In the world of electrical motor controls, two particular components – contactors and overload relays – combine to make a device known as a motor starter, which is used to start, stop, and reverse the direction of a connected load such as a motor. Motor starters can also be used to protect the connected load from a sudden increase in current or other potentially dangerous situations. This post discusses the functionality of contactors and overload relays, and how two common electrical control standards – NEMA and IEC – can be applied to such components.
Contactors
There are two circuits involved with the operation of a contactor – the control circuit and the power circuit. The control circuit is connected to the coil of an electromagnet, and the power circuit is connected to the stationary contacts.
When the control circuit supplies power to the coil, a magnetic field is produced, magnetizing the electromagnet. The magnetic field attracts the armature to the magnet, which in turn closes the contacts. With the contacts closed, current flows through the power circuit from the line to the load.
When current no longer flows through the control circuit, the electromagnet’s coil de-energizes, the magnetic field collapses,
and the movable contacts open under spring pressure.
Overload relays
Contactors are used to control power in a variety of applications. When used in motor-control applications, contactors can only start and stop the motors. They cannot sense when the motor is being overloaded, and they provide no overload protection.
Most motor applications require overload protection. Although some small motors have overload protection built into the motor, more commonly, overload relays (such as the ones shown in the accompanying graphic) are used.
Overload relays are designed to meet the special protective needs of motors. Overload relays continue to provide power during the short duration overloads that occur when a motor starts. If an overload condition persists, however, the overload relay removes power from the motor. Once the overload condition has been corrected, the overload relay can be reset to allow the motor to be restarted.
What are NEMA and IEC motor controls?
Motor starters are most often manufactured to specifications provided either by the National Electrical Manufacturer’s Association (NEMA) or the International Electrotechnical Commission (IEC).
Both NEMA and IEC motor starters have their benefits. Often the application and market use implies which version to select, as highlighted in the table below:
