The inherent characteristics of a medium such as a painted automobile body determine which wavelengths penetrate the coating and are absorbed and which wavelengths are reflected. These characteristics impact the ability of the infrared radiation to transfer heat to the receiving medium. Shorter wavelengths such as those emitted by tungsten lamps at 4,000oF (2,204oC) have a significantly greater ability to penetrate a coating than those emitted by nickel-chromium filaments operating at 1,800oF (982oC). It is the emitter temperature that determines the spectrum of energy radiated. Under ideal conditions, the emitter will be set up to radiate the spectrum of energy that will be optimally absorbed by the substrate being heated.
Various wavelengths are used to heat different materials and colors. For example, when heating transparent materials such as water or glass, longer wavelengths -- above 2 micron -- are used because shorter wavelengths have little effect. Drying paint, however, benefits from the greater penetration of shorter wavelengths.
Common Infrared EmittersIgnoring gas-fired infrared systems, there are three principle types of electric infrared emitters: ceramic, nickel-chromium, and lamps such as tungsten (T3) or halogen. Ceramic and nickel-chromium emitters are used for long- and medium-wave applications and can be considered together from a control standpoint because they have similar electromechanical characteristics. Likewise, T3 and halogen-filled T3 lamps, which are used for short-wave infrared heating applications, can be considered together due to their similar electromechanical characteristics.
Nickel-Chromium and Ceramic Emitters. Used at temperatures below 2,000oF (1,093oC) for longer wavelengths, nickel-chromium and ceramic emitters have a long thermal time constant and are, therefore, relatively easy to control. These emitters heat up slowly and have no peculiar electrical characteristics such as a high inrush current when cold. For this reason, a simple time-proportioning scheme is perfectly adequate to maintain the desired emitter temperature.
Time-proportioning control consists of a mercury, mechanical or solid-state contactor, which is turned on and off in proportion to the amount of energy needed. To extend emitter life, it is best if the base cycle time is kept low -- around 1 sec. Often, this low cycle time is not achievable using mercury or mechanical contactors, where longer cycle times of 3, 5 or even 10 sec are necessary to prevent premature contactor wear. Longer cycle times reduce emitter life.
Applying Power to Short-Wave EmittersTungsten and halogen T3 lamps have several things in common when considering the emitters' electromechanical aspects.
- The emitters have fast (100 msec) warmup.
- The emitters have low cold resistance, which means there is a high inrush current.
- The emitters offer a high operating temperature (4,000oF).
Because these emitters are extremely fast in terms of thermal time constant, they cannot be time proportioned slowly. Any significant off time will allow the filament to cool, thereby altering the radiated spectrum. It is important to maintain a constant filament temperature to produce a consistently radiated spectrum.
Phase-Angle Firing. Phase-angle firing of SCR power controllers is the traditional method for controlling short-wave infrared emitters (figure 1). With phase-angle firing, a constant 60 Hz waveform is presented to the lamp. This waveform, however, is not a simple sine wave. To modulate the power and achieve the desired emitter temperature, the sine wave is chopped up such that it conducts only during the later portion of the half-cycle.
- Fast power modulation achieves stable filament temperature.
- Soft start eliminates inrush problems.
- The firing method provides smooth, stable application of power.
When a T3 lamp's filament is cool, the electrical resistance is low; consequently, there is a potential for high current flow. The T3 lamp's high inrush current characteristic can cause several problems such as blown fuses, damage to the SCR switching devices, and stress to the transformers upstream of the system. Soft start is a technique that reduces the effects of this inrush current. When the SCR is first commanded to fire, it begins at zero phase-angle and slowly advances the firing angle to the desired level, much like slowly turning up the dimmer on the lights in your dining room. In this way, only a small amount of current flows initially, and the emitter warms up prior to the application of full power.
There are, however, some notable disadvantages to phase-angle firing:
- It generates radio frequency interference (RFI) that can interfere with other electronic equipment.
- It requires a larger transformer due to harmonics.
- It reduces power factor, thereby increasing the energy bill.
- Equipment is more costly.
Half-Cycle Firing. Similar in principle to single-cycle firing, the half-cycle firing method switches half-cycles instead of an integer number of whole cycles (figure 2). This method has the distinct advantage of cutting the off time in half. With half-cycle firing at above 50% power demand, the off time will be one half-cycle, or 8.33 msec at 60 Hz (10 msec for 50 Hz). This technique greatly reduces flicker and thereby provides a much more stable emitter temperature. One potential disadvantage to half-cycle firing is it can generate a net DC current or voltage. Care must be taken to switch an identical number of positive and negative half cycles to prevent this from occurring.
Figure 3 shows the waveform at 33% power demand. As it shows, half-cycle firing (on the bottom curve) achieve shorter off durations than single-cycle firing (on the top curve). Shorter off durations results in less flicker.
Energy Savings and RF SuppressionThe two biggest problems with phase-angle firing are eliminated with half-cycle firing. Because the power is switched on and off at zero crossings of the AC supply, RFI is not generated, so the power switching will not interfere with adjacent electronics. The second advantage is maintenance of unity power factor. In many areas of the country, public utilities charge a premium if the power factor is below 0.85. (Power factor is the relationship or phase delay between the voltage supply and the current drawn by the load.) Typically, supplying inductive loads such as motors and transformers causes a reduction in power factor. Because phase-angle firing involves switching the power to the load at the last portion of the sine wave, this also creates a situation where the current waveform is delayed with respect to the voltage waveform. The greater the delay, the lower the power factor and, therefore, the higher the energy costs. Half-cycle firing switches for the entire half cycle, so there is no phase delay for the current waveform. The current and voltage fire in synchronism.
As has been shown, switching ceramic or nickel-chromium emitters for generation of medium- or long-wave infrared energy is a simple matter of time-proportioning the emitters with conventional mercury, mechanical or solid-state contactors. Switching T3 lamps for generation of short-wave infrared energy is more difficult due to the speed of lamps' response and the low cold resistance and consequential in-rush current. While phase-angle firing is the traditional method for firing T3 lamps, half-cycle firing has distinct advantages for long-term energy savings and reduced installed cost.