Ladder Logic Basics Tips for learning the symbolic language of motor control

摘要

A specialized segment of the electrical construction and maintenance industry, AC motor control is an area that requires specific knowledge in order to troubleshoot motors effectively and ensure smooth operations. This means gaining a clear understanding of ladder diagrams and ladder logic, which enable the automation that drives motors. The combination of input devices that either manually or automatically sense a condition - and the corresponding change in condition performed by the output device - make up the core of motor control. Let's take a closer look at what's involved in learning the symbolic language of motor control. First, it's important to discuss the term "logic" for a moment. In the study of digital electronics, devices are used that operate in either an ON or OFF state. A specialized branch of mathematics called Boolean algebra analyzes this relationship with two numbers: a zero (representing the OFF state) or a one (representing the ON state). These two numbers comprise the binary number system. The most common logic functions are the AND, OR, and NOT functions. Think of a single-pole light switch in your home that controls a 100W light bulb. The switch can either be off or on, thereby representing a zero in the off state and a one in the on state. Now imagine placing two single-pole switches in series to control the same 100W light bulb. In this condition, switch No. 1 and switch No. 2 have to both be on to light the 100W bulb. This is an example of an AND operation. Figure 1 represents the AND circuit just mentioned. Logic relates to ladder diagrams because input functions in series constitute an AND operation, while input functions in parallel constitute an OR operation. You will encounter two types of ladder diagrams: the 2-wire control circuit and the 3-wire control circuit. The 2-wire control circuit is shown in Fig. 2. This circuit is used to start a motor for some industrial process. The components in a 2-wire control circuit are a maintained contact switching device (S{sub}1), a relay coil (M{sub}1), and the thermal overload relay contact (OL). The sequence of operations is fairly simple. When S{sub}1 is closed, the coil of magnetic motor starter M{sub}1 is energized and the motor starts, provided the running overload current is within the values of the overload relay OL. To stop the motor, S{sub}1 is simply opened.