Chapter 6: Electromechanical Relays
Chapter Overview
Chapter 6 will cover Electromechanical Relays. Relays are devices that control one electrical circuit by opening and closing contacts in another circuit. Relays are typically utilized in order to switch the starting coils in contactors and motor starters but are not designed to handle excessive amounts of current. Remember that a small voltage applied to a relay results in a larger voltage being switched. Worksheet information link.
Key points to remember:
There are two major types of relays:
- Electromechanical Relays (EMR)
- Solid State Relays (SSR)
Three key characteristics can describe relays:
- Number of poles
- Number of throws
- Number of breaks
When troubleshooting a relay, always start at the relay itself, beginning with the coil and any associated components.
Introduction to Relays
Relays are electromechanically controlled switches that use an electric signal to connect or disconnect a circuit through sets of properly rated contacts. Relays can be broken into two categories, each with their own unique qualities. Electromechanical relays (EMR) rely on the strengthened magnetic field formed by the presence of an electric signal flowing through a coil of wire surrounding a metal core. EMR type relays are common in most electrical systems due to their reliability, affordability and ability to hold up to the rigors of an industrial application. Solid-state (SSR) type relays have no moving contacts and are switched entirely by electronic means. The absence of moving parts increases reliability potential but the limited current that all electronic devices can withstand is the main reason electromechanically controlled relays, contactors, and motor starters are still relied on in many electrical systems.
Image source: What Is Relay? How Relay Works? (electronicshub.org)
Relay Characteristics
Relays are not designed to directly control power-consuming loads; rather, they are used to switch other load-bearing control devices such as contactors and motor starters. Relays are typically controlled by a lower voltage and current level but are intended to manage larger voltage and current-rated loads.
Solid State Relays (SSR) offer several advantages over electromechanical relays (EMR), including faster operation, no contact bounce, and no wear of moving parts. SSRs are also resistant to vibration, have a long service life, and reduce electromagnetic interference.
Reed Relays Example: Want to know more? Click the information link: What is a Reed Relay? | Standex Electronics
Reed relays, also known as proximity switches, are magnetically controlled, fast-operating, low-current-rated, fully enclosed switches. They are typically used as liquid level switches.
EMR vs. SSR Example: Want to know more? Click the information link: Solid State Relay vs. Electromechanical Relay | Arrow.com
Electromechanical relays (EMR) feature a more robust construction that can handle higher levels of current. Their contacts are interchangeable and can be replaced, if necessary, unlike solid-state relays (SSR) which are nonreplaceable and more susceptible to excessive operating currents.
Think of EMRs as the heavy-duty version of their solid-state counterparts. Although EMRs are not intended to control loads directly, their robust construction allows them to handle larger loads. These specialized relays, referred to as machine control relays, are considered the backbone of modern control systems. They are widely used to control solenoids, contactors, and motor starters. General-purpose relays, though similar, do not offer as robust construction or replaceable contacts, but they are more economical.
EMR’s all have mechanical contacts that are NEMA rated for specific amount of current levels. EMR’s are rated for an expected life expectancy of 100,000 to 500,000 operations, commonly referred to “contact life”. Contact life is the number of times the contacts can mechanically operate before failure and is affected by the material the contacts are constructed from. Silver alloy is a common material used due it’s superb electrical characteristics. In high voltage and high current situations, Tungsten is used due to its high melting point and resistance to the negative effects of arcing.
Keep in mind
While electronic components are generally considered more reliable due to their simple construction, in practice, EMR-controlled electrical devices tend to perform better in industrial environments. This is because their more robust, albeit somewhat primitive, construction makes them more durable compared to SSR-controlled devices.
Looking Deeper
Solid-state controlled devices utilize electrical semiconductors instead of physically moving contacts like those found in electromagnetically controlled relays. Solid State Relays (SSRs) are known for their accuracy, speed, and durability. They offer rapid switching speeds, exceptional reliability, and robustness, making them ideal for applications that require precise control. Furthermore, because they lack moving parts, SSRs operate more quietly and produce less electromagnetic interference.
A electromechanical switch controlled by an electromagnet to operate a set of electrical contacts.
An assembly consisting of many turns of wire wound together with the purpose of creating an electric magnet strong enough to be used to open and close sets of contacts. Referred to as the “heart” of a contactor or motor starter.