Considerations to take into account when designing PLC ladder logic

Where do we use ladder logic?

Motor control circuits are often implemented using ladder logic. It is so called because the schematic representation of the control circuit resembles a ladder with two vertical rails and a number of horizontal rungs, as shown in Figure 1. The vertical rails are energized at two different potentials, often 120 V and ground. Any available low-voltage source, either AC or DC, can be used for the control circuit.

If the motor operates at 240 V or less, it is not uncommon to obtain the control voltage directly from the power circuit, as shown in Figure 1.

If the motor operates at a higher voltage that is not suitable for the control circuit, a control power transformer can be used to feed the control circuit, as shown in Figure 2 below.

Current will ow from the left-hand rail to the right-hand rail if a closed path is provided. Some impedance is required in this closed path to prevent a short circuit. Impedance can be provided by relay coils and indicator lamps.

Fusing protects the control circuit against heavy current caused by a short circuit in the control circuit. If the control circuit is left ungrounded, both rails of the ladder can be fused, but only the left rail is fused if the right rail is grounded.

Closing the path between the rails is done by contacts placed in the rung. The contacts, which can be normally open or normally closed, are placed in series to implement a logical and function or in parallel to implement a logical or function. The contacts can be actuated by a relay coil or by automatically or manually controlled switches.

Things to consider when designing ladder logic

Some terminology must be understood before proceeding with implementing ladder logic. When a relay coil is energized, that relay is said to be picked up. When a relay picks up, its contacts change state: the normally open contacts close and the normally closed contacts open.

When a relay coil is de-energized, that relay is said to drop out. Care must be exercised so relay coils do not inadvertently drop out due to inadequate voltage. This can be a concern because the starting of a large motor can significantly depress the system voltage and thereby the control circuit voltage.

This is one reason why voltage drop during motor starting must be kept within specific parameters! If a running motor experiences a voltage drop in excess of the coil dropout voltage of its contactor, the running motor will shut down.

This problem can be mitigated in many ways, ranging from changing the motor starting method to reduce the voltage drop, to using contactors capable of withstanding larger voltage drops without dropping out, or to supplying the control circuit from a power source independent of the starting motor.

Symbols are used to represent commonly used control circuit devices on schematic diagrams. Although variations in drafting standards exist, the symbols shown in Table 1 are fairly standard throughout the industry.

Accidental grounds in the control circuit should never pick up a relay coil. In Figure 4, an accidental ground at point F would short out the three Type A contacts and pick up the coil.

Control circuits should always be designed in such a way that they fail in a safe mode.

In other words, the controlled motor should safely shut down in the event of a control circuit failure. Under no circumstances should a control circuit failure cause a de-energized motor to start.

PLC Training – Introduction to Ladder Logic

Introduction to PLC ladder logic programming. This video is an introduction to what ladder logic is and how it works.

Introduction to PLCs and Ladder Logic concepts

Overview of PLCs (Programmable Logic Controllers) and why we use them. Introduction to the basic concepts of ladder logic.

Reference // Industrial power distribution by Ralph E. Fehr (Purchase hardcover from Amazon)