Confused About Temperature Control in Thermal Processes?

Confused About Temp Control in Thermal Processes?

Here are the basics of eletromechanical contactors, solid-state relays and SCR power controllers.

By Matthew Fischer, field applications engineer, Thermal Products, Advanced Energy

Editor's Note: This article is adapted from a white paper, "Top Advantages of SCR Power Controllers Over Contactors and Solid-State Relays." Download the free, full white paper with details comparing the three devices in factors such as on-off cycles; power consumption and harmonics; utility cost reductions; control accuracy; cost efficiency; noise and transient resistance; process monitoring and diagnostics; and power consumption.

This article makes a brief comparison of electromechanical contactors, solid-state relays (SSRs) and SCR power controllers operating in a similar on-off manner.

Eletromechanical Contactors

An eletromechanical contactor is a device that opens or closes a contact, allowing electricity to be switched fully on and fully off. An electrical control signal triggers the opening or closing of the contact or contacts.

In general, an eletromechanical contactor is rated for a specific number of on-off cycles while in operation throughout its lifetime. This rating depends on the manufacturer and typically ranges from less than a million cycles at rated current and voltage to a few million cycles at rated current and voltage.

Solid-State Relays

An SSR is an electronic switch that operates without the moving parts of an electromechanical relay. Because there are no moving parts, life expectancy of the SSR is greater. Typical cycle times are 2 to 5 sec. for reasonable SSR life. Also, it can operate at a faster cycle (on-off) time than a comparable mechanical contactor.

This infographic compares the estimated total quantity of cycles of operation for an electromechanical relay, solid-state relay and SCR over 48 weeks, operating six days per week. [CLICK IMAGE TO ENLARGE]

SCR Power Control Modules

An SCR power control module is an electronic semiconductor device designed to regulate the power to an output load. It can switch the power applied to an output load quickly, such as in milliseconds, compared to an electromechanical contactor or SSR.

Typical Operation: On and Off Cycling

The electromechanical contactor and SSR both switch electricity to a load when signaled to do so. This means that both devices can turn the electrical power to the output load on and off at any point in a sinusoidal wave.

In the United States, electricity is provided at 60 cycles per second, or 60 Hz. When the power supplied to a device does not turn on or off at a zero crossover point in the sinusoidal waveform, voltage spikes occur on the sine wave and RFI, and harmonics are generated. An arc also might occur with this type of random on and off operation.

When in operation and turned completely off or completely on, both the contactor and SSR devices operate efficiently — no disturbances when fully turned on or off. However, under normal process operating conditions, the generated harmonics can cause the sine wave provided by the power company to become distorted. This might cause difficulty for the power company’s measurement equipment, making it appear that the measured or required power is greater than the actual power used. This could increase power costs.

By eliminating the effects of the RFI and harmonics associated with contactors and SSR, the SCR provides more consistent and efficient power regulation to the output load, reducing potential harmful effects and minimizing power consumption.

Closer Glance at SCR Power Control

Using SCR power control modules in electric thermal heating applications can provide several advantages, including:

  • Mitigated harmonics and RFI.
  • Removal of arcing potential across contacts.
  • Improved process control accuracy.
  • Extended heater life.
  • Optimized power consumption.
  • Noise and transient resistance.
  • Active process monitoring.

These benefits, among others, can lead to lower cost of operation and total cost of ownership.

Advanced Energy, Inc. is a participating EncompassProduct Partner in the Rockwell Automation PartnerNetwork™ program. Based in Fort Collins, Colorado, Advanced Energy provides power and control technologies, including power control modules, for high-growth, precision power conversion solutions.


The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.


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The JOURNAL from Rockwell Automation and Our PartnerNetwork™ is a bimonthly magazine, published by Putman Media, Inc., designed to educate engineers about leading-edge industrial automation methods, trends and technologies.