Formula for pi or t lowpass filter?

[Michael Moroney]

What are the formulas for a lowpass pi or T filter for a given load and
frequency? What I want to do is a lowpass filter for use between an
inverter and a power transformer, to keep the inverter's harmonics (of
60 Hz) out of the transformer. In other words, pass 60 Hz (120V, 2A) to
the load, keep the third and higher harmonics out. The attenuation is
not too critical, just want to keep from overheating the transformer.

 

[Tom Grayson]

What are the formulas for a lowpass pi or T filter for a given load and
frequency? What I want to do is a lowpass filter for use between an
inverter and a power transformer, to keep the inverter's harmonics (of
60 Hz) out of the transformer. In other words, pass 60 Hz (120V, 2A) to
the load, keep the third and higher harmonics out. The attenuation is
not too critical, just want to keep from overheating the transformer.

Mike,
I cannot help you with the actual design criteron of the filters
however I can offer a few words on Harmonic filters.

Non linear loads such as the front end of an inverter tend to create
harmonic currents which are a function of the load current and the inverter
output voltage.

These currents change with the load and a low impedance path offered to them
will certainly be taken

If there are high values of harmonic currents and your filter elements are
not rated to carry these currents, then they will certainly fail, and
catestropically, sometimes, as I have found out from Experience.

Most inverter manufacturers should have a "White Paper" dedicated to the
design of the harmonic filters , giving specific details of harmonic
currents generated for given loads.

See if you can find such things at the Manufacturers web site, but be
prepared to "Over Design" a little rather then "Just big enough".
Manufacturers tend to be a little conservitave with estimatesof harmonic
currents.

Good Luck
Tom