AC motor starters are used to turn on and off electric motors and motor-controlled equipment. AC motor starters include motor controllers, usually contactors, and overload protection. For motor loads, motors contactors are devices that connect or disconnect a motor from the power supply. Through the use of a coil and a magnetic armature frame, an electromagnetic circuit operates the power poles.
To prevent excess current draw, AC motor starters use eutectic, bimetallic, or solid-state overload protection. Eutectic overload relays include a heater coil and a eutectic (melting) tube attached to a tripping device. When an overload occurs, the heater coil melts the alloy tube, initiating a switching action. Bimetallic devices are permanently joined strips of two different metals that also use heat to protect against overloads. Heating a bimetallic device causes the strip to bend because each metal expands and contracts at a different rate. As the strip bends, the spring pulls the contacts apart, breaking the circuit. Unlike eutectic or bimetallic overload protection, solid-state relays do not use heat to trip circuits. Instead, they provide overload protection by measuring current or changes in resistance.
To prevent excess current draw, AC motor starters use eutectic, bimetallic, or solid-state overload protection. Eutectic overload relays include a heater coil and a eutectic (melting) tube attached to a tripping device. When an overload occurs, the heater coil melts the alloy tube, initiating a switching action. Bimetallic devices are permanently joined strips of two different metals that also use heat to protect against overloads. Heating a bimetallic device causes the strip to bend because each metal expands and contracts at a different rate. As the strip bends, the spring pulls the contacts apart, breaking the circuit. Unlike eutectic or bimetallic overload protection, solid-state relays do not use heat to trip circuits. Instead, they provide overload protection by measuring current or changes in resistance.
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Last Updated: Sep 2009
Last Updated: Sep 2009
AC motor starters can also provide low voltage protection or interrupt power in one or more wires. Multi-speed starters operate at a constant frequency and voltage, while reduced voltage starters operate at a constant frequency and speed. Manual AC motor starters are suitable for any application where the motor runs in only direction, at one speed, and where the draw of inrush current at start up is not a concern. Reversing voltage starters help prevent overloading the power distribution system. They also reduce wear and tear on equipment by reducing the starting torque.
AC motor starters have National Electric Manufacturers Association (NEMA) ratings and International Engineering Consortium (IEC) ratings. NEMA ratings indicate the continuous current rating and horsepower at rated voltages. IEC ratings indicate thermal current ratings or operational currents. Typically, manufacturers of AC motor starters who list NEMA sizes also include the continuous current or ampere rating for each size. NEMA ratings are numbered from 0 to 9, and include a rating of 00. IEC ratings are lettered and range from A to N.
AC motor starters have National Electric Manufacturers Association (NEMA) ratings and International Engineering Consortium (IEC) ratings. NEMA ratings indicate the continuous current rating and horsepower at rated voltages. IEC ratings indicate thermal current ratings or operational currents. Typically, manufacturers of AC motor starters who list NEMA sizes also include the continuous current or ampere rating for each size. NEMA ratings are numbered from 0 to 9, and include a rating of 00. IEC ratings are lettered and range from A to N.
