A Programmable Logic Controller, PLC or Programmable Controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures.
PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact.
Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.
The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking.
The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers.
PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.
PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs.
The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.
A motor soft starter is a device used with AC electric motors to temporarily reduce the load and torque in the powertrain and electrical current surge of the motor during startup.
This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.
Motor soft starters can consist of mechanical or electrical devices, or a combination of both.
Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter.
Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage or current input, or a device that temporarily alters how the motor is connected in the electric circuit.
Soft starters can be set up to the requirements of the individual application. In pump applications, a soft start can avoid pressure surges.
Conveyor belt systems can be smoothly started, avoiding jerk and stress on drive components. Fans or other systems with belt drives can be started slowly to avoid belt slipping.
In all systems, a soft start limits the inrush current and so improves stability of the power supply and reduces transient voltage drops that may affect other loads.
Adjustable speed drive (ASD) or variable-speed drive (VSD) describes equipment used to control the speed of machinery. Many industrial processes such as assembly lines must operate at different speeds for different products. Where process conditions demand adjustment of flow from a pump or fan, varying the speed of the drive may save energy compared with other techniques for flow control.
Where speeds may be selected from several different pre-set ranges, usually the drive is said to be adjustable speed. If the output speed can be changed without steps over a range, the drive is usually referred to as variable speed.
Adjustable and variable speed drives may be purely mechanical (termed variators), electromechanical, hydraulic, or electronic.
Process control and energy conservation are the two primary reasons for using an adjustable speed drive.
Historically, adjustable speed drives were developed for process control, but energy conservation has emerged as an equally important objective.
Adjusting speed as a means of controlling a process
The following are process control benefits that might be provided by an adjustable speed drive:
- Smoother operation
- Acceleration control
- Different operating speed for each process recipe
- Compensate for changing process variables
- Allow slow operation for setup purposes
- Adjust the rate of production
- Allow accurate positioning
- Control torque or tension
- Allow catching of spinning load (e.g., column of water) after outage.
In electrical engineering, a protective relay is an electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected.
Unlike switching type relays with fixed and usually ill-defined operating voltage thresholds and operating times, protective relays have well-established, selectable, time/current (or other operating parameter) operating characteristics.
Protection relays may use arrays of induction disks, shaded-pole magnets, operating and restraint coils, solenoid-type operators, telephone-relay contacts, and phase-shifting networks.
Protection relays respond to such conditions as over-current, over-voltage, reverse power flow, over- and under- frequency.
Distance relays trip for faults up to a certain distance away from a substation but not beyond that point.
The functions of electromechanical protection systems are now being replaced by microprocessor-based digital protective relays, sometimes called "numeric relays".
A microprocessor-based digital protection relay can replace the functions of many discrete electromechanical instruments.
These convert voltage and currents to digital form and process the resulting measurements using a microprocessor.
The digital relay can emulate functions of many discrete electromechanical relays in one device, simplifying protection design and maintenance.
Each digital relay can run self-test routines to confirm its readyness and alarm if a fault is detected.
Numeric relays can also provide functions such as communications (SCADA) interface, monitoring of contact inputs, metering, waveform analysis, and other useful features.
Digital relays can, for example, store two sets of protection parameters, which allows the behavior of the relay to be changed during maintenance of attached equipment.
Digital relays also can provide protection strategies impossible to synthesize with electromechanical relays, and offer benefits in self-testing and communication to supervisory control systems.