Control System Voltage Choices

Control engineers should take a closer look at '120 V ac vs. 24 V ac/dc' question.

By John F. Heneage, Dufresne-Henry Inc. May 1, 2001

O ne of the first decisions control system designers must make at the onset of any design process is how much system voltage their project will need. Often use of 120 V ac for control system power is based on traditional usage, simply because it has always been done that way. However, there are viable alternatives to 120-volt control systems that have evolved over the history of the control system.

A short history
Electrical control systems began replacing mechanical control systems approximately 100 years ago. In many cases, early electrical control systems used the same voltage for control as the driven device because it was convenient and easily accessible (i.e. 120 V ac.) Larger electrical motors required large magnetic contactors for starters. Early contactor designs were not very efficient, and thus required large amounts of power to generate contactor pull-in and hold-in forces. Higher voltages made these devices practical, as the required currents were lower.

Pros and cons of using 24 V ac/dc for control systems?
On the plus side:

  • Almost any control function can be implemented using 24 V ac/dc equipment.

  • Most control systems use digital (low voltage) devices today, and thus a lower control voltage is better suited to those applications.

  • Troubleshooting 24 V circuits reduces the likelihood of dangerous shock hazards (NEC Article 725, Class 2 Circuits).

  • DC circuits avoid problems with inductive coupling.

  • HVAC/R control systems already use 24 V (ac and dc).

  • 24 V dc is compatible with most analog instrument systems, which allows a common power supply to serve multiple functions.

  • Many equipment manufacturers now provide 24 V control options for their equipment.

  • Where alternate voltages/phases are required, interposing relays can be used.

  • Smaller dimension components and switches can be used.

On the minus side:

  • Industry is generally more familiar with 120 V ac.

  • A large installed and available component equipment base exists for 120-volt devices.

  • Large starters (coils) will still require 120 V ac.

  • Contact cleaning is better with 120 V ac.

  • Designers need to learn to use slightly different components.

  • Does not always require a control transformer (i.e. 120 and 208-240/120 motor circuits).

  • Some suppliers will have to modify their equipment offerings.

As electrical and electronics practices evolved, control system and power distribution practices emerged. In general, electrical control circuits used small amounts of power to control larger amounts of power for driven equipment. Modern control system designs addressed both power distribution to the driven devices (motors, solenoids, positioners, etc.) and control of those devices.

The control system is normally designed to use very little power relative to the controlled equipment. Control systems typically include discrete devices (e.g. switches and counters) and analog devices (e.g. transmitters, meters, and positioners). The power component of a control system normally controls and distributes power at higher voltages (e.g. 208, 240, and 480 V ac). Not all industrial segments, most notably the HVAC industries, have embraced 120 V ac for control purposes.

Computerization arrives
Computers were first applied to control systems in the late 1950s and were rapidlygaining acceptance by the 1970s. As with any new technology, the introduction of computers had to ‘blend’ with the older technology in order for them to be adopted/accepted. In this case, PLCs and I/0 devices were designed to replace relay ladder logic-style control systems. The new PLCs were often constructed to allow 120 V ac for I/O functions. By the 1980s, many designers began to question use of 120 V ac for control functions. Because I/O signals usually went into a digital device or out to a relay, lower voltages could be used. Control systems designers and component manufacturers now see the advantages of using low voltage. They are now adopting 24 V as standard because it is practical and safe.

Modern control systems are often very complex. Troubleshooting control systems frequently requires that they be powered up while the system is checked. The use of 24 V reduces the likelihood of injury caused by shock. A typical control system uses 24 V ac for panel and field I/O devices. This system would also use 24 V dc for analog instrumentation. And, alternately, 24 V dc could be used for panel, field I/O and analog devices. The flexibility in the choice of ac and dc or all dc or all ac covers most control situations.

Deciding what voltage level is most appropriate depends on several factors. For small systems, all the controls can be implemented using 24 V dc. For systems with motor starters, 24 V ac is available by providing a step down transformer connected to the power feeders at the starter. Systems with analog instruments and starters probably are best served by having both 24 V ac and dc available. Where 120 V ac is needed for specific applications, interposing relaying can be provided. Today, many factors are combining to help propel the adoption of 24 volts for control purposes. (See the sidebar below, which summarizes the pros and cons of the ‘best practice’ 24-volt alternative.)

John F. Heneage, P.E., is a senior program manager, Dufresne-Henry Inc. (Manchester, N.H.), a multidisciplined, national engineering firm, where he provides electrical and instrumentation design support.

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