Capacitor Duty Contactor: Working Principle, Advantages, Basics

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.

Working principle

The working principle of capacitor duty 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 duty contactors are installed in switching single-step or multiple-step capacitor banks in commercial and residential buildings.

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.

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

Circuit diagram