Patent ID: 7481274

Claim:
A system configured to heat at least a part of a subsurface formation, comprising: an electrical power supply; a heater section comprising one or more electrical conductors electrically coupled to the electrical power supply and configured to be placed in an opening in the formation, at least one of the electrical conductors comprising ferromagnetic material; wherein the heater section (a) provides a first heat output when time-varying electrical current is applied to the heater section below a selected temperature, and (b) provides a second heat output approximately at and above the selected temperature during use, the second heat output being reduced compared to the first heat output; wherein the system is configured to allow heat to transfer from the heater section to a part of the formation; and wherein the electrical power supply is configured to provide a relatively constant amount of time-varying electrical current that remains within about 15% of a selected constant current value when a load of the electrical conductors changes.