In article , Tony Miklos wrote:

Yes, the thread got too long. If you saw the photo, and know the
primary went open on 3 transformers, it sure looks like a problem on the
primary side. If the load was too much, those wires on the secondary
side would most likely have been at the very least discolored from the
heat.

This makes me think the most likely causes a

* Improperly connecting the transformer (such as using only 1 of the 2
primaries of a 120/240V dual primary transformer)

* DC flowing through the secondary. That can occur if the tranny's load
has a fullwave rectifier with one diode open. If the fullwave
rectifier has discrete diodes or a dual diode, the problem may be a bad
solder joint at one of the diodes.

DC through the secondary is well known to be able to cause transformer
cores to saturate. That can lead to the primary drawing excessive
current.

* Less likely - there is a magnet on the transformer core. That
combined with the tranny's normal magnetic flux can cause core
saturation, similar to the effect of DC in a winding although this
usually only causes part of the core to saturate.

However, a magnet on the core appears to me "probably unlikely to be
an actual problem" unless the tranny is of marginal design, line voltage
is on the high side, or the magnet is especially powerful.

There is the matter of voltage spikes on the primary blowing the
insulation. However, I consider that unlikely if nothing else has been
blown while 3 transformers blew.

--
- Don Klipstein )