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.

Implementation

A magnetic starter has a

contactor :''In semiconductor testing, contactors can also be referred to as the specialized socket that connects the device under test.'' :''In process industries, a contactor is a vessel where two streams interact, for example, air and liquid. See Gas ...

and an overload relay, which will open the control voltage to the starter coil if it detects an overload on a motor. Overload relays may rely on heat produced by the motor current to operate a

bimetal Bimetal refers to an object that is composed of two separate metals joined together. Instead of being a mixture of two or more metals, like alloys, bimetallic objects consist of layers of different metals. Trimetal and tetrametal refer to objects co ...

contact or release a contact held closed by a low-melting-point alloy. The overload relay opens a set of contacts that are

wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...

d 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 relay In utility and industrial electric power transmission and distribution systems, a numerical relay is a computer-based system with software-based protection algorithms for the detection of electrical faults. Such relays are also termed as micropr ...

s 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 A latching switch is a switch that maintains its state after being activated. A push-to-make, push-to-break switch would therefore be a latching switch – each time you actuate it, whichever state the switch is left in will persist until the switc ...

(such as a common

light switch In electrical wiring, a light switch is a switch most commonly used to operate electric lights, permanently connected equipment, or electrical outlets. Portable lamps such as table lamps may have a light switch mounted on the socket, base, or i ...

), when the power is resumed the machine will not operate until being turned on again. As a result, magnetic starters often use

momentary switch 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 typ ...

es for "off" and "on" functions, as this type of switch returns to a defined normal position when released.

Latching switch A latching switch is a switch that maintains its state after being activated. A push-to-make, push-to-break switch would therefore be a latching switch – each time you actuate it, whichever state the switch is left in will persist until the switc ...

es 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.

Operation

Typically starters are operated by two

push switch A push switch (button) is a momentary or non-latching switch which causes a temporary change in the state of an electrical circuit only while the switch is physically actuated. An automatic mechanism (i.e. a spring) returns the switch to it ...

es, 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.

Applications

Magnetic starters are commonly found on equipment drawing several horsepower or higher. Examples include woodworking machinery such as cabinet saws or

shaper A shaper is a type of machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is (archetypally) linear instead of ...

s. Machines with smaller loads, such as a

drill press A drill is a tool used for making round holes or driving fasteners. It is fitted with a bit, either a drill or driverchuck. Hand-operated types are dramatically decreasing in popularity and cordless battery-powered ones proliferating due to i ...

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. NFPA79:7.5.3

References

{{Switches Over-current protection devices Switches