What is a Capacitor Duty Contactor? Working and Advantages

A capacitor duty contactor is a crucial component in electrical systems that helps control the flow of electrical power. It is responsible for switching capacitors on and off in electrical circuits and its working principle is based on the principle of electromagnetic induction. This article aims to explain the working principle of the capacitor switching contactor and its advantages over traditional contactors. It will give a comprehensive understanding of how this device operates and the benefits it brings to electrical systems.

What is a Capacitor Duty Contactor?

The capacitor duty contactor is a specially designed contactor for switching 3-phase, single or multiple-step capacitor banks in power factor correction applications. It suppresses high switching starting currents of the power capacitors.

The connection of power capacitors in automatic capacitor banks causes high transitory overcurrents. The value of the overcurrent connection peak can achieve values of even 200 times the rated current of the capacitor.

These high currents can damage both, contacts of the contactors and capacitors. The voltage oscillations can produce problems in other circuits of the installation.

Therefore a capacitor contactor is needed in capacitor-switching applications.

New-generation capacitor systems use special types of contactors that are equipped with damping resistors to suppress the high switching currents. The resistors are switched in series with the capacitor banks.

Capacitor Duty Contactor Working Principle

The working principle of capacitor switching contactor has 2 steps:

1-When the coil is energized

The early making auxiliary contacts connect the capacitor to the network via the set of 3 resistors. The damping resistors attenuate the first current peak and the second inrush current when the main contacts begin to make. Once the main contacts are in the closed position, the auxiliary contacts automatically break.

2-When the coil is de-energized

The main contacts break ensuring the breaking of the capacitive current. The contactor can then begin a new cycle.

This working principle extends the capacitor and contactor life.

Capacitor swwitching contactors are installed in switching single-step or multiple-step capacitor banks in commercial and residential buildings.

Capacitor Duty Contactor Advantages

Using a capacitor duty contactor has many benefits for the system such as:

• Excellent damping of inrush current.
• Improved power quality. (e.g. avoidance of voltage sags)
• Extended capacitor life.
• Soft switching of the capacitor and thus longer useful service life.
• Reduced ohmic losses.
• Increased system reliability.
• The unplanned halt of production and the risk of reactive penalties decrease.

Capacitor Duty Contactor Selection Parameters

When selecting a capacitor switching contactor below product parameters should be considered:

• Rated power (kVAR)
• Rated insulation voltage (Ui)
• Rated operational voltage (Ue)
• Coil voltage
• Electrical durability
• Operating temperature
• Auxiliary contacts mounted
• Maximum switching frequency

Capacitor Switching Contactor Circuit Diagram