Scott,

?

I worked on three phase power supplies that had some premature transformer failures.

?

The engineers had us install MOVs across the primary windings for all three phases.

?

Never seen another transformer failure afterwards.

?

Dave

W9HF

?

?

Halden, your investigation is interesting and detailed. IMO, the DC [resistance only case, ignoring all inductances] is the extreme limit which won’t be reached. Inductance does not go away, especially in the wiring; and it is still there in the transformer, albeit quite reduced IF it reaches saturation. ?Core steel used in 60 Hz “power ” transformers is chosen to have soft magnetic steel, a low magnetic remnance, and a B-H curve with low hysteresis, so a fair bit of inductance remains, even IF the transformer is driven into saturation; like it most likely is when energizing at zero voltage crossovers. ?But there are some more recent high temperature cheapie transformers that have less steel, drive the flux into saturation, get hot with no load and tend to blow normal fuses when turns on, so old “typical” maximum inrush currents are not cast in stone any more.

?

I don’t see how the stuff about “already be in positive saturation” that has be floating around ?has anything to do with any realistic enegization or re enegization of? a realistic power transformer, as the steel has little remnance.

?

Yes it seems that the?? ?“STBY-->OFF-->STBY” failure mode has been overtaken by the SSR stuff, yet what caused circuit condition causes the failure has not been refined. Is ihr failure triggered when closing the power switch contacts, or opening them. Seems to me the old Halli warning was related to opening the switch, not closing! How much voltage spiking and or arcing takes place in the switch when it is opened under load as opposed to under standby?

In the event that this is the problem, the solution might be to open the switch only near zero current--- so …….. ?don

?

?

Thanks, all, for the comments and lessons!?
It seems we're converging on using zero-crossing transformers that are capable of switching whatever current the transformer would draw if only the winding's DC resistance limits the current.? I measured the HT-37 primary to be 0.616 ohms (4-wire, Keithley 2100 DMM).? A #18 AWG power cord 1.5 meters long would add 63 mohms to this and a 4A SB fuse about 31 mohms more.? The wiring from the AC socket to the breaker panel might add another 100 m ohms.? This means the maximum surge current could be as high as 150A if there is really no back emf.? The estimate from the Opto22 document suggests 10 times nominal which is much less than this.
The HT-37 transformer is probably close to 250 VA, so the [M]P120D4 is the OPTO22 recommendation.? 4A continuous and 85A for a single cycle, 66A for 3 cycles.? This seems ample capacity if using Opto22's "10 times" estimate.? If a turn-on drives the transformer into saturation, then the additional current that flows doesn't actually cause any more flux and thus no voltage surge at the HV secondary.? OTOH, a relay that turns on immediately could cause the flux to rise more quickly, to saturation or not, causing the voltage spike.? Did I get this right?
In many cases, I the zero-crossing SSR in use actually does have the capacity to handle the surge current even though the person installing it wasn't paying attention to this issue.? Thus, many people using ZC-SSRs haven't had problems.?
The part I don't understand is:
"Large inrush currents can occur during the first half cycle of line voltage if a zero-voltage SSR
happens to turn on during the positive half cycle of voltage when the core is already in positive saturation." (Opto22 document cited earlier in this thread)
How could a transformer already be in positive saturation if it has been unenergized for a while prior to turn-on?
Halden VE7UTS