Question about switching Rectified inductor with SSR.

[StutterMeThis]

Hi All,

I've been trying to use a solid state relay to switch a contactor, and have been getting very unreliable results. I was curious if I could get some help and insights.
In particular the SSR's love to stay latched onto the on state of the device. Regardless of the snubber used.

Looking up online a little bit, this seems to be mostly because of the current wave form of the device, (and the corresponding dIcom/dt). I've attached an image below, of how the current wave form essentially looks like. I believe the contactor has an inbuilt rectifier, this causes a very high dI/dt , which keeps the SSR on. However from my understanding this mostly just affects triacs, and not so much SCR's. Here is a link to the SSR I'm using, and they say they are using SCR's so i'm wondering if the high dI/dt will also cause the SCR's to latch on. (https://www.mouser.com/datasheet/2/240/media-3321277.pdf and the contactor is https://www.se.com/us/en/product/LC1D115G7/iec-contactor-tesys-deca-nonreversing-115a-75hp-at-480vac-up-to-100ka-sccr-3-phase-3-no-120vac-50-60hz-coil-open/). For context the contactor has a draw of around 20-30 mA when on.

I've read some application notes about this, but for the most part they've been pretty light on the details (https://edata.omron.com.au/eData/Relays/Y108-E1-1.pdf and http://class.ece.iastate.edu/ee330/miscHandouts/AN_GOLDEN_RULES.pdf). I was curious if there were any other books / resources that go into a bit more detail, or describe a bit more in depth what the problem is here. Also are there any rule of thumbs for what dIcom/Dt is the max acceptable. Would also love to dive into the math of how this stuff works, normally helps me better understand how to design the stuff.

Am I correct in my assessment that it is a dI/dt issue or is there something else at play?

Thanks all.

[Zero999]

Could it be back-EMF related? Try adding a MOV in parallel with the contactor's coil.

[StutterMeThis]

A move is an interesting idea. I don't have any right now so would have to buy them. I did have a similar idea, so I tried adding a RC snubber both directly across the relay and the contacts of the SSR, with little effect. One of the things is that this contractor does have a diode current shunt built in which does seem quite effective. The measured voltage rise rate was only about 50V/us (though tbh it was a bit hard to measure so I could imagine I could have affected it that way).

I had a hunch that it was maybe the dI/dt due to the rectified nature of the current which gives a really fast change (definitely way above 5 amps / ms)

[PCB.Wiz]

I've been trying to use a solid state relay to switch a contactor, and have been getting very unreliable results. I was curious if I could get some help and insights.
In particular the SSR's love to stay latched onto the on state of the device. Regardless of the snubber used.

So you are saying, once turned on, the contactor does not release ?  Does that happen 100% of the time ?
Do you have a scope to check the contactor coil waveform ?

The off state current may be enough to hold-in the contactor, the specs suggest the hold-in current is much lower than the peak pull-in current.
SCR SSR's need some current flow, to trigger the gates, so they never go to zero.

A power resistor across the contactor coil, would lower the voltage that sees in the off state.

Am I correct in my assessment that it is a dI/dt issue or is there something else at play?

It may be dV/dT more than dI/dT  - inductor loads are not that nice, the SSR releases when current crosses zero, which is when mains is close to max, so there are high dV/dT effects.