I have been doing a bit of testing over the weekend.
I replaced the resolver exciter circuit with a slightly better design.
I made a resonant circuit that runs from the logic output and then I used an audio amplifier to drive the exciter coil. This arrangement works better from the point of view that it has less effect on the exciter signal when the rotor turns.
I did a few measurements and all seems fine.
There appears to be no DC offset any more.
Here is a video of the test.
I was running 450A RMS into the motor. You can see the cables moving from the current flow.
The inverter under test has now decided to behave as it should.
The DC offset has miraculously disappeared.
This is the current measurement using a bus bar to short the terminals.
Perfect sine waves and no DC offset.
- Inverter under test with motor shorted with bus bar.
- 20211010_212134.jpg (1.89 MiB) Viewed 2821 times
This is the current measurement as above but using the output stage to short the motor but only half the windings shorted to avoid damage to the output stage.
- Inverter under test with motor shorted thru output stage.
- 20211010_212156.jpg (1.82 MiB) Viewed 2821 times
This is the current measurement with the inverter driving with 40v bus voltage.
No DC offset and output current reasonably sinusoidal. The distortion is due to the voltage drop from the diodes which I will attempt to fix at some point and may not be necessary because once working at higher voltage the distortion will be reduced.
- Inverter under test.
- 20211010_205328.jpg (1.22 MiB) Viewed 2821 times
This is the other inverter with no modifications.
You can see the current waveforms are very distorted and there is a DC offset.
- Driving inverter.
- 20211010_205406.jpg (2.02 MiB) Viewed 2821 times
I really hate these solutions when the problem just disappears without any apparent reason.
Tomorrow I will make the resolver mods to the other inverter and see what happens.
I do not see how fixing the resolver can fix the DC offset. From my understanding it is not possible for the FOC output to produce or correct a DC offset.