Your Heater Material and Design Dictate How You Control It, Part 2
In this conclusion to my two-part series on how a heater's construction materials affects how you control it, I'll look at molybdenum disilicide and tungsten heaters. Figure 2 shows how resistance varies for these units. Note that the figure shows element -- not process -- temperature.
Protect Against Excessive Current and PowerIn the face of low element resistance, you need current-limiting to protect wiring and transformer windings and to avoid nuisance fuse blowing and breaker tripping. At all values of resistance, you have to limit the power to a level that keeps the elements' watt density below the manufacturer's recommended watts per square inch. Of all the power control devices tried, the most successful has been the phase-angle controlled SCR with true power (load voltage times load current) feedback.
The temperature controller puts a control signal representing demanded power into the SCR, which delivers power in proportion, regardless of variations in line voltage and load resistance. Be sure that you also have current and power limit features on the SCR that can override any excessive level of control signal.
Molybdenum Disilicide Heaters. In figure 2, you can see there is a resistance change of some 14:1 over the working temperature range -- much more than silicon carbide but subject to only small changes with service life. This means that you have more freedom in arranging series strings of elements and less vigilance is needed in watching for signs of aging.
The SCR features recommended for silicon carbide apply equally for molybdenum disilicide heaters, but the demands on current limiting are more severe. Without it, cold-start load current would be some 14 times that at working temperature.
Tungsten Heaters. I will limit this discussion to tungsten filament lamps for short-wave radiant heating applications. The resistance change over the working range is about 17:1. Aging effects are negligible and the same SCR control methods apply as for molybdenum disilicide heaters. Transformers rarely are needed. Tubular lamps are available for standard factory voltages (115, 230, 400, etc.) up to around 10 kW rating. The high cold-inrush current can decay in a second or two because of the low filament mass and consequent fast rise to working temperature of some 4,532oF (2,500oC). Current limiting is inherent in the resistance attained at working temperature. Automatic current limit may only be necessary for the slower, high power lamps to avoid blowing the high speed SCR fuses.
With tungsten lamp heating, a temperature controller often is not used; power (therefore, heat flux) is manually adjusted and held stable by the SCR in true power control mode. The common lamp dimmer switch enables you to adjust voltage for rough and ready low power applications, but it will not stabilize power against line voltage variations. It will even exaggerate the variations.
Current and Voltage MonitoringRMS analog ammeters and voltmeters on your heaters can give valuable clues to heater condition. With nickel-chrome heaters, current will follow voltage and you can notice loss or partial loss of a heater by a drop in current. From the readings, you also can calculate power and heater resistance.
With all the other heater materials dealt with here, you will learn not to expect current to strictly follow voltage. Heater resistance will go its own way according to temperature (and age, in the case of silicon carbide). With aging silicon carbide, you eventually notice the SCR delivering full line-voltage yet not enough current to make the temperature you want. This is the time to move up another transformer tap, then finally change the element. Often, you can omit the transformer and have the phase-angle fired SCR unit limit the load voltage initially, then raise the limit as the elements age.
False Meter Readings. Many meters are average-responding and give grossly low indications when handling the shark fin waveforms of SCR phase-angle control. So, your best choice is the AC moving-iron meter. It indicates the RMS value, which represents the heating effect value of the voltage or current.