This image:

looks a lot like one I've studied on my DSO at work. The difference is scale. You seem to be dropping 20 V where I would expect more like 2 V.

So here's a whacko suggestion for you:

Is your DSO probe set to x1 when you've told the DSO it's x10?

If so then the voltage is actually 10x lower than what you think.

Then the discontinuity at 60 degrees, only under load, is actually expected. This would be where the current crosses zero. Remember, you have a highly inductive circuit here, since you are not loading the motor much, so this is mostly magnetising current.

(I don't drive a motor; I have a resistive load, so my L is explicit and known, though split into pieces with my LCL filter. Your discontinuity is at about the same point to where I see it [ edit: was different ], but I see it about 40 degrees. That's just because my load has about the same reactance as resistance; yours has about 7:4 (tan^-1 60 degrees). The shape and the "it only happens under load" aspect matches exactly what I see.

Remember that the current is actually not a pure sine wave, but a sine wave with a triangle wave superimposed. So as the current increases from negative to positive, at first only the tiniest peak of the triangle crosses zero, then more of the peak, and so on, then after a millisecond or so, most of the current is positive with only the negative peaks of the triangle dipping negative, then only the very negative most peaks cross zero, and finally the current stays positive even at the negative peaks of the triangle.

When the current is positive (depending on convention of course), it is coming out of the IGBT, so the output is about 1.5-2 V

**less than** the DC+ rail (considering the top IGBT now). When the current is negative, it is flowing through a free-wheel diode into the pack. The free-wheel diode has a voltage drop, say 0.8 V, so the output will be around 0.8 V

**more than** the DC+ rail. So you see 2-3 V difference in amplitude (depending on the magnitude of the current, and the size of the IGBTs) as the current crosses zero. When there is no or very light load, you don't see the 2 V drop across the IGBTs, and somehow you don't see the diode drop either (maybe at light loads, the diode drop is say 0.3 V and gets lost in the noise).

[ Edit: I forgot to say what the point of the above is: this explains why the discontinuity isn't abrupt, but takes about 10 ms on your slow wave (less than a millisecond with my 50 Hz waves). ]

[ Edit2: remember the current lags the voltage, here by about 60 degrees, so when the voltage is at 60 degrees, the current is only just crossing zero from negative to positive. ]

Notice that after the discontinuity, the shape of the curve is basically right, just reduced in amplitude, until the voltage crosses zero (and at that point, some other current is crossing zero, since it is 120 degrees later).

[ Edit3: my circuit has the load basically as one delta leg. So I only see two discontinuities, one at about 40 degrees, and another at 180+40 = 220 degrees. You may care to disconnect one leg of your motor to see if this makes any more sense that way. ]

But what you seem to be seeing is a 20 V drop, and that can't be right. If it's actually around 2 V drop, then this is to be expected. Apparently, this is moderately well known, and good inverters actually detect the zero crossings and compensate by increasing the PWM amplitude at just the right point. I've actually done this recently, and it works to a point. It's hard to get the timing just right, so what happens is that there is a spike at the crossover point, and the output rings badly after that point, almost to the next half cycle. (I'm doing only one phase, remember.) I actually don't mind this too much, as the 2.4 kHz ringing (from my LCL filter's resonance) is something that we want to try to compensate for in software, if possible, as a future research project.

Of course, if the amplitude of the signal is always quite large, then none of this matters, as 2-3 V is small potatoes compared to your ~300 V bus voltage (mine will be over 700 V).

So perhaps you could verify the line to line amplitude with a multimeter?

My apologies if this has already been checked; I've only had time to skim this whirlwind flurry of posts.

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