许多阻性负载应用使用固态接触器，或者在某些情况下使用合适的标准接触器执行控制。

标准接触器遭淘汰的原因是接触器本身的电气寿命磨损。相比固态设备，电子机械部件的电气寿命周期有限，而前者由于少用或不用机械部件从而寿命更长。

白皮书：用于管理阻性负载的 SMC-50 固态智能电机控制器

为使整个过程保持在特定温度下，需要某种类型的控制以循环通断接触器。这种控制可来自可编程逻辑控制器 (PLC)、单回路温度控制器或介于两者之间的设备。

标准晶闸管 (SCR) 通常使用称为“零交叉”的控制形式，基本上是在正弦波零交叉处执行通断。

通断功能可用于一个周期，或者有时可使用零交叉时基功能。如下图所示，时基功能使晶闸管保持触发达“x” (时长)，然后保持关断达“y” (时长)，以此循环往复。

时基功能使晶闸管保持触发达 x (时长)，然后保持关断达 y (时长)，以此循环往复 [单击以放大]。

如果用于电阻加热器应用，该方法需要一些计时方面的调整，以保持温度。

这最适合低热冷比的情况。例如，镍铬、铁铬和铝合金等阻性负载在温度变化时也很少有阻抗变化。零交叉功能的优势：

• 最大程度减小晶闸管开通时的负载电流
• 减少电力线上以及释放到外界的噪声干扰
• 尤其适合固定阻抗类元件

控制阻性负载的另一个方法是相位角切换或触发。此类控制能在需要时执行精准的电压控制，从而提供恒定的输出。

如下图所示，凭借阻性负载控制，可通过命令开通并在零交叉电压处关断。

凭借阻性负载控制，可通过命令开通并在零交叉电压处关断 [单击以放大]

相位角控制的优势包括：

• 为控制温度迟滞提供最快、最准确的响应时间
• 通用的阻性负载启动方法
• 每半个正弦波周期控制晶闸管触发
• 最适合高浪涌应用特性
• 负载在阻抗变化处发生老化

面对种类繁多的阻性负载，最通用的控制方法是相位角控制。

最适合进行相位角控制的阻性负载包括：

• 高压釜加热电阻
• 用于加热大型液缸的加热器元件
• 高热冷比应用
• 在升温期间呈现阻抗增大现象的钨、钼石英或其他元素

如何使用软启动器实现上述控制？

IEC 使用类别为 AC-53A 的软启动器可对阻性负载使用相位角控制，对加热器提供足够的控制，如同启动电机一样。

但软启动器的作用不是启动电机，而是在算法允许的情况下保持特定的输出电压 (全电压的 1…100% 范围内的任意电压)。

敬请下载我们的白皮书，进一步了解 Allen Bradley SMC^TM-50 软启动器如何提供用于正式实施阻性负载应用的参数设置，凭借多种集成控制选项进行相位角控制。

Whitepaper: SMC-50 Solid-state Smart Motor Controller for Managing Resistive Loads

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.

Time base function has the SCRs fire for x time, and then off for y time, then repeat [CLICK TO ENLARGE]

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.

With resistive load control, it can turn on with a command and turns off at zero cross voltage [CLICK TO ENLARGE]

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 SMC^TM-50 Soft Starteroffers parameter settings for officially performing resistive load applications using phase angle control with a multitude of integrated control options.