Industrial automation glossary
66 terms in plain language. If a term is missing or unclear, tell us and we will add it.
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- 4–20 mA loop
- An analogue signalling standard where a current between 4 and 20 mA represents a measured value. 4 mA is the bottom of the range and 20 mA the top, so a reading of 0 mA signals a fault such as a broken wire.See also: Analogue input, NAMUR NE 43
A
- Analogue input
- A PLC input that reads a continuously varying signal, usually 0–10 V or 4–20 mA, from a sensor such as a pressure or temperature transmitter.
B
- Backplane
- The bus inside a rack or chassis that connects the modules to each other and to the controller, carrying power and data between them.
- Braking resistor
- A resistor connected to a drive that dissipates the energy a motor feeds back when it slows a high-inertia load quickly, preventing DC bus overvoltage trips.See also: DC bus, Variable frequency drive
C
- Catalogue number
- The manufacturer's code that identifies one specific product variant. Allen-Bradley uses this term, for example 1756-L71. Siemens calls it an order number or MLFB.See also: Order number, MLFB
- Circuit breaker
- A protective device that automatically interrupts current when it exceeds a set level. Miniature (MCB) and moulded-case (MCCB) breakers protect cables and equipment from overload and short circuit.
- Commissioning
- Testing and adjusting new or replaced equipment so it runs correctly in the plant, including checking wiring, setting parameters and proving the control sequence.
- Constant torque load
- A load, such as a conveyor or crusher, that needs roughly the same torque at all speeds. These need drives sized for heavy duty. Compare with variable torque loads such as fans and pumps.See also: Variable torque load
- Contactor
- An electrically operated switch used to start and stop motors and other heavy loads. A small control signal on the coil switches a much larger current.
- Control Unit
- In modular drives such as the Siemens SINAMICS G120, the part that holds the control logic, I/O and communication, separate from the Power Module that handles the motor current.
- CPU
- The central processing unit of a PLC. It runs the user program, reads inputs, writes outputs and handles communication.
D
- DC bus
- The internal DC link inside a variable frequency drive, between the rectifier and the inverter. Its voltage rises when a motor regenerates energy, which can cause an overvoltage trip.
- Derating
- Reducing the allowed output of a device in demanding conditions such as high temperature or altitude. Drives and power supplies are commonly derated above their rated ambient temperature and above about 1000 m altitude.
- Digital input / output
- A PLC channel that reads or switches a simple on/off signal, for example a limit switch input or a contactor coil output.
- DIN rail
- A standardised metal rail, usually 35 mm wide, on which control panel components such as terminals, relays, breakers and power supplies are clipped.
- Direct on line (DOL) starter
- The simplest motor starter, where the motor is connected straight to the supply through a contactor. It draws a high starting current, typically six to eight times full-load current.See also: Soft starter, Star-delta starter
- Discontinued
- A manufacturer status meaning a product is no longer made or sold. Spares may still exist in the market, and an upgrade path is normally published.See also: Obsolescence
- Drop-in replacement
- A replacement part that fits and works without any changes to wiring, mounting or the program.
E
- Earth fault
- An unwanted current path from a live conductor to earth. On a drive it commonly points to damaged motor cable or motor insulation.
- EMC filter
- A filter fitted to a drive to reduce electrical noise (electromagnetic interference) conducted back to the supply or radiated by the motor cable.
- Encoder
- A sensor that reports the position or speed of a shaft, used for closed-loop speed control and positioning with servos and drives.
- EtherNet/IP
- An industrial network protocol that carries the Common Industrial Protocol over standard Ethernet. Widely used with Allen-Bradley and Rockwell Automation systems.
F
- Fault finding
- Systematically diagnosing why a machine or control system is not working, to identify the actual cause rather than just replacing parts.See also: Commissioning
- Fieldbus
- A digital communication network connecting controllers to field devices such as drives, sensors and remote I/O. PROFIBUS and Modbus RTU are examples.
- Firmware
- The embedded software inside a device such as a PLC CPU or drive. Firmware version affects compatibility with engineering software and with replacement parts.
- Full-load current (FLC)
- The current a motor draws when delivering its rated power at rated voltage. It is printed on the nameplate and is the figure used to size drives, starters and overloads.
- Function block diagram (FBD)
- A graphical PLC programming language where functions are drawn as connected blocks. One of the languages defined in IEC 61131-3.See also: Ladder logic, Structured text
H
- HMI
- Human-machine interface. A screen or panel where operators see the state of a machine or process, change setpoints and acknowledge alarms.
I
- I/O module
- A PLC module that provides inputs and outputs, either digital or analogue, connecting the controller to field devices.
- IE efficiency class
- A classification of motor efficiency defined in IEC 60034-30-1, from IE1 (standard) up to IE4 (super premium).
- Insulation resistance test
- A test, often called a megger test, that measures the resistance of motor or cable insulation to earth. It should be done with the drive disconnected, because the test voltage can damage drive electronics.
- IP rating
- A two-digit code from IEC 60529 describing protection against solids (first digit) and water (second digit). IP65 is dust-tight and protected against water jets.See also: Ingress protection
L
- Ladder logic
- A graphical PLC programming language that resembles electrical relay diagrams. One of the languages defined in IEC 61131-3.See also: Function block diagram, Structured text
- Last-time buy
- A final opportunity to order a product before it is discontinued, usually announced by the manufacturer. Used to stock spares for the remaining life of an installation.
- Lifecycle status
- A manufacturer's classification of where a product is in its life, commonly active, phase-out and discontinued. Names differ between brands.
- Line reactor
- An inductor fitted on the supply side of a drive to reduce harmonic currents and protect the drive from supply spikes.
- Load shedding
- Planned interruption of electricity supply to reduce demand. For control systems, the risk is mostly at power restoration, when surges and dips can damage equipment.
M
- Megger
- An instrument that measures insulation resistance using a high test voltage. See insulation resistance test.
- MLFB
- The Siemens machine-readable product designation, better known as the order number, for example 6ES7 313-5BG04-0AB0.See also: Order number
- Modbus
- A simple, widely supported communication protocol. Modbus RTU runs over serial RS-485 and Modbus TCP runs over Ethernet.
- Motor protection circuit breaker
- A breaker designed to protect a motor and its cable against overload and short circuit, often combining the functions of a fuse and an overload relay.
N
- Nameplate
- The label on a motor, drive or module that lists its ratings and identification numbers. The most useful starting point when ordering a replacement.
- NAMUR NE 43
- A standard that defines how analogue transmitters signal faults. Currents below 3.6 mA or above 21 mA indicate a fault, and 3.8 to 20.5 mA is the valid measuring range.
O
- Obsolescence
- When a component can no longer be bought new from the manufacturer. Obsolescence planning means identifying these parts early and deciding between stocking spares and upgrading.
- Order number
- The manufacturer's code for a specific product variant. Siemens calls it the order number or MLFB. Quote the full number, including version blocks, when asking for a replacement.See also: Catalogue number, MLFB
- Overload relay
- A device that protects a motor from sustained overcurrent by tripping a contactor. Thermal overloads use a heating element, electronic ones measure current.
P
- PID control
- Proportional-integral-derivative control, a feedback method used to hold a process value such as temperature or pressure at a setpoint.
- PLC
- Programmable logic controller. A rugged industrial computer that reads inputs, runs a program and drives outputs to control machines and processes.
- Power factor
- The ratio of real power to apparent power in an AC circuit. Motors typically run at a power factor of 0.8 to 0.9 at full load.
- Power Module
- In modular drives, the part that contains the power electronics and connects to the motor and supply. It is paired with a Control Unit.
- PROFIBUS
- A long-established fieldbus standard, widely used with Siemens systems. Cabled with a dedicated shielded cable and terminated at each end.
- PROFINET
- An industrial Ethernet standard from the PROFIBUS organisation, used with Siemens and many other systems for real-time control and diagnostics.
R
- Redundant power supply
- Two power supplies connected through a redundancy module so that one can fail without interrupting the 24 V rail.
- Retrofit
- Upgrading part of an existing installation with newer equipment, often replacing an obsolete controller or drive while keeping the rest of the system.
S
- SCADA
- Supervisory control and data acquisition. Software that monitors and controls a plant or process, collecting data from many PLCs and presenting it to operators.
- Service factor
- A multiplier on a motor's rated power that indicates how much overload it can tolerate continuously. A service factor of 1.0 means no allowance for overload.
- Servo drive
- A drive that controls a servo motor with precise closed-loop control of position, speed and torque, using feedback from an encoder.
- Soft starter
- A device that ramps up the voltage to a motor at start, reducing starting current and mechanical shock. Unlike a drive, it does not control speed while running.See also: Direct on line starter, Variable frequency drive
- Star-delta starter
- A starter that connects the motor windings in star for starting and then in delta for running, reducing starting current to about a third of direct on line.
- Structured text (ST)
- A text-based PLC programming language, similar to Pascal. One of the languages defined in IEC 61131-3.
- Surge protective device (SPD)
- A device that diverts voltage surges, from lightning or switching, away from sensitive equipment. Fitted at the incoming supply and, in stages, closer to the equipment.
- Synchronous speed
- The speed of the rotating magnetic field in an AC motor, equal to 120 × frequency ÷ poles. At 50 Hz it is 1500 rpm for a four-pole motor.
T
- TIA Portal
- The Siemens engineering software used to configure and program S7-1200 and S7-1500 PLCs, HMIs and drives.
V
- Variable frequency drive (VFD)
- A drive that controls an AC motor's speed and torque by varying the frequency and voltage it supplies. Also called a variable speed drive or inverter.
- Variable torque load
- A load, such as a centrifugal fan or pump, whose torque demand falls sharply at lower speed. These can often use a drive's normal-duty rating.See also: Constant torque load
W
- Watchdog
- A timer in a controller that detects if the program has stopped running normally and forces a safe state or restart.
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