<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Please pardon my jumping in on this issue, but I think this might be an opportunity to increase my knowledge on magnetostrsiction… I have observed that a large transformer, when initially energized at no-load, generates a loud power-frequency sound which fades out over ten to fifteen seconds… <div class=""><br class=""></div><div class="">What causes the time-decaying character of the hum sound ?</div><div class=""><br class=""></div><div class="">Jerry AB3SX</div><div class=""><br class=""><div class=""><div><br class=""><blockquote type="cite" class=""><div class="">On May 22, 2018, at 14:29 PM, Karl W4KRL <<a href="mailto:W4KRL@arrl.net" class="">W4KRL@arrl.net</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">Mark,<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">This is due to remanent flux in the transformer core. If the first half cycle of the line tries to magnetize the core in the same polarity, it will saturate the core and draw high current. One possible approach is to put a resistor in series with the line and cut out the resistor after the transformer core resets. An incandescent bulb, say 75W if you can find one, would be a good resistor.<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">That said, try an SL22 10008 NTC.<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><a href="https://www.ametherm.com/inrush-current/transformer-inrush-current.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-R5wYlOv0eMMWHP5owpuxpxYzNUo4YC3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">https://www.ametherm.com/inrush-current/transformer-inrush-current.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-R5wYlOv0eMMWHP5owpuxpxYzNUo4YC3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE</a><o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">Karl W4KRL<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>Tacos <<a href="mailto:tacos-bounces+karl.w4krl=gmail.com@amrad.org" class="">tacos-bounces+karl.w4krl=gmail.com@amrad.org</a>><span class="Apple-converted-space"> </span><b class="">On Behalf Of<span class="Apple-converted-space"> </span></b>Mark Whittington<br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>Tuesday, May 22, 2018 6:57 AM<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Tacos <<a href="mailto:tacos@amrad.org" class="">tacos@amrad.org</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>Advice sought - isolation transformer inrush current limiting<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">I've got a 1KW isolation transformer that usually trips the 15A breaker at the breaker box when I turn it on, load or no. I'm considering installing a pair of NTC thermistors on the input side of the transformer, but I've no idea how to size them or if there might be a better option.<o:p class=""></o:p></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">Any advice would be appreciated.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">Cheers.<o:p class=""></o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">-Mark<o:p class=""></o:p></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div></div></div></div></div><span style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">_______________________________________________</span><br style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">Tacos mailing list</span><br style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class=""><a href="mailto:Tacos@amrad.org" class="">Tacos@amrad.org</a></span><br style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: LucidaGrande; font-size: 16px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class=""><a href="https://lists.amrad.org/mailman/listinfo/tacos" class="">https://lists.amrad.org/mailman/listinfo/tacos</a></span></div></blockquote></div><br class=""></div></div></body></html>