t understand why i get more voltage in V1 at filter output than at input.

Regards

 

[hacksaw]

as a simplfied RLC circuit your damping ratio is right at the marging of being critically damped, so you may be getting some overshoot