(2nd post this evening)
This image shows the profile of primary current starting with a peak a little more than 4 A and decaying over about 4 seconds to about 1.6A as the filaments warm up.? In this case, there was no big current surge.
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Next, I installed the Omron SSR in series with the original mechanical switch to see what happens when the transformer turns on at zero-crossing voltage and off at zero-crossing current (almost the same time as voltage zero-crossing).? Now that we have entire half cycles at the start and end, the surge current, when it occurs, is much stronger:
Yes, that's 49A!
These surges occur only when the last cycle before turn-off is of the same polarity as the first cycle after turn-on.? This is consistent with the explanation that remanence and saturation are the causes.? The core is left magnetized with one polarization when the transmitter is turned off, and the first cycle once turned on seeks to increase that magnetization further.? Since it hits the limit, there is no further conversion to magnetism available, so the current is limited only by the resistance in the primary, the current-sampling resistor, the wiring in the transmitter, and the wiring from the outlet to the circuit breaker panel.
Sometimes, there's no such surge and the measurement looks like this:
A few other notes:
I solved the problem with the images.? It doesn't have anything to do with groups.io.
The familiar THONGGGG! sound occurs when one of these big surges occurs.
Yes, Jim, the current and voltage are approximately in phase.? You are correct that in a purely reactive circuit they would be 90 degrees out of phase.? Evidently, a loaded transformer is not a purely reactive circuit.
I have not yet looked for voltage surges in the secondary and thus have not looked for possible correlation between such surges and different conditions in the primary.
This transmitter has certainly survived many turn-on surges in the 11-24 A range.? I'm reluctant to subject it to surges of twice that magnitude.? I'm concerned that if there's a nick in the primary winding, such surges might cause it to open.? At this point, it appears better to keep using the mechanical switch instead of installing a zero-crossing SSR to manage the transformer switching.?
But I'd prefer to never have any such surge at all.? To achieve this, I have designed a circuit to ensure that the first cycle is always of the opposite polarity of the last one the previous time the transmitter was on.? I need to order? (and receive)? a part, so it'll be a while before I report the results on that.
Cheers,
Halden VE7UTS
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