In With SSR? Out With EMR?

Are There Applications for Which There Are Justifiable Reasons not to Make This Change?

By Control Design Staff

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We're ready to start changing out electromechanical relays (EMRs) with solid-state replacements (SSRs) in our installed base during troubleshooting or routine customer service calls, as well as use them in an upcoming new-generation machine line.

We're receiving mixed reaction from the installed base and even from a few potential new customers that we're after. The device prices are higher, and we have some heat to dissipate, but given the solid-state device's advantages in service life, noise generation and arc resistance, we think we'll provide notably better performance. Are there applications for which there are justifiable reasons not to make this change?

—From June '14 Control Design

Answers

EMRs Have Their Place

There is always a tendency to resist change, even while solid-state relays could work in many applications. However, there are some types of applications in which electromechanical relays are definitely preferred, including whenever:
leakage current from the SSR is a concern;

  • heat could be an issue;
  • normally closed contacts are required;
  • a relay with more than one pole is desired.

You should consider what devices are connected to the control and load sides of the relay. Electromechanical relays are far more forgiving than solid-state relays if voltage or current spikes are possible. A quick specification review should be performed when substituting devices. Electromechanical relays typically will have a higher contact-current rating and varying pilot-duty ratings compared to the comparably sized solid-state version.


Bryan Knapp,
Product Manager for General-Purpose Relays,
Rockwell Automation

Also Read: Relays Fall, Expected to Get Back Up in 2013

Solid for Solid-State


Solid-state relays (SSRs) are different in operation from mechanical relays that have movable contacts. SSRs employ semiconductor switching elements such as thyristors, triacs, diodes and transistors. SSRs consist of these electronic parts with no mechanical contacts. Therefore, SSRs have a variety of features that mechanical relays do not incorporate. The greatest feature of SSRs is that they do not use switching contacts that will physically wear out. SSRs are ideal for a wide range of applications due to the following performance characteristics:

  • They provide high-speed, high-frequency switching operations; 
  • They have no mechanical contact failures;
  • They generate little noise;
  • They have no operation noise.

However, SSRs do have some limitations that need to be considered when designing the relay into an application. These limitations include the following characteristics:

  • They're never truly off;
  • a small leakage current might be present at the load;
  • When they fail, SSRs will mainly fail as a short and no longer control the load that is operating;
  • A multi-pole SSR is two or more SSRs in parallel and might not operate in tandem like an electro-mechanical, multi-pole relay will;  
  • They generate heat during operation.

Since their introduction, SSRs have gained acceptance in industrial applications. These applications include circuits that previously had been the main operating area of the electromechanical relays (EMR) or the contactor. The SSR is increasingly employed in industrial process control applications, such as temperature control, lamps, solenoids and valves. For those EMR applications requiring two- or three-pole operation, today's SSR technology can incorporate two or three poles into one unit as opposed to using two or more single SSRs. The benefit of this is that the application can switch all sets of contacts at the same time like an EMR.

The main cause of application failure with SSRs is not properly mounting the SSR using a heat sink (Figure 1). The load characteristic is also a concern that will cause application issues with SSRs. The current requirement characteristics of the load should be carefully considered when using SSRs as a switching solution (Figure 2).

Using SSRs with resistive loads, DC loads, lamp loads, capacitive loads, or motor and solenoid loads all provide challenges that can be overcome in circuit design. The possible cost difference and the increased heat that can be generated by SSRs do not prohibit the SSRs installation. SSRs can be installed in almost all applications currently using an electromechanical relay. With proper circuit design, I would suggest that the SSR has more benefits for use in industrial applications then an EMR.


Bruce Fink,
Product Engineer,
Omron Automation & Safety


SSRs Aren't Perfect

The semiconductor-based SSR has many advantages over electromechanical relays, but two factors inherent to SSRs—leakage current and operational temperature limits—can preclude using them for some applications. Leakage current is caused mainly by the SSR's snubber circuit. When in the "off" state, the relay will exhibit a small amount of leakage current, typically a few mA. Although slight, this current can keep some loads from turning off, especially ones with a high impedance such as a small solenoid or a neon lamp, which have relatively small "hold-in" currents. Leakage current also can cause circuits with SSRs that are switched off (electrically open) to still have potentially hazardous voltages on the outputs, in particular for SSRs that switch high voltages.

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