| Electronics > Repair |
| Inverter drive module overvolt fault |
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| Eamon:
ok, so with a much longer cable, I have just over 2 ohms phase to phase now instead of 1.1 ohms but it runs and triggers the fault the same. I can use the scope on the desk now so will try to get some traces. Just have to work out the safest way to go about this as the scope is a USB scope with earth ground and the inverter is mains earth connected with the common ground as the -DC I've ordered a USB isolator but that will take a couple of weeks to arrive I think. I am reading that I should probably be using an isolated transformer to power the inverter? The full load of the inverter would require a fairly expensive transformer, but as it faults at around 30Hz which is well below full load I will probably get away with something smaller. Also the scope can be powered externally rather than USB powered. The inverter has a 15V line for the fan (relative to -DC/common ground), If i were to convert that down to 5V the scope uses would it be the way to go as that would put the scope and the inverter on a common ground? (assuming I use the USB isolator) |
| capt bullshot:
An USB isolator is fine. Some of them have capability to power up the connected USB device - don't know if this is enough for the AD. If not, I'd recommend a quality external power supply for the AD, these should have enough isolation to cope with -DC potential (don't use a lab supply). I wouldn't recommend powering the AD from the inverters internal supply, as 1. it might not provide enough power, and 2. the AD powers down when you disconnect the inverter mains supply, this might be really inconvenient. Third reason: You'd connect the AD GND to -DC anyway in this scenario, but you'd better ensure no power supply current flowing through this connection as this might induce "interesting things" to your measurements. Alternatively, use an isolation transformer and connect -DC to protective earth. The AD should work then without any further isolation (in theory at least, in practice one could discover what kind of funny things EMI is able to kid on you ;) in such kind of setup). The isolation transformer must not be rated for the full power of the inverter, as at the lower motor frequency the load is lower and your typical isolation transformer can be short term overloaded by single digit multiples of their rated power. It just heats up faster, if you cut off the power before it overheats and wait for it to cool down again, it'll work. |
| Eamon:
Ok, I think the isolation transformer is the way to go and I will then tie -DC to earth, along with the Analog discovery's ground. I came across a couple of step up/down transformers for cheap and had the thought that I could step down through one and up with the other to get isolation but turns out they are both autotransformers so no luck there, however the input can be switched between 240v, 220v, 200v and 110v with both "110v" and "220v" outputs, net effect is that if I switch it to 240v input, I get 210v on the "220v" output rather than my normal 230v so I can try what you originally suggested of feeding it ~20v lower voltage, I need to source a US style plug first, but I'll give that a go while I wait for the isolation transformer. |
| Eamon:
Ok, I haven't given up on this and I'm finally in a position to use the oscilloscope on the inverter (isolation transformer, USB isolator). Also, while waiting for the USB isolator to arrive I've painstakingly mapped all the ADC pins on the MCU. I've put probes on all the ADC inputs and none appear to exhibit the pattern reported by the "AC Plate voltage" value reported by MODBUS when I run the compressor :( That said, there is another characteristic that might provide some clues; when powering off the inverter there's enough power in the capacitors that the 3.3v rail holds for another 15 seconds or and continues to power the MCU and I can continue to read the values for this period. The DC voltage drops in a fairly linear fashion until about 160v at which point the MCU cuts out BUT the reported "AC Plate voltage" drops immediately to 190v and stays exactly at that until the MCU cuts out. In fact, regardless of what input voltage I feed the inverter (using a step up/down transformer I have been able to generate 250v, 217v and 210v in addition to my current line voltage of 227v) and consequently where the "AC Plate voltage" starts when the power is cut (it always seems to read 14v lower than I measure at the terminals with a DMM), it always drops abruptly to exactly 190v for the remaining duration before the MCU cuts out. One reason this behavior is useful is that I have probed every pin of the MCU while powering off the inverter from idle and the only two pins that show any real change between power off and the MCU cutting out are pins 6 and 9. Pin 6 (ADCINA7) drops from about 1.6v in a linear fashion to about 0.8v before the MCU cuts out (pin 6 is connected to DC+ through 4 x 470kΩ resistors and I'm fairly confident this is where the DC voltage measure is taken) pin 9 (ADCINA2) drops from about 2v much faster, more of an exponential decay. Maybe not directly converted to the "AC Plate voltage" but certainly suggests its it involved. My guess is that because the "AC Plate voltage" does not directly track any input it is some sort of calculated value (with a floor of 190v) but I'm at a bit of a loss as to where next? I was thinking that maybe I should I try manipulating the voltage on pin 9 with an external source and see what it reports as the "AC Plate voltage"? What sort of supply would be suitable for this? |
| capt bullshot:
The supply must be capable to override the voltage at pin 9. So it needs to have a lower output impedance than the circuitry that brings the signal to pin 9. I'd recommend to investigate / reverse engineer the circuit driving this pin at least partially to see if it's driven from a resistive divider or an OpAmp output. In the latter case, be careful when applying external voltage to that node, as it could (thermally) overload the driving OpAmp. For a supply, keep in mind it has to be able to source and sink current to overdrive the voltage, and should be of low output impedance. In a former job, I've used a purpose designed amplifier to achieve that, but there was low risk, as I easily could get a fresh or repaired target board if I blew the device by mistake. For whatever reason (most possibly some kind of reversed murphys law) I didn't blow up a target. https://www.eevblog.com/forum/testgear/show-the-homemade-equipment-you-are-using-now/msg1983056/#msg1983056 |
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