A three phase inverter is usually driving the bridge legs with in-phase PWM ("unipolar scheme"). This means that you have a considerable PWM frequent common mode voltage superimposed to the output, e.g. 1/3 of the bus voltage peak-to peak. From an EMC viewpoint it's surely desirable to connect the filter capacitors to DC bus ground (or midvoltage). But inductor and output stage currents will be increased, and particularly a reactive current component added.If you consider a bit of RF hash flowing down through a filter capacitor, it arrives at the "neutral" point, then where does it go? Up through the other LPF caps into the other FETS? - they may be off? Also the neutral point is neutral wrt the 50 HZ, not to the PWM carrier, because even if the PWM carrier is exactly timed so each pulse is 120 degrees time separated from the others, they will not balance out as they are off equal amplitude (unlike time shifted sine waves). The filter caps should be returned to the PSU 0V, to complete the RF path.
Don't do that, leave the transformer star point floating. Usually a three phase inverter superimposes a 3*f common mode component to increase the available output for a specific bus voltage, it would be shorted by a three-phase transformer with connected star point.i tested the circuit by connecting the neutral of primary side of the transformer to the neutral of three capacitors
Measure the phase-to-phase output voltage, it should be DC-free.Also can you please guide me how can i check for possible DC inbalance?
Don't do that, leave the transformer star point floating. Usually a three phase inverter superimposes a 3*f common mode component to increase the available output for a specific bus voltage, it would be shorted by a three-phase transformer with connected star point.
Measure the phase-to-phase output voltage, it should be DC-free
At least for the start. I wonder if you need a filter on the transformer primary side at all? I would think about utilizing the transformer leakage inductance as filter inductance (either additional or without a dedicated inductor).so should i leave the star point of the low pass filter capacitors floating as well?
Yes, presumed you achieve stable operation with transformer. Otherwise connect a resistive test load. The unloaded output should be DC-free as well.Phase to phase voltages on the primary side of the transformer?
Frank i tested the circuit by connecting the neutral of primary side of the transformer to the neutral of three capacitors and this was connected to the ground of DC which is also connected to the PSU 0V. I had 3A fuses placed in between the Rectifier's DC ouput and Inverters DC bus. These fuses instantly were blown away as i powered on my inverter.
The other possibility of two low side mosfet's being on would also cause the imbalanced current flow when i am directly connecting the load acrosse the inverter.? Although i checked and this isnt the case.
FvM
Thank you for sharing your knowledge over this.
As i mentioned above i tried connecting the DC bus ground to the neutral of the capacitors and primary side transformer neutral but it wasnt of much use.
I will be uploading the three gate control pwm waveforms for you to look through and guide me if i made some error while implementing them correctly. Also please let me know whatever you think is needed regarding my design or waveforms at any point, in-order for you to help me find a solution to my problem.
Also can you please guide me how can i check for possible DC inbalance?
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Below is the Ouput across the capacitors of the low pass filter connected in Y configuration. The Voltages are measured phase to phase. The Mosfets are not heating up over 100W load per phase.
View attachment 104911
Thankyou for your insights chuckey
The driver IR2109 comprises of an internal dead time to avoid both the high and low side Mosfets be switched on at the same time.
This maybe the cause of leakage inductance. What is your recommendation to solve this problem?
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View attachment 104877
The above attached diagram of a three phase inverter shows a ground between the two sources Vs/2, in comparison to my design these are two capacitors connected in series so that they charges upto Vs/2 respectively.
The diagram below of the three phase low pass filter shows Neutral of the three phase RLC filter to be grounded. Should i be connecting this neutral point to the ground mentioned in the above diagram of inverter. Maybe this is causing the problem?
View attachment 104878
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