Apparatus and method for inductive heating
A system and method for inductive heating, in which a power source provides current pulses with high-frequency harmonics to a heater coil, the coil generating a magnetic flux for inductive heating of an article. The high-frequency harmonics enhance a relative proportion of inductive heating, compared to resistive heating of the heater coil. Providing these high-frequency harmonics, occurring above the border frequency of the heating system, enables the system to deliver an increased proportion of inductive heating, compared to resistive heating, without requiring an increase in the Root Means Square (RMS) current in the coil. Providing better coupling between the coil and the core, such as by embedding the coil wholly or at least partially in the core, and providing a magnetic yoke to close the loop with the core, can provide a significantly decreased border frequency. This reduction of the border frequency then can be utilized to provide larger amounts of energy in the high-frequency harmonics of the current pulses, and thus provide a greater percentage of inductive heating without increasing the current in the coil. The current pulses preferably have steeply varying portions, such as a steeply varying leading edge and/or trailing edge.
1. A heating apparatus comprising:
a heater coil for inductive heating; and
a power source for supplying to the heater coil non-sinusoidal current pulses having steeply varying portions providing high frequency harmonics in the heater coil, wherein the heater coil generates a magnetic flux for inductive heating of an article.
2. The apparatus of claim 1 , wherein the heater coil is inductively coupled to a load which includes the article.
3. The apparatus of claim 2 , wherein the load includes a closed loop for the magnetic flux.
4. The apparatus of claim 3 , wherein the load includes a core and a yoke which form the closed loop.
5. The apparatus of claim 2 , wherein the load includes a core and a yoke and the heater coil is disposed between or embedded within at least one of the core and yoke.
6. The apparatus of claim 2 , wherein the load includes a core having a passageway for a flowable material.
7. The apparatus of claim 6 , wherein the core heats the flowable material.
8. The apparatus of claim 6 , wherein the heater coil is positioned in the core so that heating is concentrated in the passageway.
9. The article of claim 2 , wherein a portion of the load has discontinuities or restrictions to a flow of eddy currents for concentrating inductive heating in another portion of the load.
10. The article of claim 1 , wherein the article forms at least part of a closed loop for the magnetic flux.
11. The apparatus of claim 1 , wherein the power source includes a low or line frequency current source.
12. The apparatus of claim 1 , wherein the power source is variable for adjusting the energy content of the current pulses supplied to the heater coil.
13. The apparatus of claim 1 , wherein the heater coil is inductively coupled to a load which includes a core, and the heater coil is at least partially embedded in the core.
14. The apparatus of claim 1 , wherein the heater coil is wrapped around the article.
15. The article of claim 1 , wherein the heater coil is mounted on the surface of the article.
16. A method of supplying current to a heater coil of a heating apparatus for inductive heating, the method comprising: supplying to the heater coil non-sinusoidal current pulses having steeply varying portions providing high frequency harmonics in the heater coil, wherein the heater coil generates a magnetic flux for inductive heating of an article.