A magnetic starter is an electromagnetically operated switch which provides a safe method for starting an electric motor with a large load. Magnetic starters also provide under-voltage and overload protection and an automatic cutoff in the event of a power failure.
A magnetic starter has a contactor and an overload relay, which will open the control voltage to the starter coil if it detects an overload on a motor. [1] [2] The overload relay opens a set of contacts that are wired in series with the supply to the contactor feeding the motor. The characteristics of the heaters can be matched to the motor so that the motor is protected against overload. Recently, microprocessor-controlled motor digital protective relays offer more comprehensive protection of motors.
Due to the electromagnet in the contactor, if power to the machine should fail the contactor will automatically disengage. Unlike machines with an ordinary latching switch (such as a common light switch), when the power is resumed the machine will not operate until being turned on again. As a result, magnetic starters often use momentary switches for "off" and "on" functions, as this type of switch returns to a defined normal position when released. Latching switches do not have this feature and therefore are normally not used with a magnetic starter.
Motor control contactors can be fitted with short-circuit protection (fuses or circuit breakers), disconnecting means, overload relays and an enclosure to make a combination starter. Several combination starters and other switchgear and control devices can be grouped in a common enclosure called a motor control center.
Typically starters are operated by two push switches, a "start" switch which is normally-off (i.e. push-to-make) and a "stop" switch which is normally-on (i.e. push-to-break).
When the motor is not running, although line voltage is available, no current is drawn by the starter or motor.
When the "start" button is pressed, the motor is not powered directly, rather the electromagnet in the contactor is energized. The magnetic switch in the contactor then engages, simultaneously switching current to the motor and providing self-sustaining current to maintain its own state. Thus when the start button is released, the magnetic switch remains engaged and the motor remains running.
Pressing the "stop" button breaks the circuit to the contactor which consequently de-energizes its electromagnet thus cutting current to the motor.
Magnetic starters are commonly found on equipment drawing several horsepower or higher. Examples include woodworking machinery such as cabinet saws or shapers. Machines with smaller loads, such as a drill press or most handheld tools normally use only a switch instead. Magnetic starters are stock components for many machines, and aftermarket starters are also available for use as replacements or for retrofitting older machines.
Federal OSHA recognizes NFPA 79 2007 Edition standard 7.5.3 states "Upon restoration of the voltage or upon switching on the incoming supply, automatic or unintentional restarting of the machine shall be prevented when such a restart causes a hazardous condition. [3]
A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof.
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of switch is an electromechanical device consisting of one or more sets of movable electrical contacts connected to external circuits. When a pair of contacts is touching current can pass between them, while when the contacts are separated no current can flow.
Ladder logic was originally a written method to document the design and construction of relay racks as used in manufacturing and process control. Each device in the relay rack would be represented by a symbol on the ladder diagram with connections between those devices shown. In addition, other items external to the relay rack such as pumps, heaters, and so forth would also be shown on the ladder diagram.
A starter is a device used to rotate (crank) an internal-combustion engine so as to initiate the engine's operation under its own power. Starters can be electric, pneumatic, or hydraulic. The starter can also be another internal-combustion engine in the case, for instance, of very large engines, or diesel engines in agricultural or excavation applications.
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by overcurrent. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.
The reed switch is an electromechanical switch operated by an applied magnetic field. It was invented in 1922 by professor Valentin Kovalenkov at the Petrograd Electrotechnical University, and later evolved at Bell Telephone Laboratories in 1936 by Walter B. Ellwood into the reed relay. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near it. When the magnetic field is removed, the contacts in the reed switch return to their original position. The "reed" is the metal part inside the reed switch envelope that is relatively thin and wide to make it flexible, resembling the reed of a musical instrument. The term "reed" may also include the external wire lead as well as the internal part.
A motor controller is a device or group of devices that can coordinate in a predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and electrical faults. Motor controllers may use electromechanical switching, or may use power electronics devices to regulate the speed and direction of a motor.
A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.
In an electric power system, a switchgear is composed of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply.
Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on. Alternating-current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power converters also often have inrush currents much higher than their steady-state currents, due to the charging current of the input capacitance. The selection of over-current-protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The over-current protection must react quickly to overload or short-circuit faults but must not interrupt the circuit when the inrush current flows.
A contactor is an electrically controlled switch used for switching an electrical power circuit. A contactor is typically controlled by a circuit which has a much lower power level than the switched circuit, such as a 24-volt coil electromagnet controlling a 230-volt motor switch.
A wire spring relay is a type of relay, that has springs made from drawn wires of nickel silver, rather than cut from flat sheet metal as in the flat-spring relay. This class of relays provided manufacturing and operating advantages over previous designs. Wire spring relays entered mass production in the early 1950s.
A starter solenoid is an electromagnet which is actuated to engage the starter motor of an internal combustion engine. It is normally attached directly to the starter motor which it controls.
A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source and utilizing an electric brush for contact.
A network protector is a type of electric protective device used in electricity distribution systems. The network protector automatically disconnect its associated distribution transformer from the secondary network when the power starts flowing in reverse direction. Network protectors are used on both spot networks and grid networks. The secondary grid system improves continuity of service for customers, since multiple sources are available to supply the load; a fault with any one supply is automatically isolated by the network protector and does not interrupt service from the other sources. Secondary grids are often used in downtown areas of cities where there are many customers in a small area.
In electrical engineering utilization categories are defined by IEC standards and indicate the type of electrical load and duty cycle of the loads to ease selection of contactors and relays.
In electrical engineering, the Korndorfer starter is a technique used for reduced voltage soft starting of induction motors. The circuit uses a three-phase autotransformer and three three-phase switches. This motor starting method has been updated and improved by Hilton Raymond Bacon.
A motor control center (MCC) is an assembly to control some or all electric motors in a central location. It consists of multiple enclosed sections having a common power bus and with each section containing a combination starter, which in turn consists of motor starter, fuses or circuit breaker, and power disconnect. A motor control center can also include push buttons, indicator lights, variable-frequency drives, programmable logic controllers, and metering equipment. It may be combined with the electrical service entrance for the building.
This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.
In engineering, a solenoid is a device that converts electrical energy to mechanical energy, using an electromagnet formed from a coil of wire. The device creates a magnetic field from electric current, and uses the magnetic field to create linear motion. In electromagnetic technology, a solenoid is an actuator assembly with a sliding ferromagnetic plunger inside the coil. Without power, the plunger extends for part of its length outside the coil; applying power pulls the plunger into the coil. Electromagnets with fixed cores are not considered solenoids. In simple terms, a solenoid converts electrical energy into mechanical work. Typically, it has a multiturn coil of magnet wire surrounded by a frame, which is also a magnetic flux carrier to enhance its efficiency. In engineering, the term may also refer to a variety of transducer devices that convert energy into linear motion, more sophisticated than simple two–position actuators. The term "solenoid" also often refers to a solenoid valve, an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid or linear solenoid. Solenoid bolts, a type of electromechanical locking mechanism, also exist.
