Lesson 1: Classification of  Automatic Controllers

Classification of Automatic Controllers 

Factors to be considered when selecting motor controllers include the required types of starting, stopping, reversing, running, and controlling speed and sequence. Other factors that influence the selection of a controller include the electrical service, environmental conditions, and electrical codes and standards. 

The motor may be controlled directly or manually by an operator using a switch or a drum controller. Remote control uses contactors, relays, pushbuttons, sensors, and possibly electronics. 

Motor Control Circuits 

Motor control circuits are an effective way to reduce cost by using smaller wire and reduced-amperage devices to control a motor. Imagine trying to wire a pushbutton station for a 100A motor using 3 AWG conductors. Many smaller motors use the same size conductors for both control and power circuits, but as the horsepower increases it becomes impractical to do so, Figure 1–1. Motor control circuits are often connected to lower voltages than the motor they control to make it safer for operators and maintenance personnel. 

A motor control circuit, for the most part, is simply a switch (or group of switches) and a motor. If you keep the word “switch” in mind, it helps keep the intimidating subject of “motor control” in its proper context. 

For example, the following can be considered motor controls: A time clock that operates a pool or sprinkler pump is nothing more than an automatic switch. At a preset time, a set of contacts open or close (turn off or on). 

Motor Control Language 

Electrical symbols, words, and line diagrams provide the information necessary to understand the operation of motor control circuits. Used together, they create a type of motor control “language” that is used to transfer information and ideas quickly and efficiently.

The symbols in motor control schematics represent devices, power conductors, control conductors, conductor connections and terminals, and sometimes the motor itself. 

The words, phrases, and abbreviations in a schematic are generally accepted terms that represent functions, describe actions, and list sequences of operation. In many cases, the symbols and words have a similarity to the items they represent. The basic types of schematics are shown in Figure 1–3. Parts A, B, and C of that figure illustrate three different methods of representing the same control circuit. 

Ladder Diagrams (Figure 1–4) 

Ladder diagrams are also called “line diagrams” or “elementary diagrams.” They’re used to represent the function of the control circuit and the associated devices, but don’t show the components of the control circuit in their actual positions. As control circuits become more complex, a ladder diagram can be less complicated to read than a wiring or connection diagram. For example, in Figure 1‑4, notice the set of contacts marked M under the start pushbutton. The M contacts marked 2 and 3 are actually located in the motor starter fairly close to the coil, as shown in Figure 1–3B, and the normal physical appearance of these contacts often look as shown in Figure 1–3C. (Notice that all three examples of the M contacts are shown with a blue background in Figure 1–3. 

Connection Diagrams (Figure 1–6) 

Connection diagrams, or wiring diagrams, show the components of the control circuit in a semblance of their actual physical locations. The start-stop push-button station is shown more as an actual device in the control circuit wired to a set of contacts marked 2 and 3. In Figure 1–4, the wires on each side of the M contacts trace back to points 2 and 3 on either side of the start pushbutton. The contacts marked M in Figure 1–4are the same contacts as those marked 2 and 3 in Figure 1–6 

Different manufacturers of control devices, as well as books about motor controls, use different methods of showing the control circuit wiring. For example, in Figure 1–3B1, the control wiring from the start-stop pushbutton station runs to the actual connection points 1, 2, and 3. As the wiring diagrams become more complicated, the optional method shown in Figure 1–3B2 is frequently used to show the connection points for the start-stop pushbutton station. Here, in Figure 1–6, instead of running the control wires to the actual connection point, arrowed lines represent connections to be made by the installer. 

Pictorial Diagrams (Figure 1–8) 

Pictorial diagrams are often used in educational mate-rial and as exploded views or cut-away views in installation and maintenance literature. Pictorial diagrams help students see actual devices and components used in motor control circuits and how they relate to the symbols used in ladder diagrams and wiring diagrams. For example, the start pushbutton we mentioned earlier has wires run to the device that contains the set of contacts M in Figure 1–4, and to contact points 2 and 3 in Figure 1–6.