| Electronics > Repair |
| Inverter drive module overvolt fault |
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| capt bullshot:
The capacitance code 225 means 22 and five zeros pF -> 2200000pF -> 2.2uF |
| Eamon:
Ok, so it's going to be a little while before I can get a replacement capacitor for C53A, but I have a much larger electrolytic capacitor (labeled 820uF but tests at 730Uf with my DMM) here that has a suitable voltage rating so I was wondering if I could wire it in parallel to test. I assume there is a good reason that they used a metalised-polypropylene capacitor with a relatively small value here instead of a larger electrolytic capacitor, but can I add an electrolytic capacitor in parallel in this circuit with a much larger capacitance and see if that prevents the fault? Also, if I do just end up de-soldering C53A to test it, is the capacitance test function on a DMM sufficient? |
| Eamon:
Well this is frustrating; I've ended up de-soldering C53A and testing it and it looks good so I'm back to the drawing board for the root cause of the problem :( I've tested the C53A capacitor with both the DMM (which reports 2.18uF) and as it turns out the analog discovery has an Impedance Analyzer function that I used to measure the capacitance from 10Hz up to 100kHz and it was within 1%-2% of the 2.2uF nominal value between 10Hz and about 50kHz and within 10% from there up to 100kHz so I am reasonably confident that the capacitor is good, at least at low voltage (AD2 test was done at 5v). Is it possible/likely that the capacitor might still be bad but only at AC line voltage? Is there any other test I should do before I put the capacitor back in circuit? I can manipulate the reported AC voltage by varying the voltage at pin 9 between about 1.8v to 2.5v and this corresponds to linear change in reported voltage between 190v to 255v, but everything from 2.5v to 3.3v remains at 255v like it was a ceiling value (3.3v was as high as I was prepared to go as that is MCU datasheet say to keep the ADC inputs below 3.3v), but when running the compressor, the reported voltage jumps higher than this (usually up to about the DC voltage) and I observed the voltage at pin 9 actually dropping (but getting noisier) when the compressor kicks in and the "AC voltage" jumps up. Where to next? |
| shakalnokturn:
I doubt the capacitor would behave badly at higher voltages, nothing else I'd check before re-fitting. Sorry for giving you hope, at least it's checked. So if in "bench test" conditions the AC voltage measurement is behaving correctly and I would suppose that 255V is a sensible high limit anyway, then the problem must be noise getting to the wrong place. Others have previously mentioned some of these points: -MCU supply voltage(s) decoupling (ADC Vref. voltage if externally generated or decoupled.) -Snubber capacitors/networks on the motor output. -"Y" capacitor on the small SMPS. -Wrong grounding. |
| florentbr:
--- Quote --- as I varied the voltage, the reported AC voltage varied between a floor value of 190v (at 1.83v or lower) up to a 255v ceiling (2.45v or higher) --- End quote --- The output of the bridge rectifier DB1 should be 320v DC (flat) when the PFC is off (no load) and 230V DC (rectified sine, 320v peak) when the PFC is ON . Maybe it's simply the software reporting the voltage as Vavg/1.41 when the PFC is turned off and just Vavg when ON. Assuming the PFC is failing to pull the coils L1/L2 to ground with Q1/Q2, that would explain the ACPlateVoltage rising from 220v to 320v and why the InvertDCVoltage doesn't rise. I don't think it's a sensing issue since the reported values are stable and they match either Vrms 230v or Vpeak 320v. Check the voltage on the gate/drain of Q1/Q2 with and without load. |
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