There are statements on the web, for example from Star K, to the effect that an induction cooktop cannot be used on Shabbat. My question: are those statements outdated? Consider this: One could purchase an induction cooktop that has a timer. There are even brands that have a separate timer for each element. Before Shabbat, set a timer to turn the element off, say, after 45 minutes (to coincide roughly with the start of dinner). Place a pot (e.g., of soup) on the hot element before Shabbat starts. Wait until the element turns off, and voilà -- you can now retrieve the pot and its contents. Does that make sense? Granted this is a very limited Shabbat use (only Friday night), but would the described use of induction cooktops on Shabbat be halachically permitted? Would the following be a halachic issue in the above situation? if you set the timer too late, and it does not turn off when you're ready to eat, you might be tempted to remove the pot with the induction element still on.
If I understand, your question doesn't seem to be about induction cooktops so much as it is about timers: can I leave my pot on an open element before shabbos, with a timer that will turn it off some point after shabbos starts?
At first glance, we fall back on the Mishna Shabbos 3:1 -- a pot left on a burning stove before shabbos always requires that the flame be covered, to prevent someone from being tempted to fiddle with it. I'm not aware of anyone who's suggested (or even considered) that this Talmudic requirement is lifted if you have the fire on some kind of a timer. (Or more simply, there's only enough fuel on the fire to last half an hour into shabbos.) The temptation to fiddle with it would still be there.
Although induction cooking offers a koshering benefit, the cooktop cannot be used on Shabbos or Yom Yov because the cooking connection is made once the pot is put onto the coil area. Similarly, one would not be able to remove the pot from the cooktop on Shabbos or Yom Tov because one would be “disconnecting” the magnetic field by removing the pot.
Rav Shlomo Miller expressed concern of adjustment and chazara issues on Shabbos with induction cooking. http://www.yeshiva.co/ask/?id=447 Any use of a burner, an open flame or other uncovered source of heat, e.g. an electric burner is forbidden according to all opinions, even for keeping food warm that had been fully cooked before Shabbat. (Mishna B’rura Siman 253, Sa‘if 100, Sh’mirat Shabbat Kehilchata Perek 1, Sa‘if 26.) The induction magnetic field requires the connection directly between pot and stove, thus making a blech unusable.
The following question was submitted to star k March 2017:
Many induction cooktops have inbuilt timers on all 4 burners so you can program them to keep food warm and to turn off at a specific time. What if the timers were set before shabbos and then turned off to coincide with the seuda? Once off they would stay off for the rest of shabbos. Is this permitted?
Star-K's Rabbi Avrohom Mushel's answer: It should be OK if the control is covered.