Motor Starters and Contactors
14 Magnetic-Motor Starters
For the control of three-phase motors, magnetic contactors are used to open and close the power contacts in line with the motor. This allows for separation of the control circuit from the power circuit, providing greater safety for the operator and ease and convenience of wiring for the installer. Magnetic contactors also provide low-voltage protection (LVP) in the case of a power outage.
Magnetic contactors must also have built-in overload protection if they are to be used to control motors. The most common controllers for three-phase motors are the across-the-line magnetic starter, meaning that the motor is started with full line voltage.
NEMA vs. IEC
The difference between NEMA and IEC contactors is in their certifications and ratings. NEMA (National Electrical Manufacturers Association) is recognized in North America.
IEC (International electrotechnical commission) is recognized in both North America and Europe.
As a general rule, NEMA equipment is more expensive and robust than IEC equipment, but IEC equipment is more versatile. And because is often cheaper, IEC equipment is more commonly seen in modern installations.
Contactor Construction and Operation
A magnetic motor starter has two basic parts: the magnetic contactor and an overload relay.
The magnetic contactor is a solenoid relay consisting of stationary contacts that are wired in series with the lines to the motor, an inductive coil wrapped around a magnetic core and a movable armature attached to moveable contacts. When an electric current is passed through the coil of wire, a magnetic field is created. This field, in turn, draws the armature towards it, causing the movable contacts to bridge the gap of the stationary contacts and thus energizing the motor. A spring is constantly trying to open the contacts but as long as there is a voltage present at the coil, magnetic forces will overcome the strength of this spring.
When there is a power outage, however, and the current through the coil drops below a threshold, the spring will push open the contacts. If power is restored, the motor load will not re-energize but will instead require further input from the operator. This type of control is called three-wire control and it provides low-voltage protection (LVP).
For the control of three-phase motors, contactors are built with three sets of horsepower-rated contacts. Additional auxiliary contacts can be included as well. Relay contacts are usually silver-plated to improve their conductivity and although single-break contacts are used, most industrial quality relays use double-break contacts to improve their interrupting capability.
Coils are generally designed to activate at approximately 85% of their rated voltage and will not deactivate until the voltage falls below approximately 85% of the rated value. Usually a coil can withstand an overvoltage of up to 10% without damage to the coil.
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