Three phase rectifier with thyristors not working as expected

Question

I'm trying to simulate the following circuit with a resistive load:

The control circuit provides three signals, and are the turn on signals for the thyristors:

the first one (yellow) α1 = α + π/6: the second one (red) α2 = α + 5π/6 and the third one (green) α3 = α + 3π/2

This is the isolation circuit:

And this is the main circuit

For α = 0. the following voltage results on the load, in yellow is VRN:

For α = π/2 The resulting signal is this, with VRN, VSN and VTN:

But that's not the right signal, it should start from 2π/3. Why the resulting signal doesn't match the corresponding pulse time?

These are the pulses, and the VRN signal. For α = π/2 the blue pulse should activate the thyristor at 2π/3, but the resulting signal that has been previously shown doesn't seem like it's working that way, why?

I've changed the connections as suggested in the comment and answer. The output signal is closer to the expected, but it seems like there's some sort of noise on the signal

This is the circuit now:

And this is the output signal for α = 0, α1 = π/6

For α = π/2, this is the signal:

In the last image, the negative part shouldn't be there

I've tried with the 15V power source connected to ground but for α = π/2 the output signal is this

What changes can be done to improve the output signal, is there something else I'm missing?

Answer 1

When you use an isolated power source for an isolated driver, the power source negative must not be connected to ground. Instead, it must be connected to the cathode of the SCRs. Otherwise, the isolated driver will simply connect a 15V signal wrt ground to the gate of the SCR. Since the cathode of the SCR can be at a voltage higher than 15V, the driver will not be able to push a pulse of current into the gate of the SCR.