today's solid-state power supplies range from 60 to 90 percent. Those of earlier vacuum-type power supplies and motor generator systems may be 50 percent. Applying this to the previous example of 3.88 kW and knowing the power supply conversion efficiency is 90 percent shows that the power supply requirement is now 4.31 kW.
Coil efficiency and transmission Losses. Determining coil efficiency and transmission losses from the power supply of the work coil are the next steps in establishing the final kilowatt requirement of the application.
Solenoid coils, sometimes referred to as helical coils, are efficient designs because they encompass the part for maximum coupling pf the magnetic field to the part being heated. Typical efficiencies of solenoid cells are from 85 to 95 percent when close-coupled to the part.
Coil coupling is the air gap the workpiece and the induction work coil. As the air gap increases between the coil and the workpeice, the coupling efficiency decreases, as the two are inversely proportional (See Figure 2).
With other coil designs such as pancake or conveyor-style, parts are proximity-heated. Additional power input is required to maintain the desired rate.
Referring to the original example of 3.88 kW having a 90 percent conversion efficiency (4.31 kW), having a conveyor-style work coil of 50 efficiency would require a power supply with an output rating of no less than 8.62 kW.
Induction heating equipment manufacturers have computer programs to calculate the slightest change in coil design to optimize the equipment selections and application efficiencies (see figure 3).
The operating output frequency of the power supply can be critical to an application's success. Typically, smaller diameters or thin-wall parts are heated with higher-frequency equipment, while larger masses of heavy-wall parts are heated with frequencies as low as 3 kHz.
Once again, referring to the previous example of the tube and fitting assembly, calculating the depth of penetration of the magnetic lines of flux will establish the current flow in the hex head fitting at a lower or higher frequency by using the following formula:
d=3,160
where: d= Reference depth of the flux field in inches
(RHO)
= resistivity of the material in micro-ohm inches
=Permeability
=
Operating Frequency of the power supply
Therefore, reference depth at 10 kHz=
d=3,160 
d=.199"
Figure 2 (Above)- This shows the air gap between
the workpiece and the induction work coil.
Figure 3 (Below)-This table is an example of the
results of a coil calculation program.
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