Industrial Motor Control Using Contactors

Motor contactors are large, high-current rated electromechanical relays. These are special relays in the sense that, they have heavy duty contacts that can handle the motor’s inrush current over many start and stop cycles. The contactor is actuated by an electromagnetic solenoid (coil) which pulls the contacts closed when energized. They also have an arc-suppressing cover to quench the arc formed when the contacts open under load.

Let’s consider a three-phase contactor connected to a three-phase motor (with fuses for overcurrent protection) as demonstrated in the figure below:

Energizing terminals A1 and A2 magnetizes the electromagnet coil, causing all three switch contacts to close at the same time, sending three-phase AC power to the motor. De-energizing the coil causes it to de-magnetize, releasing the armature and enabling a return spring inside the contactor to snap all three contacts to the open (OFF) position.

If you are designing a system, take note of the fact that, AC contactors should never be used to break the current of a loaded DC power circuit. It is more difficult to quench the arc from a DC power load because there is no AC “zero crossing” to interrupt the current flow. DC contactors are designed to specifically handle DC current. You will also notice that, they have embedded magnets, or special blow-out coils, that are used to stretch the arc long enough to break the DC current flow.

Also read:

• Power MOSFET Motor Control
• How Servo Motors are used in Process Control
• Speed Control of a DC Series Motor
• Permanent Magnet DC Motors – Operation & Applications

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