3.3 Choice of which type of current limit to use
Either type of current limit may be used in the system described in 3.2, but proportional current limit is much better than fixed current limit. The following is an explanation of why this is so.
Fixed Current Limit works well with those types of heaters whose resistance increases with rising temperature, such as tungsten, because the current limit operates only when the temperature is low and the controller is calling for maximum power. Such a system should be designed so that the supply voltage and current limit setting interact at the point on the maximum power curve corresponding to the maximum resistance (point B in Figure 5).
When the resistance of those heaters is low, so is the temperature and hence the heat loss so that the power available (reduced by current limiting action to a fraction of the power limit) is still sufficient to raise the temperature. As the temperature increases, the available power increases and so automatically compensates for the increasing heat loss. By the time the operating temperature is reached and the temperature controller is reducing the power demand, the current limit is no longer being used.
When fixed current limit is used with silicon carbide elements (at least when new) it is likely that the resistance will be close to the minimum when the load reaches operating temperature, so that the current limit will be operating as the temperature approaches setpoint. This means that there will be a discontinuity in the proportional band at the point of transition from voltage control to current control, and the point of transition changes with changing signal. Within this region the current-control signal has no effect (until the point of transition reaches the operating point). This will be certain to cause “overshoot” when setpoint is reached. Also the reduction of resistance causes an increase in loop-gain, so that if the control loop was tuned when the resistance was cold it may begin oscillating as it heats up. Unfortunately, any increase in proportional band to compensate for this will increase the overshoot.
Thus proportional current limit is better than fixed current limit because proper control action is maintained throughout the proportional band. The operation simply transfers between voltage and current control as the load resistance passes the critical value (represented by the 0Q in Figure 6). There is no discontinuity in the proportional band, and the variation in loop-gain is much less than fixed current limit.
It should be noted that voltage control, with either fixed or proportional current limit, suffers from the disadvantage that the maximum permissible power dissipation can be achieved at only one value of load resistance (represented by 0A in Figure 5 and 0Q in Figure 6). At any other value of load resistance the maximum power available must be lower, so that the heater required is larger than that implied by the specified power limits.
3.4 “True-power Control with Current Limit”
True-power control has the ability to reach the maximum permissible power level at all values of load resistance. This enables smaller heaters to be used in many applications.
As illustrated in Figure 7, the supply voltage and current limit setting can be set at or near the limits of the resistance range, thus allowing full power (i.e. maximum permitted power) to be dissipated at all possible values of resistance. Note also that current limit is not necessary for normal operation; it serves only to protect against faults such as incorrect connections when changing elements.
4. Design Procedure
To optimize the design of a system using silicon carbide elements it is likely to be necessary to repeat the calculations with different elements, different supply voltages, etc. Thus by a process of iteration the best combination for the particular application can be found. Eurotherm has developed software tools to assist engineers in the optimization of power control systems, the calculation of top voltage on transformer and thyristor current ratings. Appendix 1 shows a table with examples of designs for several different applications.
Warning
Elements connected in series and/or parallel will not dissipate equal amounts of power unless their resistances are identical. Therefore such elements should be replaced as a set. It is unlikely that these elements will age, or heat up, at the same rate, and therefore the life of the set may be reduced due to over-powering of one element. This problem can be avoided by providing a separate power controller for each element.
The resistance of these elements varies with both temperature and time. The nature of these variations depends on the particular grade of material and manufacturer. For most types, the resistance is high when the material is cold, decreasing as temperature rises, reaching a minimum at typically between 1000°F and 2000°F, then increasing again as its temperature rises further. When kept at a high temperature, the resistance of the material increases with age.
This method can provide some degree of automatic comparison for variation of load resistance. The SCRs are triggered in the “phaseangle” mode, and there are two internal control loops: voltage control and current limit. The voltage control loop is designed to keep the mean square of the voltage applied to the load proportional to the control signal. The current control loop can override the voltage control loop, and is designed to prevent the RMS value of the current from exceeding a fixed level (irrespective of control signal). This has also been called “Threshold Current Limit”.
