Quiz 8-2: Control Voltages @🏠

What is Voltage?

Voltage is the amount of electrical pressure created by a power source that pushes electrons (current) through a circuit. This electrical pressure is measured in volts.

High voltages are required to run various things in an industrial environment. Typically, electric motors in a plant require 240 or 480 VAC in order to run. Things like actuators and feed screws may require less voltage to operate, and things like heating elements or welding applications may require even more. 

Control Systems 

Voltage

As we've seen in earlier courses, a control system man ages the operation of all those various devices in the industrial environment that does not use high voltages. Control systems use lower voltages for their operation.

Below is a typical motor control circuit that shows how a 480 VAC motor can be turned off and on by a control circuit that uses 24 VAC. The operator of this control circuit never gets close to the 480 VAC that is needed to run the motor. 

Figure 1. Typical Motor Control Circuit.

Not that long ago, the typical voltage used for control systems was 120 VAC. Now, the most common voltage encountered in a control system is 24 VDC. Some of the reasons for this transition include:

• troubleshooting 24 VDC circuits reduces the likelihood of dangerous shocks
• DC circuits help you avoid problems with induction
• smaller dimension components and switches can be used
• solid-state devices are now everywhere, and solid-state devices use low-voltage DC power

Solid-State

A solid-state device is an electronic device in which electricity flows through solid semiconductors. Switching that used to occur using mechanical switches now occurs in a Solid-State device without any mechanical parts. It switches using no contacts, but by entirely electronic means. Solid-State devices normally use DC power.

PLC I/O can be solid-state, but it doesn't have to be. Many of you used the Micrologix 1000 in the PLC-1 course. You may have noticed that you could hear a click when you turned an output on with that PLC. Those were not solid-state outputs. If they were solid-state, you would not hear a mechanical click when you turn on the output.

Many of the sensors you will work with in this course are solid-state. We'll talk more about those when we get to them.

Diagrams

Before we move on, there are a few new symbols you will see. Below are the symbols to use for PLC I/O in a schematic.

Figure 2. PLC I/O Schematic Symbols.

Figure 3 shows a schematic much like the power and control circuit diagram shown above in Figure 1. This time, our circuit is wired to a PLC.

Figure 3. Control Circuit Diagram Wired to a PLC.

Remember that making the "jog button" actually jog the motor is done in the PLC program.

A typical control system uses 24 VDC to run the controls, then replaces the control transformer with a 24VDC power supply.

Figure 4. Control Transformer with a 24VDC Power Supply 

Here's a link to these notes in PDF format.