The Basics of Control Relays

The Basics of Control Relays

Inside the world of control relays, including how they’re constructed, how they work, and where they’re used

Over the years, control relays of various types have been used by the hundreds — even thousands — to control nearly every function in commercial and industrial processes. Today, many of those applications have been supplanted by programmable logic controllers (PLCs) and so-called “smart relays,” which are actually more like small PLCs than relays. Nevertheless, relays still play an important role in today's electrical systems.

Photo 1. A DC relay showing pole-piece inside of the coil; the contacts were removed to show the coil.


Relays are used to isolate one voltage level from another. A PLC may be used to control the operation of a medium-voltage motor, perhaps 2,300V or 4,160V. A relay is used to energize the starter, which, in turn, switches the motor voltage while the PLC controls the relay. Wired to provide a control sequence, relays may also be used for simple control schemes where a PLC would prove uneconomical. Troubleshooting relays can be handled in short order, without having to return to the maintenance shop for the computer necessary to analyze the control sequence within the PLC.

DC relays

DC relays consist of wire wound on a bobbin, which is placed over a ferromagnetic core. A hinged contact assembly is positioned over the core (Photo 1). When current is applied to the coil, flux is induced in the ferromagnetic core, causing the contacts to close.

Photo 2. An AC relay showing the shading ring (at the arrow) in split pole-piece inside the coil. Again, the contacts were removed to show the coil.


AC relays

AC relays are manufactured similar to their DC counterparts. If AC current is applied to a DC relay, the relay will pulse at the frequency of the AC current. To overcome this problem, the core is equipped with a shading ring on one-half of the core (Photo 2). The shading ring acts like a shorted secondary winding in a transformer, causing the flux in that half of the core to be 90° out of phase with the flux in the other half. The result is that the flux in the core never falls to zero, allowing the relay to energize the contacts.


When depicted in drawings, relay contacts are shown de-energized — that is, with the power off the coil. Types of contact symbols are shown in Fig. 1.

Relay types

There are many kinds of relays available, several of which we'll discuss now.

Plug-in relays

Also known as ice-cube relays, plug-in relays are inexpensive, widely available, and used for control circuits (Photo 3). Contacts are usually of the normally open/normally closed (NO/NC) or Form C style, in quantities of one, two, three, or four poles per relay. The relays are built with fixed quantities of contacts. Plug-in relays plug into sockets; sockets may be mounted directly to a panel or they may be mounted on DIN rails. Some tiny relays are so small they fit into a lineup of sectional terminal blocks, appearing nearly like a terminal. Coil voltages are typically 6V to 240V(AC) and 6V to 110V(DC).

Contact ratings for plug-in relays are usually available to 240VAC maximum and 24VDC to 30VDC maximum. Ampere ratings range from less than 1A to 30A. Note that DC voltage and current ratings may be reduced from AC ratings. Because DC voltage never passes through zero as AC voltage does, greater arcing occurs when the contacts open. The voltage must be reduced due to the narrow gap between contacts. Current ratings are reduced as well for the same reason. Some contacts may be horsepower-rated to operate fractional horsepower motors.

Fig. 1. Normally open contacts are termed Form A contacts, normally closed contacts Form B contacts, and normally open/normally closed contacts Form C.


Care must be exercised when applying contacts with low-current circuits. When relay contacts operate, they depend on a certain level of current to remove oxidation. Relays to be used with low currents must have contacts rated for the current level. A contact rated for 10A, for example, would not be acceptable when used in a circuit of only a few milliamperes. Relay data sheets usually specify a minimum load current.

Some plug-in relays are equipped with light-emitting diode (LED) indicators, which provide an indication that voltage is applied to the coil. Although the LED indicator does not prove the coil will operate, it does prove the presence of voltage.

Test buttons, a useful feature on some plug-in relays, allow manual actuation of the relay contacts. Manual actuation can be helpful when you're troubleshooting circuits while voltage is not applied to the coil.

Machine tool relays

Conventionally, the term “machine tool relay” is applied to NEMA-style relays. Today, IEC relays, often called “control relays,” are also applied for the same purposes. In this article, the term machine tool relay will be used interchangeably for both NEMA-style and IEC-style relays.

Machine tool relays are available with quantities of contacts from two to 12. The base unit contains two to four contacts. Additional decks may be added in quantities of four, up to a maximum of 12 contacts. Contacts are NO (Form A) or NC (Form B). Contacts for machine tool relays are double-break contacts, which consist of two fixed contacts and one set of movable contacts. By employing double-break contacts, contacts can have higher voltage ratings than those of plug-in relays. Contacts can be rated to 600VAC and 240VDC. Be sure to check the ratings for individual relays. Coils are available from 6VAC to 600VAC, and from 6VDC to 240VDC. Machine tool relays may be mounted directly to a mounting panel or on DIN rail.

Some NEMA-style machine tool relays have fixed contacts, both in quantity-per-deck and type (NO or NC), while others have convertible contacts. Convertible contacts are housed in individual cartridges, which can be removed and flipped over to convert from NO to NC. Additional cartridges also can be added. Nearly all IEC relays contain fixed contacts, both in quantity-per-deck and type.

Machine tool relays may have auxiliary devices, such as time delay modules (solid-state or pneumatic) and magnetic latching attachments, which may be added by the user. In addition, the ability to add a time delay attachment allows the user to avoid having to add a separate time delay relay to the control system.

Photo 3. Plug-in control relays with sockets, also known as “ice-cube” relays.


Latching relays

The contacts for both plug-in and machine tool relays remain closed (or open, as the case may be), as long as voltage remains on them. Removing voltage causes the coil to release the contacts. Magnetic latching relays, which feature a closing coil that operates to energize the relay, are also available. When voltage is removed, the relay's contacts remain in the last position. A separate coil is provided to toggle the relay to the opposite position. Plug-in relays must be purchased with the magnetic-latching feature. Some machine tool relays may have latching attachments added to the relay while others are ordered with latching attachments.


Control relays are often used for isolating one voltage level from another. In motor control centers, a variable-frequency drive (VFD) may have its own 24VDC supply for powering its own inputs. The user may wish to operate controls from 120VAC due to long lengths of field wiring. A plug-in control relay works handily to provide the necessary isolation between the two voltage levels. This concept is illustrated in Fig. 2 (click here to see Fig. 2).

Machine tool relays may be employed where higher voltages are involved, because large starters often require a great deal of current to operate their coils. The coil will be operated from the 480VAC line voltage, while the operator controls are operated from 120VAC or 24VDC for safety purposes. A machine tool relay, with contacts rated to 600VAC, can be employed to operate the starter coil from the 480V power source used for the motor power. Figure 3 (click here to see Fig. 3) illustrates this principle.

Note in each of the foregoing examples that the schematic symbol “CR” is used for each relay type. A distinction is not made between the types of relay through the schematic symbols. Reference should be made to the bill of materials for the assembly to determine the actual type components used.

Although they're not used in the same quantities they once were, prior to the advent of the PLC, relays remain an important element in the control of many products. Because they're still found wherever electrical controls are employed in homes, commercial facilities, and industrial facilities/processes, it's essential for electrical professionals to thoroughly understand them.

Bredhold, MS, is an application engineer with Eaton Corp., Louisville, Ky. He can be reached at [email protected].

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