[SOLVED] Phase detector circuit not working properly

[Bob Edge]

View attachment Phase Transformers.pdfView attachment transformer board.pdf

Hi,
I have two circuits, one is a simplified version of the other. It is used for zero cross detection in a welding machine.

The trouble is that the stripped down version is giving me zero crossing signal 30 degrees before the other circuit.

Can anyone please take a look see if you can see what I'v done wrong?

Many thanks.
Bob...

 

[dick_freebird]

You've removed a lot of what looks like a timing cap and hysteresis
from the circuit. Why?

What is the truw phase relation you need, and if you have consistency but
skewed, can you make that up somewhere else (like a one-shot down the
line, before whatever it's supposed to be triggering)?

Way back in the day I used simple blocking cap, reset resistor and comparator
scheme for SCR based AC-DC motor control. Not 3-phase though, I don't know
what you're trying to do here. What is the true phase relation (mains -> trigger)
you are looking for? True zero needs either a lead network to comp the time
delay through the detector / gain, or 360 minus a smidgen of lag. A motor
control might not care so much as near zero turnon doesn't give you any
power and may cost if you connect to a line that's lower than back EMF in
the moment. We used the C-R network to get trigger at "peak or later"
(back when nobody but the power company complained to you about power
factor).

 

[Bob Edge]

Hi,

The older circuit was an analogue current feedback welder, the circuitry removed was used to create a ramp. I only need a digital signal, since I use microcontrollers to figure out the phase angle delay, and firing the thyristors.

It is the line voltage I need to detect zero crossing, not phase to phase voltage. I could simply add a 30 degree delay to the program, but I just wanted to know what I was doing wrong. It is the mistakes that teach new things in electronics.

Bob.

 

[FvM]

I must confess, I didn't realize it at first sight.

As a major difference, the first circuit has the transformer primary star point connected to neutral and the present hasn't. In combination with the non-harmonic (rectifier and filter capacitors) load, this can't work as a phase detector. You either need to connect the star point or provide a mostly resitive load to the transformers.

 

[Bob Edge]

Hi,
Thanks for that.
The first circuit only had the neutral points connected together externally on a connector, not to the supply. I just changed it to be on the board.
Maybe I need to load the transformers more. They are at half their rated load at the moment, with the two load resistors.

Another difference is the transformers I used are different manufacturer, and smaller since they do not provide a power supply like the old circuit did.
I would have thought the winding was the same though, or things would really be way out.

 

[FvM]

I should add, that resitively loaded transformers are also involving a (lagging) phase error. Anyway, the floating star point creates errors with non-harmonic transformer load. To know what's going on in your circuit, use an oscilloscope, compare secondary waveforms and phase voltages. There are many ways to overcome the problem, e.g. using optocouplers connected to the primary voltage or transformers with delta connected windings on the primary.

 

[Bob Edge]

Hi,
Now solved the problem.

I removed the load resistor on the positive side of the rectifier, added back to back diodes to the secondaries of the transformers, then added resistors from the centre of these diodes to the positine of the rectifier.

The value of these resistors affects where the zero cross happens, so I tuned the value to agree with the line voltage on the scope, and there we go, a perfect zero crossing circuit. Thanks for all your help on this one.

Bob...