In article , Dave M wrote:
Don Klipstein wrote:
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.


That's not true. an open rectifier does not allow DC current to flow
through the secondary. It's just 1/2 of the power line cycle. During the
opposite half of the cycle, no current flows in the secondary. True, it's
unidirectional current, but it's an intermittent current, not constant DC.
If your statement were true, then half-wave rectifiers wouldn't be feasible.

A unidirectional pulsing waveform has a DC component.

That analyzes to a sum of DC, fundamental frequency AC, and AC at
harmonic frequencies. The average as averaged over a whole cycle is the
DC component.

It is fairly well known that a transformer driving a halfwave rectified
load can run into core saturation problems from the DC component in the
unidirectionally pulsing current waveform.

--
- Don Klipstein )