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Jim Stewart

wrote:

There is no variable control for the HF. It is either on or off.
Those little line filters are pretty good at filtering noise at high
frequencies coming back to the line, but a thyristor phase control
injects harmonics at low multiples of the mains frequency, which can
require more aggressive filtering.

Switching heavy inductive currents with phase
control is the devil's own work in line-conducted
interference.

Years ago, I installed a very sensitive electron
beam lithography machine at a semiconductor
house in Silicon Valley. We had horrible, non-
reproducible problems with it until we scoped
the 3-phase coming in. The epitaxal reactor
on the other side of the building was making
the input power look like something out of Jimi
Hendrix's guitar. Both machines were essential
so the e-beam machine was only used for critical
jobs when the reactor was off.