Should it be a square wave centered at 0V?

Or can it be a square wave centered at + 7.5V and swing from 0 to 15V?

 

My guess is to oscillate around 0 since having a DC offset would just heat up the work coil more?

 

A sine wave will be the most efficient. And yes, you don't want a DC component.

 

Using the Chinese made 1000 Watt ZVS Induction Heater Notes this link is pretty good. Several years ago I played around with one of these units and I found the notes, including schematic and scope images to be useful.

Ron

 

I used to service industrial HF systems and they were all AC at the work coil.
In fact some even had an HF transformer at the work jig.
Max.