Ulf,
there are two fundamentally different kinds of SSRs. One type uses
thyristors (TRIACs or SCRs), the other uses back-to-back MOSFETs.
These two (or three!) types have very different behavior. Not only that
MOSFET SSRs can handle AC and DC while the others are usable with AC
only, but also in that their handling of inductive loads is very
different. Generally TRIACs are poor with inductive loads, antiparallel
SCRs are much better, and MOSFET SSRs might be best if rated for
inductive loads, but that needs a special circuit inside them to avoid
damage from inductive kick.
Thyristor SCRs will switch on either the instant you apply a control
signal, or the next voltage zero crossing after that, depending on their
type, and will always switch off at the current zero-crossing. With a
purely inductive load (and the primary of a very lightly loaded
transformer is very close to that), the current zero-crossing is 90
degrees out of phase with the voltage zero-crossing. This leads to some
head-scratching.
TRIAC SSRs will often trigger far more easily in one polarity than in
the other. This is likely the cause for your transformer hum: It's
getting only a semicycle instead of the full grid waveform, so there is
a huge DC component present, and the transformer core gets hopelessly
saturated, with the magnetic flux leaking out of the core.
Another possible reason for hum is that the SSR is switching on too much
after the zero crossing, so that there is a hard voltage step inside
each half cycle. The harmonics contained in such a step tend to be very
loud.
You will need to look for SSRs rated for highly inductive loads, and
preferably of the back-to-back MOSFET type. Or else use old-fashioned
mechanical relays.
One last thing: In some cases you can cure such thyristor misbehavior by
adding snubber networks. You might want to try this first. Simply add a
100 ohm resistor in series with a 100nF capacitor, of adequate AC
voltage rating, directly across the output side of each SSR, right at
the SSR. With some luck that might tame them.
Manfred
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