Basics of PLCs

Basics of PLCs - SIEMENS (photo by Systemy Sterowania PLC; plc24.pl)

Programmable Logic Controller (PLC)

A Programmable Logic Controller (PLC), also referred to as programmable controller, is the name given to a type of computer commonly used in commercial and industrial control applications.

• PLCs differ from office computers in the types of tasks that they perform and the hardware and software they require to perform these tasks.

While the specific applications vary widely, all PLCs monitor inputs and other variable values, make decisions based on a stored program, and control outputs to automate a process or machine.

This course is meant to supply you with basic information on the functions and configurations of PLCs with emphasis on the S7-200 PLC family.

Basic PLC Operation

The basic elements of a PLC include input modules or points, a Central Processing Unit (CPU), output modules or points, and a programming device. The type of input modules or points used by a PLC depend upon the types of input devices used. Some input modules or points respond to digital inputs, also called discrete inputs, which are either on or off. Other modules or inputs respond to analog signals.

These analog signals represent machine or process conditions as a range of voltage or current values.

The primary function of a PLC’s input circuitry is to convert the signals provided by these various switches and sensors into logic signals that can be used by the CPU. The CPU evaluates the status of inputs, outputs, and other variables as it executes a stored program. The CPU then sends signals to update the status of outputs.

• Output modules convert control signals from the CPU into digital or analog values that can be used to control various output devices.

The programming device is used to enter or change the PLC’s program or to monitor or change stored values. Once entered, the program and associated variables are stored in the CPU. In addition to these basic elements, a PLC system may also incorporate an operator interface device of some sort to simplify monitoring of the machine or process.

In the simple example shown below, pushbuttons (sensors) connected to PLC inputs, are used to start and stop a motor connected to a PLC output through a motor starter (actuator). No programming device or operator interface are shown in this simple example.

Hard-Wired Control

Prior to PLCs, many control tasks were performed by contactors, control relays and other electromechanical devices. This is often referred to as hard-wired control.

Circuit diagrams had to be designed, electrical components specified and installed, and wiring lists created. Electricians would then wire the components necessary to perform a specific task. If an error was made, the wires had to be reconnected correctly. A change in function or system expansion required extensive component changes and rewiring.

Advantages of PLCs

PLCs not only are capable of performing the same tasks as hard-wired control, but are also capable of many more complex applications. In addition, the PLC program and electronic communication lines replace much of the interconnecting wires required by hard-wired control. Therefore, hard-wiring, though still required to connect field devices, is less intensive. This also makes correcting errors and modifying the application easier.

Some of the additional advantages of PLCs are as follows:

• Smaller physical size than hard-wire solutions.
• Easier and faster to make changes.
• PLCs have integrated diagnostics and override functions.
• Diagnostics are centrally available.
• Applications can be immediately documented.
• Applications can be duplicated faster and less expensively.

Siemens PLCs

Siemens makes several PLC product lines in the SIMATIC® S7 family. They are: S7-200, S7-300, and S7-400.

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Basics of Motor Control Centers – SIEMENS

Basics of Motor Control Centers - SIEMENS (on photo: New Motor Control Center and power distribution board up to 7000 amperes)

Motor control

Power distribution systems used in large commercial and industrial applications can be complex.Power may be distributed through:

1. Switchgear,
2. Switchboards,
3. Transformers, and
4. Panelboards.

Power distributed throughout a commercial or industrial application is used for a variety of applications such as heating, cooling, lighting, and motor-driven machinery.

Basic Motor Control

Wherever motors are used, they must be controlled.

Basic motor control

In Basics of Control Components you learned how various control products are used to control the operation of motors.

• The most basic type of AC motor control, for example, involves turning the motor on and off. This is often accomplished using a motor starter made up of a contactor and an overload relay.

The contactor’s contacts are closed to start the motor and opened to stop the motor. This is accomplished electromechanically using start and stop pushbuttons or other pilot devices wired to control the contactor.

The overload relay protects the motor by disconnecting power to the motor when an overload condition exists. Although the overload relay provides protection from overloads, it does not provide short-circuit protection for the wiring supplying power to the motor. For this reason, a circuit breaker or fuses are also used.

Typically one motor starter controls one motor. When only a few geographically dispersed AC motors are used, the circuit protection and control components may be located in a panel near the motor.

Motor Control Centers (MCC)

In many commercial and industrial applications, quite a few electric motors are required, and it is often desirable to control some or all of the motors from a central location. The apparatus designed for this function is the motor control center (MCC).

• Motor control centers are simply physical groupings of combination starters in one assembly.

A combination starter is a single enclosure containing the motor starter, fuses or circuit breaker, and a device for disconnecting power.

Other devices associated with the motor, such as pushbuttons and indicator lights may also be included.

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Basics of DC Drives – SIEMENS

Basics of DC Drives - SIEMENS (on photo: The Sinamics DC Master cabinets - can be directly connected to three-phase systems up to 3 AC-950 V, and in the basic version they cover a power range between 6 and 2500 kW.)

Controlling a DC Motor

A thyristor bridge is a technique commonly used to control the speed of a DC motor by varying the DC voltage. Examples of how a DC rectifier bridge operates are given on the next few pages. Voltage values given in these examples are used for explanation only. The actual values for a given load, speed, and motor vary.

It is important to note that the voltage applied to a DC motor be no greater than the rated nameplate. Armature windings are commonly wound for 500 VDC. The control logic in the drive must be adjusted to limit available DC voltage to 0 – 500 VDC. Likewise, the shunt field must be limited to the motor’s nameplate value.

Basic Operation

A DC drive supplies voltage to the motor to operate at a desired speed. The motor draws current from this power source in proportion to the torque (load) applied to the motor shaft.

100% Speed, 0% Load

In this example an unloaded motor connected to a DC drive is being operated at 100% speed. The amount of armature current (Ia) and unloaded motor needs to operate is negligible. For the purpose of explanation a value of 0 amps is used.

The DC drive will supply only the voltage required to operate the motor at 100% speed. We have already learned the amount of voltage is controlled by the gating angle (COSα) of the thyristors. In this example 450 VDC is sufficient. The motor accelerates until CEMF reaches a value of V_a – I_aR_a.

Remember that V_a = I_aR_a + CEMF. In this example I_aR_a is 0, therefore CEMF will be approximately 450 VDC.

100% Speed, 100% Load

A fully loaded motor requires 100% of rated armature current at 100% speed. Current flowing through the armature circuit will cause a voltage drop across the armature resistance (Ra). Full voltage (500 VDC) must be applied to a fully loaded motor to operate at 100% speed. To accomplish this, thyristors are gated earlier in the sine wave (36.37°).

The DC drive will supply the voltage required to operate the motor at 100% speed. The motor accelerates until CEMF reaches a value of V_a – I_aR_a.

Remember that Va = I_aR_a + CEMF. In this example armature current (Ia) is 100% and Ra will drop some amount of voltage. If we assume that current and resistance is such that Ra drops 50 VDC, CEMF will be 450 VDC.

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Basics of Control Components

Control

Control components are used in a wide variety of applications with varying degrees of complexity. One example of a simple control circuit is a circuit that turns a light on and off. In this circuit, the control component is often a single-pole switch.

Control circuits used in commercial and industrial applications tend to be more complex than this simple circuit and employ a broader variety of components. However, the function of these circuits is often the same, to turn something on and off. In some cases, manual control is used. More often, automatic control circuits or circuits that combine manual and automatic control are used.

Manual Control

A simple on-off lighting control circuit illustrates an example of manual control. Manual control requires someone to use a switch to turn something on or off. The device being turned on or off may be a light, as in the previous example. However, many other devices are also controlled manually. For example, a manual starter can be used to start and stop a motor.

Automatic Operation

While manual control of machines is still common practice, many machines are started and stopped automatically or by some combination of manual and automatic control. Automatic control occurs when circuits can turn something on and off without human interaction.

Control Components

A wide variety of components are used in control circuits. This includes components that vary in complexity from indicator lights to advanced systems that monitor, protect, and control AC motors. In some cases, the interaction of these components is dependent only on how they are wired to each other. This is sometimes referred to as hard-wired logic. Increasingly, however, these components are wired to a control system, such as a programmable logic controller or variable speed drive. In such cases, the interaction of the circuit components is dependent both on wiring and the software stored in the controller.

The complete range of Siemens control components is too extensive to be fully addressed in this course. However, this course will give you a good start.

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Basics of Control Components

Control

Control components are used in a wide variety of applications with varying degrees of complexity. One example of a simple control circuit is a circuit that turns a light on and off. In this circuit, the control component is often a single-pole switch.

Control circuits used in commercial and industrial applications tend to be more complex than this simple circuit and employ a broader variety of components. However, the function of these circuits is often the same, to turn something on and off. In some cases, manual control is used. More often, automatic control circuits or circuits that combine manual and automatic control are used.

Manual Control

A simple on-off lighting control circuit illustrates an example of manual control. Manual control requires someone to use a switch to turn something on or off. The device being turned on or off may be a light, as in the previous example. However, many other devices are also controlled manually. For example, a manual starter can be used to start and stop a motor.

Automatic Operation

While manual control of machines is still common practice, many machines are started and stopped automatically or by some combination of manual and automatic control. Automatic control occurs when circuits can turn something on and off without human interaction.

Control Components

A wide variety of components are used in control circuits. This includes components that vary in complexity from indicator lights to advanced systems that monitor, protect, and control AC motors. In some cases, the interaction of these components is dependent only on how they are wired to each other. This is sometimes referred to as hard-wired logic. Increasingly, however, these components are wired to a control system, such as a programmable logic controller or variable speed drive. In such cases, the interaction of the circuit components is dependent both on wiring and the software stored in the controller.

The complete range of Siemens control components is too extensive to be fully addressed in this course. However, this course will give you a good start.

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Basics of AC Motors

AC Motors

AC motors are used worldwide in many applications to transform electrical energy into mechanical energy.

There are many types of AC motors, but this course focuses on threephase AC induction motors, the most common type of motor used in industrial applications. An AC motor of this type may be part of a pump or fan or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer.

Siemens manufactures a wide variety of AC motors. In addition to providing basic information about AC motors in general, this course also includes an overview of Siemens AC motors.

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NEMA Motors

Throughout this course, reference is made to the National Electrical Manufacturers Association (NEMA). NEMA develops standards for a wide range of electrical products, including AC motors.

• For example, NEMA Standard Publication MG 1 covers NEMA frame size AC motors, commonly referred to as NEMA motors.

Above NEMA Motors

In addition to manufacturing NEMA motors, Siemens also manufactures motors larger than the largest NEMA frame size. These motors are built to meet specific application requirements and are commonly referred to as above NEMA motors.

IEC Motors

Siemens also manufactures motors to International Electrotechnical Commission (IEC) standards.

• IEC is another organization responsible for electrical standards. IEC standards perform the same function as NEMA standards, but differ in many respects.

In many countries, electrical equipment is commonly designed to comply with IEC standards. In the United States, although IEC motors are sometimes used, NEMA motors are more common.

Keep in mind, however, that many U.S.-based companies build products for export to countries that follow IEC standards.

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Basics of AC Drives

Totally Integrated Automation

Totally Integrated Automation (TIA) is more than a concept. TIA Automation is a strategy developed by Siemens that emphasizes the seamless integration of automation products.

The TIA strategy incorporates a wide variety of automation products such as programmable controllers, computer numerical controls, Human Machine Interfaces (HMI), and drives which are easily connected via open protocol networks.

PROFIBUS DP

An important aspect of TIA is the ability of devices to communicate with each other over various network protocols, such as Ethernet and PROFIBUS DP.

• PROFIBUS DP is an open bus standard for a wide range of applications in various manufacturing and automation applications.

Siemens AC drives can easily communicate with other control devices such as programmable logic controllers (PLCs) and personal computers (PCs) through the PROFIBUS-DP communication system and other various protocols.

Mechanical Basics

In many commercial, industrial, and utility applications electric motors are used to transform electrical energy into mechanical energy. Those electric motors may be part of a pump or fan, or they may be connected to some other form of mechanical equipment such as a conveyor or mixer. In many of these applications the speed of the system is determined primarily by its mechanical design and loading.

For an increasing number of these applications, however, it is necessary to control the speed of the system by controlling the speed of the motor.

Variable Speed Drives

The speed of a motor can be controlled by using some type of electronic drive equipment, referred to as variable or adjustable speed drives.

• Variable speed drives used to control DC motors are called DC drives. Variable speed drives used to control AC motors are called AC drives. The term inverter is also used to describe an AC variable speed drive. The inverter is only one part of an AC drive, however, it is common practice to refer to an AC drive as an inverter.

Before discussing AC drives it is necessary to understand some of the basic terminology associated with drive operation. Many of these terms are familiar to us in some other context.

Later in the course we will see how these terms apply to AC drives.

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