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24 VDC vs 120 VAC Controls

Around the world, several control voltages are used for industrial automation, such as 120 VAC, 220 VAC, 240 VAC, 24 VDC, 125 VDC, and 48 VDC. The decision-makers, such as control engineers and plant staff choose control voltage depending on many factors such as system design, safety considerations, cost, distance, speed, sizing, performance, etc.

This article discusses the two most common control voltages; 24 volts direct current (VDC) and 120 volts alternating current (VAC) controls. That will help you choose the control voltage best suited for your system.

A) Safety Considerations

The conventional 120VAC control design typically focuses on the protection of the equipment and preventing fires rather than personnel protection. NFP and NEC are now emphasizing the need for personnel protection, which is achievable by focusing on these two factors.

Lowering System Voltages: 

An electrical hazard can be avoided by lowering system voltages to a safe level. In comparison with the higher values of 120VAC, 24VDC is a significantly safer limit.

120VAC is hazardous and perhaps fatal. This risk is made worse by the fact that the widespread availability of 120VAC has exposed us to the hazard, allowing many electricians and maintenance personnel to underestimate 120VAC potency. The major advantage of 24VDC over other voltage levels, on the other hand, is safety. The voltage is low enough that the shock risk is almost non-existent.

Reducing Arc Energy: 

When the available energy level to feed an arc flash may be lowered, it should be regarded as a technique for improving safety. The arc flash hazard from 120VAC is significant. There are no arc flash risks with 24VDC.

In general, the circuit type and the level of contact voltage have a significant impact on the subsequent harm to individuals. Personnel is not as at risk from DC control circuits as they are from AC control circuits. Because a direct current of five (5) milliamperes (mA) has the same impact as two (2) milliamperes (mA) of AC current.

B) Performance Over Long Distances: 

With 120 VAC controls, voltage loss over long distances is rarely an issue, and current requirements are also lower than in lower voltage systems.

Unlike 120 VAC, 24 VDC has voltage drop issues over a longer distance. These issues can be addressed by using larger wires or increasing the control voltage level.

However, new PLC schemes with remote or distributed input /output architectures are now available. In this configuration, input /output racks are placed near the equipment and interface with the PLC using a protocol such as Ethernet /IP. This decreases the length of wire lines necessary, resolves voltage drop concerns, and saves money on installation expenses.

C) Signal Isolation and Wiring:

Proper grounding and wiring methods are critical for preventing electromagnetic interference (EMI) in systems, which can cause difficulties with low power I/O and communication wiring.

The impact of EMI is negligible in 120 VAC controls. That’s why field device wiring in 120 VAC systems is often coupled with a power wire. This saves money on wiring by removing the requirement for separate control and power connections to these devices.

In 24 VDC controls, an EMI-induced voltage can be strong enough to give a misleading signal, yet the same EMI-induced voltage may not impact a higher voltage circuit (120V). That’s why separate power conductor routing to field devices may be necessary for 24 VDC systems.

D) Protection Against Voltage Dips: 

Voltage dips are frequent in industrial control processes and are usually produced by powerful motors starting on underpowered or excessively loaded supply systems. 120 V AC control systems without additional protection are significantly more sensitive to voltage dips than DC control systems.

In 24 VDC controls, the DC power supply contains capacitors in their filters that store energy. DC coils also have significantly better ride-through capabilities because most dips are less than 10 cycles long.

Figure 1: Relative speed of DC devices compared with AC equivalents

E) Control Speed:

While designing the control system for industrial applications, its ability to detect and respond to events rapidly is typically a primary concern. The system should be able to stop a process immediately and reliably protect the equipment and operating personnel. It is recommended for design engineers to carefully evaluate any effect that control voltage has on the control speed of the system.

Figure 1 depicts the device control speed for different components depending on DC device speeds comparable to AC device speeds.

  • Solid-state output sensors and controller input/output modules with 24 V dc control are significantly quicker than ac alternatives. 
  • Electromechanical relays with 120 VAC control are significantly quicker than DC alternatives. 
  • The speed of small general-purpose relays is comparable in both AC and DC versions.  
  • Larger relays are slower in 24 VDC controls.  

Incorporating arc suppression diodes into these components would increase drop-out time and reduce relative speed even further. It is important to highlight that these device speed variations are mostly only relevant to control systems that use direct wired or I/O (input/output) local to the controller device. 

F) Reliability:

For 120 VAC controls, a step-down transformer or UPS is used. Both are very reliable components. Besides, 120 VAC works better than 24 VDC in really unclean regions because this level of voltage can overcome the resistance generated by dirt.

Aside from the voltage loss, a DC source has a reliability concern. If not backed up by station batteries, a DC source supplied only through a rectifier power supply may be less reliable since the power supply itself is more prone to failure than an ac control transformer.

G) Leakage Current: 

All semiconductor-based switching devices must have an off-state leakage current and a minimum specified latching current. Neglecting these factors can result in control issues such as equipment not turning on or, worse, failing to turn off a control signal.

The several 120 VAC control devices have quite large leakage currents, typically ranging from 1 milliampere (mA) to 3 milliamperes. This situation is worsened in 120 VAC field devices, which require less current than their 24 VDC counterparts. When turned off, indicator lights might glow dim, or general-purpose relays might not trip. In these scenarios, connecting a properly sized resistor parallel to the load device will limit leakage current to tolerable levels.

Most DC sensors and output cards with 24 VDC controls have very low leakage currents, often less than 1 milliamperes (mA). The leakage currents at these levels usually cause fewer interface issues with output devices such as relays, indicator lights, and solenoids. However, issues may develop with dc input devices that demand a low input current and high input impedance.

H) Physical Device Size:

The size reduction of control panels and equipment has increased the demand for smaller control devices. In general, the nature of DC voltage is enabling the manufacturers to reduce the size of devices with 24 VDC controls. This is mainly because 24 VDC control units do not require shading coils and laminations. Furthermore, implementing microprocessor technology on a DC removes the rectification phase and allows decoupling of the control and logic circuits, decreasing device size even further.  

Depending on the nature of voltage, each component category can fall under a different size range, as mentioned below. 

  • Sensors in 24 V dc versions are available with more sizes variety than ac variants. For example, many manufacturers produce cylindrical AC proximity sensors in 12 millimeters, 18 millimeters, and 30 millimeters sizes. The same manufacturers provide dc versions of the same device in size varying from 3 millimeters to 30 millimeters. 
  • The exterior size of general-purpose relays is usually the same for both DC and AC. 
  • In dc versions, the contactors, starters, and industrial relays are typically larger. 
  • The maximum input /output density for 24 VDC controls is 64 inputs per module, but the maximum for 120 VAC is generally 16. 

In general, 24 VDC devices are usually 40% smaller as compared to the standard motor-control devices. 

I) Length of Control Cables:

The control cable’s length is an important factor to select the control voltage and design. Cable impedance can affect long distances in two ways: 

Line Resistance and Inductance:  

The effect of line impedance changes inversely with control voltage magnitude. At lower voltages, the influence of line impedance is more. 

For a 120 VAC system, the series impedance of the control cables have an effect when only inrush current is present in the system. 

For 24 VDC systems, there is no persistent inrush, although the operational current for a DC device of the same class is roughly five times higher. Voltage drops in long cables are eliminated by using a well-controlled DC power supply. For 24 VDC controls, an increase in the cable size can lower the line impedance and allow for longer control wire distances. 

Effective Line Capacitance: 

The effective line capacitance of the control wire is considered only in AC control systems. Long cable capacitance may prevent contactor coils from dropping out when the stop contact is opened. Capacitance is often not an issue for 120 VAC controls with wire runs of less than two miles. This effect does not exist in 24 VDC controls since capacitance only conducts AC to the conduit. 

J) Cost:

Cost is always an important factor in assessing whether or not a specific solution is acceptable. Costs associated with both controls are discussed below. 

Hardware Cost:  

  • Several 24 VDC devices are less expensive than their 120 VAC counterparts. Generally, the smaller low-power electronic equipment such as sensors and IEC pilot devices are more economical.  
  • 24V dc versions of large electromechanical devices are typically more expensive than 120V ac equivalents. 
  • Control systems that use 24V dc control necessitate the addition of interposing relays or other interface devices, which increases both hardware & wiring costs. 

Installation Cost:  

Installation costs are significantly lower for 120 Vac than for 24 Vdc because the latter requires a power supply in addition to simple AC transformers. 

Wiring Cost: 

The wire gauge used in direct current systems is often smaller than that used in alternating current systems. Because smaller gauge wire is often less expensive, this directly leads to lower costs. 

K) Side by Side Comparison: 

120 VAC Controls 24 VDC Controls 
High risk of being deadly Almost no risk of being deadly 
The danger is not limited to the control panels. It is present on the factory floor in every piece of equipment that is connected to the control system. No such risk is associated with this. 
Demands the installation of conduit to any sensors, valves, or other components. Eliminates the need for conduit to be run to valves, sensors, and other devices. Cables are suitable. 
Necessitates the use of personal protective equipment (PPE) when working on any enclosure or wire. PPE is not required. (But it is recommended to use PPE to add extra layer of safety) 
Requires shielding for any analogue signals and is more likely to induce voltages on I/O signals. Does not induce voltages on I/O signals. 
It works better in unclean areas because the voltage can overcome the resistance induced by dirt. It does not work better in dirty areas. 
When a short occurs, it will burn a wire or blow a fuse, making it easier to locate the source of the problem. When a 24VDC short occurs, the power supply is often pulled ‘low,’ causing damage to the supply. The short/issue isn’t usually discovered at the point of failure. 
More Reliable  Less Reliable 
Don’t cause Electrolysis  Generate Electrolysis in humid environments 
Less resistant to control circuit drop out induced by high inrush More resistant to control circuit drop out induced by high inrush 

Conclusion: 

120 VAC is commonly used in typical industrial applications because of its familiarity and widespread use as the standard control voltage for motor starters and current control panels. Furthermore, many personnel prefers using the 120 VAC controls because of the factors such as low initial cost, simple design, easy accessibility of components, and ability to derive voltage by using a step-down control transformer. However, precautions must be made to manage the shock risk. It’s also much more vulnerable to control equipment failure owing to voltage dips.

24 VDC control is increasingly used, particularly in process control applications. It is inherently safe, which is a significant advancement in electrical safety. This allows for a reduction in lockouts and maintenance time, as well as the possibility to undertake hot work in some scenarios. When used correctly, devices are exceedingly reliable, generally less expensive than ac equivalents, and have faster time response. DC provides protection against dropout caused by voltage sags caused by capacitors in power sources. It enables direct interaction with process control equipment while requiring only a few interposing relays.

The priority on worker safety, the requirement for improved performance, and the necessity for comprehensive interfaces with control systems for automatic control all give 24 VDC a major advantage in new control applications. With costs continuing to fall, it is quickly becoming the control voltage of choice.

This entry was posted on June 13th, 2022 and is filed under Electrical, Hardware Comparison, Uncategorized. Both comments and pings are currently closed.

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