Okay, I know it's now 2007 but I've researched this too and there's lots of good info on this here, but not in one place. I am plagarizing a variety of posts here and in the BB on this subject for this reason. If you have an induction cooktop from the 1990s, and the large burner no longer works, it uses power module part number WB27X5491 or WB27X5492 (check your parts list in the manual or for your model online, some GE and Kenmore models definitely have them), this could likely be your problem:
A bad voltage rectifier (Part No. 36MB100A-ND) which can be replaced with a newer one at around $10. You will probably have to order it online, you probably will not find it in a local hardware or RadioShack-type store. This is a part INSIDE the power module listed above, which is a whole assembly of many parts connected to two burners either on the right or left side of your cooktop. You have to disconnect your cooktop from the wires in your wall, take off the top (remove the upper grill screws and take off the grill and then the whole top can pop out), remove the power module (take out four screws), and take the power module apart some to find this part. Apparently, the rectifier is about 1" square and is easily replaceable with a screw driver - be sure and label the leads before removing the old one. One corner is marked with a PLUS sign as a reference. Then you put the power module back together and put it back in the cooktop. You then replace two fuses under the control panel that correspond to that power module, part numbers MDA-15 and MDA-20, which may or may not be locally available but certainly can be purchased online. You then put the top back on and hook the wires back up. If this is your problem, it should be solved with this procedure.
Apparently, most manufacturers and repair people when diagnosing this problem will tell you that you need a whole new power module when diagnosing this problem, but they don't sell them anymore, or if you can find them, they are VERY expensive. But usually the problem is this or some other small part inside the power module. There are no diagrams of how the whole power module is put together or a parts list of the pieces inside that I can find anywhere. So if you do this repair yourself you don't have any visual reference to go by.
Now let me put in the disclaimer that I have not actually performed this repair, but I have seen at least six posts from people who have successfully solved their problem with this. I have gotten as far as looking inside and seeing that the power module is much more complicated than I want to deal with, as I have no formal electrical training. But if you are comfortable with taking these kinds of things apart and putting them back together without a diagram, and don't mind the time it takes to do it, this could be an cheap easy fix to your problem. I have ordered the parts from digikey.com at a total of $40 for two recitifiers and four fuses, and I am having a local repairman come do it for around $100-150.