In recent years, there has been a steady evolution toward the use of solid-state components in industrial control. This does not mean that all electromechanical devices will be replaced, but they will share a smaller part of the industrial control spectrum in the future.
This section covers the drawings encountered in many different industrial control situations, from basic motor control to computer control. Almost all industrial control drawings follow some drawing standard or recommended practice, and these standards will also be described.
In general, as mentioned previously in the text, there are several types of drawing that will be associated with all types of control. They are: (1) the elementary diagram or schematic, (2) the wiring diagram, (3) the layout, (4) the block plan, (5) the parts list, and (6) the assembly drawings. However, some types of industrial control have associated specialized drawings, which will also be covered. An interesting phenomenon takes place in industrial control drawings; namely, as the control circuits become more sophisticated and complex, the drawings tend to become simpler. This will become obvious as the section proceeds.
A word of caution should be inserted here. Although this section presents industrial control drawings in discrete categories (e.g., electromechanical, solid-state logic, etc.), in reality, these types of control are sometimes used together. This can, in some instances, present nightmares to the drafter putting them down on paper. Basic concepts will show how to ease this interfacing chore.
2 Basic Motor Control
Most designers and drafters doing industrial control work start by “cutting their teeth” on some type of motor control circuit. Before delving into the types of drawing used in motor control circuit design, it is a good idea to first understand the functions of motor control and some of the components associated with motor controllers. There are certain definite functions that are performed or governed by motor control. These are: (1) starting, (2) stopping, (3) running, (4) speed regulation, and (5) protection. Starting technically refers to rotating the motor from zero speed to maximum (breakdown) torque speed. Running refers to propelling the motor from maximum torque speed to load speed (usually faster). These two functions are accomplished by one device in most motor control circuits. Speed regulation is done by some device such as a rheostat in a motor control circuit. Motor control circuits provide protection for both the power source and the motor. An example of one of the motor control functions is “reduced voltage starting,” which reduces the current inrush during the starting period of a motor or machine. This eliminates or minimizes the shock of a quick start on the driven machine, the reduction of voltage in the line or system to which the machine is connected, and the dropping out of synchronism of synchronous motors on the line. Reduced-voltage starters, which pro vide smooth accelerated starting without a serious drop in line voltage, are available.
A large motor in a New England plant requires 56s starting time under normal load. An oil-well pump in Texas will suffer serious damage if its rotor locks and the motor is not tripped from the line in 20s. A conveyor drive motor in a Florida potash plant can withstand 25 percent overload for 30 mm, but a compressor motor in Missouri may burn up in 3 mm at 25 percent overload. Each of these motors must be protected from abnormal overload currents, which creep in from time to time.
An electric motor controller is a device, or group of devices, which controls the electric power delivered to the motor. Motor controls range in complexity from a simple manual motor starter to an elaborate motor control center. Some basic control devices which may be built into motor controllers, as well as other industrial controls, are described in the following paragraphs.
A circuit breaker has the function of interrupting the flow of power in a circuit under normal and abnormal conditions. Although it might be used as a power (disconnect) switch, its primary function is to protect the line against abnormally high currents. Hence, it is designed for infrequent operation only. Just as frequently, a line switch and a set of fuses are used in place of a circuit breaker. The switch is used as the disconnect, while the fuses protect the line against faults.
A contactor is a device for repeatedly establishing and interrupting an electric power circuit. In essence, it is a specialized type of relay. Usually operated magnetically, it can be operated in line or can be remotely governed by pilot devices or relays. The overload relay is used to interrupt maximum over load current or to remove the power supply from a starter and motor under a normal overload. Pushbutton switches are used to energize or deenergize the motor controller, and thus startand stop the motor, respectively. Transformers are used to reduce the line (power-source) voltage to the control-circuit voltage. Sometimes, transformers are not used, but most designers prefer to have their control voltages at 120 V ac or less for safety reasons. Many motor controllers have indicating, or pilot, lights on them to show whether the motor is running or stopped.
The types of drawing encountered when doing motor control work are the elementary diagram, the wiring diagram, the layout, the motor-control-center layout, and the motor-control-center schedule. FIG. 1 shows two types of motor controller. FIG. 1 shows the elementary diagram of a motor controller. This diagram may appear a little strange to the reader because of the unusual symbols. Unusual symbols can appear because of two factors: (1) different devices are often used in industrial controls than in computer and communication equipment, and (2) different, or alternative, symbols are often used for components that are commonly used. A number of alternative symbols are used in diagrams of this type. These include the contacts and heaters. The unit shown in this diagram is a combination motor starter. Combination means that it contains both the motor starter and a power-source disconnect (switch). Usually in industrial control, combination motor starters are used.
In recent years, there has been a steady evolution toward the use of solid-state components in industrial control. This does not mean that all electromechanical devices will be replaced, but they will share a smaller part of the industrial control spectrum in the future.
This section covers the drawings encountered in many different industrial control situations, from basic motor control to computer control. Almost all industrial control drawings follow some drawing standard or recommended practice, and these standards will also be described.
In general, as mentioned previously in the text, there are several types of drawing that will be associated with all types of control. They are: (1) the elementary diagram or schematic, (2) the wiring diagram, (3) the layout, (4) the block plan, (5) the parts list, and (6) the assembly drawings. However, some types of industrial control have associated specialized drawings, which will also be covered. An interesting phenomenon takes place in industrial control drawings; namely, as the control circuits become more sophisticated and complex, the drawings tend to become simpler. This will become obvious as the section proceeds.
A word of caution should be inserted here. Although this section presents industrial control drawings in discrete categories (e.g., electromechanical, solid-state logic, etc.), in reality, these types of control are sometimes used together. This can, in some instances, present nightmares to the drafter putting them down on paper. Basic concepts will show how to ease this interfacing chore.
2 Basic Motor Control
Most designers and drafters doing industrial control work start by “cutting their teeth” on some type of motor control circuit. Before delving into the types of drawing used in motor control circuit design, it is a good idea to first understand the functions of motor control and some of the components associated with motor controllers. There are certain definite functions that are performed or governed by motor control. These are: (1) starting, (2) stopping, (3) running, (4) speed regulation, and (5) protection. Starting technically refers to rotating the motor from zero speed to maximum (breakdown) torque speed. Running refers to propelling the motor from maximum torque speed to load speed (usually faster). These two functions are accomplished by one device in most motor control circuits. Speed regulation is done by some device such as a rheostat in a motor control circuit. Motor control circuits provide protection for both the power source and the motor. An example of one of the motor control functions is “reduced voltage starting,” which reduces the current inrush during the starting period of a motor or machine. This eliminates or minimizes the shock of a quick start on the driven machine, the reduction of voltage in the line or system to which the machine is connected, and the dropping out of synchronism of synchronous motors on the line. Reduced-voltage starters, which pro vide smooth accelerated starting without a serious drop in line voltage, are available.
A large motor in a New England plant requires 56s starting time under normal load. An oil-well pump in Texas will suffer serious damage if its rotor locks and the motor is not tripped from the line in 20s. A conveyor drive motor in a Florida potash plant can withstand 25 percent overload for 30 mm, but a compressor motor in Missouri may burn up in 3 mm at 25 percent overload. Each of these motors must be protected from abnormal overload currents, which creep in from time to time.
An electric motor controller is a device, or group of devices, which controls the electric power delivered to the motor. Motor controls range in complexity from a simple manual motor starter to an elaborate motor control center. Some basic control devices which may be built into motor controllers, as well as other industrial controls, are described in the following paragraphs.
A circuit breaker has the function of interrupting the flow of power in a circuit under normal and abnormal conditions. Although it might be used as a power (disconnect) switch, its primary function is to protect the line against abnormally high currents. Hence, it is designed for infrequent operation only. Just as frequently, a line switch and a set of fuses are used in place of a circuit breaker. The switch is used as the disconnect, while the fuses protect the line against faults.
A contactor is a device for repeatedly establishing and interrupting an electric power circuit. In essence, it is a specialized type of relay. Usually operated magnetically, it can be operated in line or can be remotely governed by pilot devices or relays. The overload relay is used to interrupt maximum over load current or to remove the power supply from a starter and motor under a normal overload. Pushbutton switches are used to energize or deenergize the motor controller, and thus startand stop the motor, respectively. Transformers are used to reduce the line (power-source) voltage to the control-circuit voltage. Sometimes, transformers are not used, but most designers prefer to have their control voltages at 120 V ac or less for safety reasons. Many motor controllers have indicating, or pilot, lights on them to show whether the motor is running or stopped.
The types of drawing encountered when doing motor control work are the elementary diagram, the wiring diagram, the layout, the motor-control-center layout, and the motor-control-center schedule. FIG. 1 shows two types of motor controller. FIG. 1 shows the elementary diagram of a motor controller. This diagram may appear a little strange to the reader because of the unusual symbols. Unusual symbols can appear because of two factors: (1) different devices are often used in industrial controls than in computer and communication equipment, and (2) different, or alternative, symbols are often used for components that are commonly used. A number of alternative symbols are used in diagrams of this type. These include the contacts and heaters. The unit shown in this diagram is a combination motor starter. Combination means that it contains both the motor starter and a power-source disconnect (switch). Usually in industrial control, combination motor starters are used.
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