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Ground Problem cause SPI to fail

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kappa_am said:
Looking to picture posted on Post #3, could you please explain to me why when power rectifier's diodes conduct, the midpoint voltage of the DC link varies respect to the negative rail of control board's power supply while DC link voltage is constant?
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Perhaps the capacitor voltage floats when referred to earth potential. I made an illustration (illustration is modified from image shown here https://www.electronics-tutorials.ws/diode/diode_6.html).
Red color show the situation during positive peak and blue color is for negative peak.

bridge rectifier common mode illustration.png

How much your DC link midpoint voltage varies?
 

In a TN network with earthed N line, the midpoint carries 0.5*UL1 against earth ground. Potential of the control circuit ground (if floating) depends on coupling capacitances against different nodes. In case of doubt, there will be considerable switching frequent voltage.

Possible isolation failure can be best checked by Hipot test.
 

Following pictures show positive rail, midpoint and negative potential compared to control section power supply's negative rail, respectively.
photo_2018-03-17_22-13-04.jpg
photo_2018-03-17_22-13-12.jpg
photo_2018-03-17_22-12-54.jpg
(the frequency is the line frequency)
amount of deviation depends on the time interval that diodes conduct. I reduced grid voltage by variac in such a way that the diodes did not conduct, and I did not see the deviation and all things were as normal. still beats me.
 

So, what can I do to avoid SPI failure? when I connect both negative rails together, the SPI gets failed. Can it be solved by taking out Y capacitor of DC power supply?
 

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