T HE I NPUT V O LTA G E

Usually, PLC power supplies require input from an AC power source; however, some PLCs will accept a DC power source. Those that will accept

a DC source are quite appealing for applications such as offshore drilling operations, where DC sources are commonly used. Most PLCs, however, require a 120 VAC or 220 VAC power source, while a few controllers will accept 24 VDC.

Since industrial facilities normally experience fluctuations in line voltage and frequency, a PLC power supply must be able to tolerate a 10 to 15% variation in line voltage conditions. For example, when connected to a 120 VAC source, a power supply with a line voltage tolerance of ± 10% will continue to function properly as long as the voltage remains between 108 and 132 VAC. A 220 VAC power supply with ± 10% line tolerance will function properly as long as the voltage remains between 198 and 242 VAC. When the line voltage exceeds the upper or lower tolerance limits for a specified duration (usually one to three AC cycles), most power supplies will issue a shutdown command to the processor. Line voltage variations in some plants can eventually become disruptive and may result in frequent loss of produc- tion. Normally, in such a case, a constant voltage transformer is installed to stabilize line conditions.

Constant Voltage Transformers. Good power supplies tolerate normal fluctuations in line conditions, but even the best-designed power supply cannot compensate for the especially unstable line voltage conditions found in some industrial environments. Conditions that cause line voltage to drop below proper levels vary depending on application and plant location. Some possible conditions are:

• start-up/shutdown of nearby heavy equipment, such as large motors, pumps, welders, compressors, and air-conditioning units

• natural line losses that vary with distance from utility substations • intraplant line losses caused by poorly made connections • brownout situations in which line voltage is intentionally reduced by

the utility company

A constant voltage transformer compensates for voltage changes at its input (the primary) to maintain a steady voltage to its output (the secondary). When operated at less than the rated load, the transformer can be expected to maintain approximately ± 1% output voltage regulation with an input voltage variation of as much as 15%. The percentage of regulation changes as a function of the operated load (PLC power supply and input devices)—the higher the load, the more fluctuation. Therefore, a constant voltage trans-

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former must be properly rated to provide ample power to the load. The rating of the constant voltage transformer, in units of volt-amperes (VA), should be selected based on the worst-case power requirements of the load. The recommended rating for a constant voltage transformer can be obtained from the PLC manufacturer. Figure 4-20 illustrates a simplified connection of a constant voltage transformer and a programmable controller.

To AC Source Constant Voltage

Transformer Primary

Secondary CPU

Processor

Memory

Power Supply

AC Input Module

AC Output Module

Figure 4-20.

A constant voltage transformer connected to a PLC system (CPU and modules).

The Sola CVS standard sinusoidal transformer, or an equivalent constant voltage transformer, is suitable for programmable controller applications. This type of transformer uses line filters to remove high-harmonic content and provide a clean sinusoidal output. Constant voltage transformers that do not filter high harmonics are not recommended for programmable controller applications. Figure 4-21 illustrates the relationship between the output voltage and input voltage for a typical Sola CVS transformer operated at different loads.

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oltage (% of nominal) 70 25% Full Load

60 50% Full Load

Output V

100% Full Load

Input Voltage (% of nominal)

Figure 4-21. Relationship of input versus output voltages for a Sola unit.

Isolation Transformers. Often, a programmable controller will be installed in an area where the AC line is stable; however, surrounding equipment may generate considerable amounts of electromagnetic interference (EMI). Such an installation can result in intermittent misoperation of the controller, especially if the controller is not electrically isolated (on a separate AC power source) from the equipment generating the EMI. Placing the controller on a separate isolation transformer from the potential EMI generators will increase system reliability. The isolation transformer need not be a constant voltage transformer; but it should be located between the controller and the AC power source.

T HE I NPUT V O LTA G E