dc42:
I don’t find your description of the arrangement very informative. Can you provide a schematic? How was the HiPot test carried out?
If you use the SSRs to break the 5A current flow to the coils, you can expect a large inductive spike, quite possibly more than 220VDC depending on the inductance and capacitance of the coils and the core material. When switching an inductive DC load with an SSR or any other device, you should use either a flyback diode (possibly with a resistor in series, if you need the current to decay faster), or a R-C snubber network (but that may not be practical with a 5A 15V load).
Sorry about that but I’ve been nowhere near a scanner or camera with which to upload a schematic, and the majority of my work is away from computer as well so I haven’t gotten to make a proper schematic on there either. I’ll throw a crappy drawn one up now, I’ll come up with a better cleaner one later.
Also the HiPot test is carried out by applying 2000VDC to the A,B,C nodes, relative to the core (and some thermistors built into the unit)
DVDdoug:
What’s limiting the current? Are you sure it’s limited to 5A?
I don’t understand the capacitance either… I agree, we need to see a schematic. 2000V from a capacitor across the SSR will probably kill it.
If there is a capacitor across where the high-pot voltage is applied, you can get “unlimited” current when it’s discharged, just as you can get “unlimited” voltage when you break the current path through a coil.
I’d say you need to discharge the capacitor as a routine part of high-pot test, and I’d say discharge needs to be an automated part of the high-pot test to protect both humans and the SSR.
P.S.
You might consider using a regular 'ol mechanical relay. Mechanical relays are generally more rugged and more tolerant of abuse. A solid state relay can last forever if it’s not abused but it can also be killed instantly.
A mechanical relay will eventually wear-out, but it’s usually cheaper to replace and it’s usually in a socket.
You might need a relay driver to power the mechanical relay coil, depending on what’s currently driving the SSR.
The current is primarily limited by the resistance of the inductor coils themselves. But basically its hooked to a power supply and the operator dials the voltage and amperage to 15@5 when hooking up the first unit and then keeps those settings throughout the tests. It is able to jump a bit over 5A from unit to unit, however if one were far enough out of spec to have that drastic of a change, it would have been caught at an earlier test.
Me I don’t understand the capacitance either. Checking it with a voltmeter on about all the points I can’t find a C higher then 200fF which really shouldn’t amount to anything.
On the automation side of discharging the unit, I’ve thought about having our testing bed pull all terminals to ground after the full test but (in the developer’s infinite wisdom) they never added a feature like that.
As far as the mechanical relay, the boss says no. (for now, probably after another relay breaks it might be a doable compromise)
retrolefty:
Controlling inductive loads with A SSR can be very problematic as the resulting voltage and current go out of phase with each other and that can cause major problems with turning on and off of the internal triac/SCRs. Carefully calculated and sized “snubber” circuits are the general solution for using SSRs with inductive loads. Or go with a good quality electromechanical relay instead.
Lefty
Yeah DC only for tests, the unit is operated in DC as well. As far as a snubber circuit, I looked them up, but that whole carefully calculated part gets kinda in the way (ME intern, not EE, which leads me to wonder why I’m the one fixing this also)
I’ve been thinking to opt for a flyback diode, however I wonder where exactly to put in. I could attach it across the terminals of the PSU, or the relay (or perhaps I could throw them everywhere, flybacks for everyone)
Note in the schematic, the selector switch is rated at 125VAC@25A, though it is of much better make then the relay is. The emergency stop switch is also rated for 125VAC@25A, but is kinda cheap. Neither of these have failed (yet)
Scan1.PDF (75.1 KB)