Guru
Joined: 09/08/2007
Location: AustraliaPosts: 4406
| Posted: 01:16pm 15 Mar 2017 |
I can tell you all about those chokes, I have several here pulled from 1.5Kw Inspire grid tie inverters. Not identical but almost so.
This choke, along with the toroidal transformer, and a 5uF capacitor form a very clever composite filter to turn the PWM into a nice smooth sine wave.
The 5uF capacitor and the transformer secondary form a tuned circuit resonant at roughly 1.5 times 50Hz. The reason it is not tuned to exactly 50Hz, is that would cause a massive uncontrollable build-up of voltage under no load.
By tuning it to 1.5 times the frequency, any stored resonant energy in one cycle appears exactly out of phase during the following half cycle.
This 75Hz resonant circuit, particularly the 5uF capacitor will have a rapidly decreasing impedance at frequencies higher than 75 Hz, and will greatly attenuate the 20 Khz PWM and its harmonics.
To assist this, and not shunt the PWM bridge with a very large capacitor, a series choke is fitted that has a low impedance at 50 Hz, but a very high series impedance at 20 Khz.
This combination is an excellent way to turn high frequency PWM into a 50 Hz sine wave with very high attenuation of the higher frequency components.
There is also the hidden advantage that the circulating energy between the transformer secondary and capacitor, even though it is not resonant, reduces the effective magnetising current (as seen by the PWM bridge) and no load loss created by the transformer.
The choke you have is not a common mode choke, although it looks exactly like a common mode choke. The phasing of the windings will be different.
Your choke will be a split choke with half the turns on each side, which is a much nicer way to do it than the Inspire choke which is all on only one side of the PWM bridge.
The coke I have here is marked 2.7mH, but it actually measures closer to 3.4mH. The inductance is not that important. I cannot remember what I measured as the saturating current, but it was pretty high, 40 to 50 amps as I recall. So it must have a very large air gap buried in there somewhere.
Cheers, Tony.