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Benefits of Soft Starters on Resistive Loads

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A lot of resistive load applications use solid-state contactors or in some cases, properly sized standard contactors to perform the control.

The downfall of standard contactors is the duty cycle wear and tear of the contactor itself. Electro-mechanical components have a finite duty life cycle compared with solid-state devices that have a much longer life due to little or no mechanical parts.

To keep a process at a certain temperature, some type of control is be 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.

Standard silicon-controlled rectifiers (SCRs) will generally use a form of control known as zero cross, basically turning on and off at the zero cross of the sine wave.

The on-and-off function could be for one cycle or could sometimes use a zero cross time base function. Time base function has the SCRs fire for "x" time, and then off for "y" time, then repeat, as shown in the figure below.

If used on a resistive heater application, this method needs some adjusting in timing to keep a temperature.

These are 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.The advantage of zero cross functionality:

  • Minimizes load current when turning on the SCR
  • Reduction of noise put both on the power line and what is released in the air
  • Great for constant resistance type elements

Another option of controlling resistive loads is phase angle switching or firing. This type of control provides precise voltage control when it is needed and thereby giving a consistent output.

With resistive load control, it can turn on with a command and turns off at zero cross voltage, as shown in the figure below.

Advantages of phase angle control include:

  • Provides 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
  • Best used in high inrush application characteristics
  • Aging loads where the resistance changes

With the variety of 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 the 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…100% of full voltage, if the algorithm allows.

Download our whitepaper and learn more about how the Allen Bradley SMCTM-50 Soft Starter offers parameter settings for officially performing resistive load applications using phase angle control with a multitude of integrated control options.

Bill Bernhardt
Bill Bernhardt
Senior Commercial Engineer, Rockwell Automation

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