Two types of control, zero cross (SCR) and phase angle, can be used to cycle solid-state contactors on and off to keep a process at a certain temperature.
By Bill Bernhardt, senior commercial engineer, Rockwell Automation
Many resistive load applications use solid-state contactors or, in some cases, properly sized standard contactors to perform control. The standard contactor’s downfall is the duty cycle wear and tear of the contactor itself. Electromechanical components have a finite duty life cycle compared with solid-state devices, which have a longer life because they contain few or no mechanical parts.
To keep a process at a certain temperature, some type of control is needed to cycle the contactors on and off. That control could come from a programmable logic controller (PLC), temperature single-loop controller or something in between.
Zero Cross Control
Standard silicon-controlled rectifiers (SCRs) generally will use a form of control known as zero cross, which basically turns the SCR on and off at the sine wave’s zero cross.
The on-and-off function can be for one cycle or sometimes can use a zero cross time base function. Time base function has the SCRs fire for "x" time and then turn off for "y" time and then repeat, as shown in Figure 1.
If used on a resistive heater application, this method needs a timing adjustment to keep a temperature.
It’s best-suited for low hot-to-cold ratios. For example, resistive loads such as nickel chromium, iron chromium and aluminum alloy have very little change in resistance as the temperature changes.
Zero cross functionality offers some advantages:
- It minimizes load current when turning on the SCR.
- It reduces noise on the power line and what is released in the air.
- It works well for constant resistance-type elements.
Phase Angle Control
Another option to control resistive loads is phase angle switching or firing. This type of control provides precise voltage control when it is needed, thereby giving a consistent output. With resistive load control, it can turn on with a command and turn off at zero cross voltage, as shown in Figure 2.
Phase angle control offers:
- The fastest and most accurate response time for controlling temperature hysteresis.
- A versatile resistive starting method.
- Control of the SCR firing each half cycle of the sine wave.
- Suitability for high inrush application.
- Aging loads in which the resistance changes.
Among the possible resistive loads, the most universal control method is phase angle control.
Resistive loads best-suited for phase angle control are:
- Heating resistors in an autoclave.
- Heater elements to heat a vat of a liquid.
- Applications with a high hot-to-cold ratio.
- Tungsten, molybdenum quartz or other elements that show an increase in resistance during increases in temperature.
How does this tie to soft starters?
Soft starters with an IEC utilization category of AC-53A can use phase angle control on resistive loads providing enough control to the heaters, as they would when starting a motor. Instead of starting the motor, the soft starter can maintain a certain output voltage anywhere from 1% to 100% of full voltage, if the algorithm allows.
Download the free Rockwell Automation white paper to learn about how the Allen-Bradley® SMCTM-50 Soft Starter offers parameter settings for performing resistive load applications using phase angle control with numerous integrated control options.
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