According to :
"PaPaPeng" wrote in message

What will happen if I plug it into a 220Vac power supply instead...

Smoke. How about a series diode or a 240 V lamp dimmer?

Or put both plates in series, if the powers are different.

If the element power consumption is different, if you put them
in series, the higher power one will produce _less_ heat than
with 120V just before the other element burns out.

If you tried this stunt with a lamp dimmer (even a 240V one),
you'd blow it up too. The dimmer would have to be rated for
something on the order of 4000 watts. Lamp dimmers are
usually 600W, occasionally 1Kw.

Dimmers don't adjust the voltage, they adjust how much of the sine
wave is "omitted" from the feed - more or less determining duty
cycle. Dimmers lop out the lower voltage part of the curve, so,
the elements would still be seeing 240V.

A series diode would almost work, except that it only cuts the
power by one half, you need it reduced by three quarters.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.