by STANLEY ZINN, Induction Heating Consultants
As presented at the Induction Heating Technology & Applications Conference, April 12-13, 2000; Arlington Heights (Chicago), Illinois
The need for radio frequency (RF) induction operations are increasing as thinner metals and shallower case requirements occur. While the theoretic basis of induction heating is the same for all frequencies, the power supplies and their operations are considerably different for RF. This paper explores those differences, and provides information on how to properly select RF equipment for various applications.
The accepted definition of Radio Frequency heating encompasses those frequencies above 50 kHz. The frequency most people associate with the Radio Frequency (RF) range is 450 kHz. This was the center of the high frequency band initially allotted for industrial use by the FCC.
Almost all tube type RF power supplies used the common designation of 450 kHz as their operating frequency. In fact, power supplies operated in a range between 200 and 750 kHz. In tube-type power supplies the design of the coil and the shape and mass of the part changed the tuned inductance and the power supply frequency changed accordingly. In contrast, the solid state power supply searches for the optimal frequency.
Today, solid state power supplies operate in the range of 50 - 750 kHz. There are supplies operating above this frequency in nominal ranges of 2-3 Megahertz (MHz) and up to and including 60-70 MHz, frequencies which have been usually associated with dielectric heating. High frequencies are selected to provide a specific shallow depth of case or for heating thin materials.
At low and medium frequencies, the deep penetration of the magnetic field provides are utilized for through heating of materials. Typical applications are forging and melting. Frequencies to 10 kHz are also used for these applications. However, frequencies above 8 kHz find major application in heat treating.
At the lower frequencies, most loads are uniform in cross section. This permits the user or manufacturer to utilize computer programs for coil design with a measure of certainty. Usually, they do not require further modification when installed on the equipment.
Where the part configurations are not symmetrical, computerized coil information can generally only be used for an approximation of the shape of the magnetic field. Modification of coil designs is a necessary part of this process. With RF power supplies, the modifications that must be made are generally more complicated and must be verified by actual laboratory tests.
In order to fully utilize the available power of Radio Frequency equipment, tuning of the coil and load to the power supply is extremely important. What are the factors that affect this tuning of the coil and load to the frequency range of the power supply?
One major factor is the Hysteresis loss in magnetic materials. Hysteresis losses are caused by reversal of the magnetic field due to frequency. As the field reverses it causes the molecular structure of the metal to realign to match the polarity of the field. The molecules however, being of a greater mass cannot respond as quickly to the changes in the field and thus friction generates a loss we call Hysteresis.
As the frequency increases, these magnetic reversals increase in speed and due to the friction generated Hysteresis losses are greater. Accordingly, Hysteresis affects RF applications to a much greater degree than lower frequencies.
At approximately 1340°F magnetic materials lose their magnetic properties and react as non-magnetic materials. This effect is known as the "Curie Point" of the material. The change from magnetic to non-magnetic state produces a significant shift in frequency. The reason for this is the change in the coil/ part inductance as Hysteresis occurs.
The combination of the coil, the part and the capacitance set the Resonant Frequency for the application. This combination of components is referred to as the "Tank Circuit". To achieve maximum energy transfer to the part, the tank circuit resonant frequency must be within the frequency capabilities of the equipment. When the Curie Point is reached, then tank circuit frequency will shift radically.
Modern inverters, at all frequencies, try to tune to the tank circuit resonant frequency. This is most important as we pass through Curie.
The tank circuit inductance is composed jointly of the coil and the part. The coil itself does not change during the heating process and its inductance remains constant. However, changes in the part, as it heats, affect the tuned frequency also causing it to shift. At the lower frequencies, these shifts can greatly affect the actual depth of penetration of the magnetic field and must be accounted for.
|Temperature||Resistivity||449 kHz||450 kHz||451 kHz|
|1400°F||8.85||0.014 in||0.014 in||0.014 in|
|1800°F||9.74||0.0147 in||0.0147 in||0.0147 in|
Penetration Depth in Steel at 450 kHz
|Temperature||Resistivity||9 kHz||10 kHz||11 kHz|
|1400°F||8.85||0.099 in.||0.094 in.||.089 in.|
|1800°F||9.74||0.103 in.||0.098 in.||.094 in.|
This variation in part resistivity amounts to a change in depth of penetration of 9.5% over a 2 kHZ span.
Penetration Depth in Steel at 10 kHz
At RF frequencies, a 1 kHz change in operating frequencies produces nominal, if any change in penetration depth. A change of approximately 30 kHz would be required before it produces a change in depth similar to that for the lower frequency.
There is also a shift due to changes in the resistivity of the part as it heats. These shifts are substantially greater at the lower frequencies and can be seen in the variation of depth of magentic penetration at these frequencies. The high frequency RF fields appear to be more stable in this respect.
The overall inductance of the coil and part play a large part in determining the operating frequency in the RF range. With low and medium frequency systems, small changes in coil or lead dimension do not cause large frequency shifts. However, at RF frequencies coil inductances are critical to achieving a good match. Lead inductance in particular can radically change the tuning frequency of the system and leads must be kept short and have minimal inductance. Low inductance leads will further enable the coil/part combination to provide maximum energy transfer.
At the lower frequencies, large inductances are necessary to tune to the power supply frequency. Small parts that are heatable at the lower frequencies require smaller coils and therefore a transformer must be used to match this frequency. At the RF frequencies, with small parts, tuning can normally be attained connecting the coil directly to the power supply. Accordingly overall efficiency of the system is greater.
Lead structures between the tank circuit and coil in RF equipment become critical between the tank capacitors and the coil and represent an unneeded inductance that prevents full voltage from appearing across the operating area of the work coil.
This inductance should be less than 10% of the operating coil inductance. This is normally accommodated through the use of "fishtail" construction which is a low inductance lead.
As frequencies go higher, the overall inductance of the tank circuit must be minimized. In many instances the total coil is a hole drilled in a water cooled copper block. If the part tries to draw the system within the operating frequencies of the power supply. In particular, lead lengths and their resultant inductance must be minimized for maximum energy transfer. Electrically the leads more power from the supply than can be delivered, the hole in the coil (the inductance) can be opened to decouple the part.
In some instances, the total inductance of this type of coil can be so low that the resonant frequency is above that of the power supply. Introducing additional inductance via a hole in series with the coil can be used to lower the frequency. Opening the hole gradually will bring the system in to the proper frequency range.
Coils for the higher frequencies are generally machined and provide a stable heat pattern that is not subject to mechanical damage or deformation. They are also, therefore, easily replicated as necessary.
The components of RF heat stations are considerably smaller than those required for lower frequencies. Typically, the remote heat station incorporates solid state capacitors, that form the complete tank circuit with the coil and part. The tank circuit carries the greatest current in the system and the short paths between the capacitance and the coil act to reduce the losses that would normally occur in a high frequency system.
A production heat station is wide x high x ong, and weighs approximately 20 pounds with water flowing in the cooling circuits. Dependent on whether the circuitry is series or parallel connected, the entire tank can operate as much as 200 foot from the power supply with negligible loss. In the parallel circuitry approach, a special cable uses field canceling construction to reduce transmission losses. The cable construction is highly flexible (you canliterally tie it in a knot). Further because the field around the cable is effectively canceled, the transmission line can be simply run through steel conduit.
Due to the light weight and flexibility of the system , tooling concepts are now broadened. Where normally work is raised in to the coil for heating, the coil/tank assembly can be easily lowered into position of the workpiece. This eliminates the requirement for individual lift fixtures on a conveyor. Instead, a light duty mechanism, configured to the tank circuit, permits the coil to come into position on the part during the dwell cycle. The resultant system is faster in operation and much less expensive to fabricate.
As one instance, the heat station is mounted on a multi-position automatic screw machine. If the dwell time of the machine is less than the heating cycle for hardening the part, heat treating may be done in situ. A water based cutting fluid is used as a quench medium to simplify fluid processing. The work piece can then be locally hardened before it is cut from the bar stock. This could easily be applied to the fabrication of inner and outer races for bearings or similar parts.
Where space is at a cost premium, as in a clean room for processing semiconductors, the heat station can be in a Class 100 or class 10 environment while the power supply is remotely located. Since no electrical contact is made or broken, it would be possible to utilize this system within a hazardous area, as well.
In order to determine the proper power supply for RF operations, it is necessary to understand that the power supply is similar to a commercial power generating station. As appliances are added to the system, they draw power from the supply. With the RF power supply, the power is available but the system must be tuned to draw the maximum power. Rarely, however is full power achieved. "
Induction loads draw power from the supply just as an electric appliance draws power from the power line.