Vacuum contactors

👉 Content :

Introduction
Construction and Working of vacuum contactors
Performance
Application ( Is vacuum contactor is suitable for 3ph 120 hp SCR induction motor? )

👉 Introduction :

In a conventional motor starter and motor control panel all control equipment are operate in air. Air contain moisture, dust particles this cause corrosion and affect the performance of the equipment.

and reduce the reliability and increase downtime.

In concept of Vacuum contactor contact operate in closed vacuum cylinder. Vacuum provide high dielectric strength .Vacuum technology is becoming viable alternative for reducing downtime in harsh, dirty environments.

In this article we discuss about vacuum contactor and its use for motor control.

👉 Working and Construction of vacuum contactors :

Working

Commonly used contactor for motor starter and control applications are “air break” contacts , i.e the “make/break” action of contacts are takes place in open air.

In a vacuum contactor, the electrical contacts are in a vacuum bottle .This sealed environment in which the switching takes place allows for a fast, clean “make/break” action of the contacts in harsh, dirty environments.

The vacuum bottle prevents the external environment from affecting its operation.

When closed contacts carrying current are separated, the current continues to flow through space, which is observed by the eye as an arc.

This arc harms the contact surface material and performance of system .

Once in vacuum contactor, the contacts are opened within the vacuum, then an arc can be generated among the contacts through the metal vapors ionization in the contacts.

But, the arc can be quenched easily as the electrons, ions & metallic vapors are generated throughout the arc quickly condense over the outsides of the CB contacts, so the dielectric strength can be quickly recovered.

The most important feature of a vacuum is that once the arc is generated within the vacuum, then it can be extinguished quickly because of the quick improvement rate in the dielectric strength of the vacuum.

Construction:

vacuum contactor

Since , Arc burn on contact so the material and the construction set of contact is design for fast arc quenching.

Material of contact used in vacuum contactor is harder than of air break contactor , for low voltage vacuum contactors dual material alloy is use e.g. WCu (Tungsten Copper) or WAg (Tungsten Silver).

This material helps to reduce the possibility of welding. The contacts are larger allowing heat to dissipate more quickly. The reduced arc time reduces contact erosion. With minimal erosion , the contacts remain clean and dependable.

The vacuum contactor comprises a steel arc chamber in the center-symmetrically arranged ceramic insulators. The pressure inside the vacuum interrupter is maintained below 10^-4 torr .

The moving contact is connected to the control mechanism by stainless steel bellow.

The arc shields are supported o the insulating housing such that they cover these shields and is prevented from condensing on the insulating enclosure.

The possibility of a leak is eliminated due to the permanent sealing of the vacuum chamber for that a glass vessel or ceramic vessel is used as the outer insulating body.

👉 Performance:

1) Contact reliability:

Sealed environment , hard contact material and high dielectric strength helps in fast quench of arc produce between contact leads to,

High contact performance
Free form harm full environment
Improved arc extinguishing above 600V
restricting voltage is low for capacitive loads
contact material and construction helps the arc extinguish quickly, and less damage is experienced.

2) Mechanical Reliability:

Contact experience minimum mechanical movement in vacuum chamber, typically 0.08 inch . Less movement gives high mechanical durability .

3) System Reliability:

Vacuum contactors are design for the specific inductive load switching for motor load applications. This helps reduce the possibility of insulation failure on downstream motors and transformers in comparison to typical air-break contactors. If a problem does occur with the contact or inside the vacuum bottle, the bottles are replaceable.

4) Overall benefits:

Quiet operation, reduced maintenance and down-time, long service life.

👉 Application :

Whether based on system load or harsh environment with possible contaminants, the

vacuum contactor is designed with tough applications in mind. The vacuum contactor

design has incorporated specific requirements of inductive load switching for motor load

applications.

Typical Product Applications:

• Mining

• Oil Well Pumping

• Waste Water Treatment

• Rock Crushing/Cement

• Petrochemical

• Pulp and Paper

• Earth Movers

We discus about this question, Is vacuum contactor is suitable for 3ph 120 hp SCR induction motor?

120 hp induction motor is high current and inductive machine. If we use air break contactor due to high current and highly inductive load the arc length and arc time is more so high risk of welding of contact,

now if we use vacuum contactor , due to high dielectric strength of vacuum arc extinguish fast so arc time and arc length is less .

so it is good to use vacuum contactor for 3ph 120 hp SCR induction motor.

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